Class AB-1 v. Class C Amps
C. R. McCally
Whats the difference???
Thanks,
c...@airmail.net
Bill Nelson
C. R. McCally (c...@airmail.net) wrote:
: Whats the difference???
There are two major differences. When driven to peak performance, the
Class C amp will have more output - as Class C amps are more efficient
than class AB1. The second is that Class C amps are not suitable for
AM or SSB, unless you modulate the OUTPUT of the amp - rather than apply
a modulated signal to the input of the amp. As far as I know, no CB amp
uses such a modulation technique.
--
Bill Nelson (bi...@peak.org)
Steve Eklund
cod...@aol.com writes: > Greetings!
Lots of good stuff clipped
> Steve Ecklund - take note! CB radios with AM and SSB capability and use a
> regulated collector design - AM mode is considered Class C, due to the
> lowered base and collector voltage bias into the Driver and Final is now
> in the cutoff region. SSB is operated with the driver and final now into
> a linear region from the bias voltages being raised to just to the knee of
> it's operating range [0.7 volt] making the amp section a Class AB type -
> because the waveform being amplified is used into the transistion region
> and maintains linearity over a wider range of signal levels. This is also
> why you have adjustable bias "pots" to control the operating region of the
> radios transmit section - it allows you to adjust the operating
> characteristics of the final and driver when they are replaced. Not all
> transistors in the same designation operate the same - they are flawed and
> the radio can compensate for that.
Noted! I still can't understand why AM is different because it contains
both upper and lower sidebands. Does the carrier act as the basis
for the amplifier's "window" between amplifying threshold and saturation?
I always thought that AM and SSB both had to have at least class AB
for lowest distortion and best reproduction.
>These two points, drive level and bias - is some of the important reasons
>for your decision, or to investigate further in your search to reason out
>the differences between classes of amps. Pretty much any "Long-Haired"
>book for Transistor theory and applications will describe these in more
>detail - better than I can.
>Hope this helps!>
I doubt anything can describe it better than you can! Good
post Andy!
73 from Steve
Bad Dog
In article <32DC17...@airmail.net>, c...@airmail.net says...
>
>Whats the difference???
>
>Thanks,
>c...@airmail.net
look in the ARRL handbook at your local library and they tell the secret
about how they class amplifiers ,Class C is really meant for use on CW
not on Audio but it works but you really can hear the differance , A
Class AB-1 is better for Audio and sounds clean over the airwaves and a
Class C sounds like 200000000% harmonic distortion, in other words it
sounds like crap ... AB-1 is the way to go and it has less channel
spatter than the Class C. Class B is great for audio but class A is the
best for Audio.
cod...@aol.com
Greetings!
In article <32DC17...@airmail.net>, "C. R. McCally" <c...@airmail.net>
writes:
>Whats the difference???
>
>Thanks,
>c...@airmail.net
In simple terms, Class AB-1 is a more "fidelity" - or true to input
[waveform] than a Class C.
The difference is important, or not necessary - it depends upon your
needs.
If you use SSB for CB, then AB-1 is the preferred way to go.
If you don't use SSB - and transmit on AM or FM or use CW [Morse code] the
output of the radio is sufficient to drive doth types.
There is a long winded reasoning behind it - I'll try to keep it simple...
Think sine waves...
AB-1 amps use most of the wave before it swings into the negative region
[below 0] - and is considered "truer" to reproducing the actual input
because it operates over a larger range of input signal wave. This Class
uses more if the input wave to recreate the output wave.
The AB-1 amps are biased [designed to keep the transistor in it's "on"
state] so that most of the input wave is reproduced at the output of the
amp. This reduces filtering and "coloring" of the output and affects the
purity of the received signal at a remote location. There is more
inefficiency to this - because the transistor doing the amplification
consumes more current to be wasted as heat in resistance and overcoming
it's limitations - but the end product requires less filtering at the
outputs of the amp stage. With less filtering, some efficiency is recouped
from losses in extra filtering that is needed with Class C amps, because
AB's are cleaner.
Another factor of AB-1 amps, are their efficiency at low input signal
levels - very useful for reproducing SSB signals under amplification. The
type of bias used allows very low signals to be greatly amplified. Used
for "dirty" amplification of analog or voice signals with less distortion
components that aren't as easily detected and can be effectively filtered
out.
Steve Ecklund - take note! CB radios with AM and SSB capability and use a
regulated collector design - AM mode is considered Class C, due to the
lowered base and collector voltage bias into the Driver and Final is now
in the cutoff region. SSB is operated with the driver and final now into
a linear region from the bias voltages being raised to just to the knee of
it's operating range [0.7 volt] making the amp section a Class AB type -
because the waveform being amplified is used into the transistion region
and maintains linearity over a wider range of signal levels. This is also
why you have adjustable bias "pots" to control the operating region of the
radios transmit section - it allows you to adjust the operating
characteristics of the final and driver when they are replaced. Not all
transistors in the same designation operate the same - they are flawed and
the radio can compensate for that.
Class C amps, are considered "efficient" due to the type of reproduction
they produce. Remember the sine wave? Class C amps use the upper 3/4 of
the wave to amplify the signal. This looks choppy, and takes some
considerable skill for the engineer to design a filter circuit to equalize
the attack and decay times of the output filter to make the output more
linear [as a true sine wave without harmonics]. This helps describe the
"coloring" effect this class of amps produce.
Part of the efficiency is also from the lack of bias used to drive the amp
- there is none. To turn on the transistor, the signals have to acheive a
voltage and current level, that when applied to the amplifying transistor
- turns it on. It causes current and voltage to flow and starts
amplification.
Class C is more like a switch versus "linear" or fidelity reproduction
you'd find in Class B/AB/A types.
The transistor [any type for an amp] driven like this, needs a signal
level thats within it's operating range to maintain some linearity - and
these "operation windows" are a very narrow range of voltages - either
fully off when it drops low too far, or full saturation and distortion
from operating beyond it's linear range.
An extra amount of filtering is needed to smooth out the produced
waveform, and it's tricky. Because of saturation and cutoff levels induce
switching transients [noise spikes] that are full of harmonics and takes,
as said before, considerable skill in determining the actual bandpass and
components needed to reproduce the required waveform to recreate the
amplified signal in respect to the input waveform.
These two points, drive level and bias - is some of the important reasons
for your decision, or to investigate further in your search to reason out
the differences between classes of amps. Pretty much any "Long-Haired"
book for Transistor theory and applications will describe these in more
detail - better than I can.
Hope this helps!
:+> Andy <+:
... You could be a CB'er if...
... you're in a Class, all by yourself...
Joe
Andy -- I concur with Steve. Excellent post on the difference
between AB-1 and C class amplifiers. Might want to save the
post to a file for future FAQ.
Joe
----------------------------------------------------------
Amateur Radio: BV/N0IAT Taipei TAIWAN Republic of China
http://www.transend.com.tw/~joentam/index1.html
ex. 7J1AOF (Japan) YU3/N0IAT (Slovenia) KA0ZDH (Novice)
Licensed Radio Amateur since 1986. Comments are mine only.
----------------------------------------------------------
Brian Ellsworth
On Thu, 16 Jan 1997 14:02:21 GMT, Steve Eklund <wa3...@compuserve.com> wrote:
>cod...@aol.com writes:
>Lots of good stuff clipped
>> regulated collector design - AM mode is considered Class C, due to the
>> lowered base and collector voltage bias into the Driver and Final is now
>> in the cutoff region. SSB is operated with the driver and final now into
>> a linear region from the bias voltages being raised to just to the knee of
>> it's operating range [0.7 volt] making the amp section a Class AB type -
>
>both upper and lower sidebands. Does the carrier act as the basis
>for the amplifier's "window" between amplifying threshold and saturation?
>I always thought that AM and SSB both had to have at least class AB
>for lowest distortion and best reproduction.
>
Class C is not suitable for AM or SSB when used as a 'linear' amplifier. The
simple reason class C is used in the final of CB radios is that the modulation
occurs IN the final not before it. When the radio is in the SSB mode the
modulation occurs prior to the final stage requiring the final to maintain a
more 'linear' response.
-be
Steve Eklund
bi...@PEAK.ORG (Bill Nelson) writes: > cod...@aol.com wrote:
>
> ... deleted ... A description better than most I have seen here about
> various classes of amplification.
>
> : An extra amount of filtering is needed to smooth out the produced
> : waveform, and it's tricky. Because of saturation and cutoff levels induce
> : switching transients [noise spikes] that are full of harmonics and takes,
> : as said before, considerable skill in determining the actual bandpass and
> : components needed to reproduce the required waveform to recreate the
> : amplified signal in respect to the input waveform.
>
> filtering, or other method, to recover the original modulating waveform.
> You may be able to smooth it - and thus get rid of the worst of the
> harmonics - but it will always remain distorted. A class C amp cannot
> produce an undistorted waveform.
>
> Class C is fine for CW and FM. It is NOT good for AM, although many amps
> seem to use such biasing.
>
> --
> Bill Nelson (bi...@peak.org)
>
>
They want to advertise the most bang for the buck. The higher
the wattage on dead key, the more sales they will get. Forget
distortion, most of the people who buy these amps don't care.
They only care about how high their meter reading is, and
the manufacturers only care about how much money is in their
pocket.
Steve
Bill Nelson
Bill Nelson
Brian Ellsworth (ka...@ziplink.net) wrote:
: Class C is not suitable for AM or SSB when used as a 'linear' amplifier. The
: simple reason class C is used in the final of CB radios is that the modulation
: occurs IN the final not before it. When the radio is in the SSB mode the
: modulation occurs prior to the final stage requiring the final to maintain a
: more 'linear' response.
Not in any CB radio that I have seen. To have faithful reproduction of the
waveform with a Class C amp, the modulation would have to be applied to the
output of the final amp. This would require very high modulation power, and
is actually used by some AM radio stations.
--
Bill Nelson (bi...@peak.org)
Bill Nelson
Steve Eklund (wa3...@compuserve.com) wrote:
: Noted! I still can't understand why AM is different because it contains
: both upper and lower sidebands. Does the carrier act as the basis
: for the amplifier's "window" between amplifying threshold and saturation?
: I always thought that AM and SSB both had to have at least class AB
: for lowest distortion and best reproduction.
You are correct, and he is wrong. Class AB1 and (rarely) AB2 produce
distortion that is deemed acceptable on SSB and AM. Only Class A can
be made truly distortion free - unless you use what would be called
"plate modulation" in the old days. Class C is fine for CW or FM,
where the amplitude if the signal does not change with modulation.
--
Bill Nelson (bi...@peak.org)
cod...@aol.com
Greetings folks!
Steve...I'll learn to spell your name [mane?] better next time [Shucks!]
;-)
Another point here...
I don't use linears, because of what Steve put bluntly, and improper
design overlooking too many variables to strive for the extra wattage [and
they do make a pretty good Bacon Cheeseburger when operated in their
second office - and with the addition of a fan, a great Jenn-Aire<TM?>
grill ;-] They waste my time and effort by masking the true efficiency of
an antenna system - and generate other problems we've all seen many posts
about...IMHO
In article <32df6e0b...@news.ziplink.net>, ka...@ziplink.net (Brian
Ellsworth) writes:
>>Noted! I still can't understand why AM is different because it contains
>>both upper and lower sidebands. Does the carrier act as the basis
>>for the amplifier's "window" between amplifying threshold and
saturation?
>>I always thought that AM and SSB both had to have at least class AB
>>for lowest distortion and best reproduction.
>>
>
>Class C is not suitable for AM or SSB when used as a 'linear' amplifier.
The
>simple reason class C is used in the final of CB radios is that the
>modulation
>occurs IN the final not before it. When the radio is in the SSB mode the
>modulation occurs prior to the final stage requiring the final to
maintain a
>more 'linear' response.
Hi guys...I'm thinking aloud here, so correct me if I'm wrong...I didn't
expect the type of response that this thread has generated...so I need
some room to help brush up on some theory...remember...it's only
thoughts...
There is another factor and it deals with the mixing of the signal from
the balanced modulator to the mixer. A small carrier signal, centered on
the audio IF, is generated here, at the modulator. It's the actual IF
frequency detected [or balanced by an offset adjustment] with audio added.
Then sent to be filtered for purity, then onto the mixer, to be combined
-excited by the PLL VCO section, and amplified. At the same time the audio
present at the Mic amp section, which then is also modulated at the
collectors of both the driver and final - help create the proper envelope.
The collectors are regulated by the Mod amp being switched fully on, and
allowing full voltage to appear at the collectors in SSB modes. This is
only allowing full bias and drive being made available to the collectors
[perhaps modulation is a loose term - consider it fully biased to generate
the proper current drive with limitations being corrected at the ALC
control settings] - which control the AF attenuation at the mic amp
stages.
AMC works much the same way, but requires a carrier being present and when
it's [in AM mode] modulated, the tap - located at the output matching
network, it's rectified and a control signal is generated and this level
control is returned to the Mic AMP section, and in some cases, the
balanced modulators AF input comparator, and acts as a floating gain
control of the comparators' audio input.
Because of the centering of the balanced modulator - a signal is always
presented to the mixer and contains audio information. When SSB is used -
audio information is offset [usually 2.5KHz] and a voltage offset at the
modulator is compared and the resultant signal is mixed with this IF and
the PLL derived frequency from the VCO loop - into the mixer. The voltage
offset is used to "Balance" the equality of the carrier level signal
normally generated at the Modulator - with the audio IF. The voltage
offset also helps with proper offset deviation centering when a different
frequency is presented to the mixing input - it helps keep the offset
frequency centered. The deviation of the IF at the modulators output is
an important factor.
A filter is used, centered on the expected output of the balanced
modulator - and due to the frequency shift occuring because of the offset
audio - only one portion of the dual sidebands generated at the modulator
can pass through. The modulators' signal is crystal controlled by the
audio IF and when the audio IF deviates from the voltage offset - the
output of the modulator is also shifted in frequency. It's this "shift"
that makes SSB work without sending a carrier - for the modulation
presented to the mixer is filtered and will contain "inverted", or
"positive" [I know I shouldn't talk like this ;-] waveforms of the shifted
[and filtered to the correct bandpass] audio IF. The true [zero beat] IF
is a signal that is centered on the crystals' IF output. When audio is
presented to the audio input, the IF frequency is then modulated and would
contain information reflected in both above and below the center IF
frequency.
An inherent design problem is the proper shift above and below the center
IF to remove part of the undesired audio spectrum portions - like bass and
treble - yet have enough fidelity to be intelligble speech. These
spectral componenets can affect the stability of the transmitted outputs.
Many manufacturers address this issue on many fronts, some design inputs
with limited spectrum response desired before amplification. While others
filter the audio input, and use a low pass filter network on the mic amp
section. And others use a compression technique to limit dynamic range to
a constant level [like AGC] and use negative feedback heavily to help in
filtering and reducing harmonic components that can affect the
transmittters stability and output. It depends on the ingenuity and
creativity, along with efficiency - of the engineers' design plan.
However, there are flaws in every design and the tolerances of the
components used can add to the complexity of the problem.
Back to the thought thread...
It's how the balanced modulator works, and is why we have carrier balance
- to center the carrier generated, to the audio mixing thats' occuring at
both the modulator and at the collectors [envelope] in the TX section.
Without the carrier balance - we would create dual sideband no carrier
type signals. And depending on the tuning of the audio IF - one side
would dominate over another [Asymmetrical]. This helps to tie into what
Bill Nelson was talking about - and may help explain how this technique
can work. [At least I hope so...correct me Bill if I'm wrong]
The mixer otherwise would generate a signal in porportion to the audio
information occuring on both sides of center on the balanced modulator, as
if it was on AM. With this waveform - the mixer takes the correct offset
and amplifies the mixed audio IF of the proper offset , under SSB mode,
and ignores [with the help of the modulator] the center and opposing
sideband and this then is mixed with the frequency synthesis generated at
the PLL and excited to the bandpass that the mixer output is sent into.
Then into the 1st and 2nd stage RF amps [if applicable - some only have 1
stage].
What does this have to do with the 2 different Amp classes?
Well, I had to tell you the above to get to the below...;-)
Remember the AF modulator section going to the Collectors? In AM mode, the
Mixer is only generating low level carrier and modulation envelope. To
control the gain of the envelope, the AM modulator generates a voltage -
and the current needed by AM to produce the modulation is supplied by a
control signal from the Mic amp section and the AMC controls - which
develops the [gulp] "swing" regulation. The AM modulator only supplies
regulated voltage with extra current gain control provided by the AMC/MIC
amp section - some voltage gain is acheived by fluctuating the audio
envelope signal when it's applied to the collectors.
Thats' why we have two PNP transistors in the AM mod section - one
controls the gain of the other [Darlington Pair] and presents the proper
voltage and current gain at the collectors - and is why it's called High
Level modulation and only works on AM - we are controlling the gain at the
outputs by providing a regulated control - and is controlled [regulated]
by audio to form the proper level of envelope.
In SSB modes, the control is bypassed and audio is not presented to one of
the transistors - it's turned fully on to present full voltage to the
collectors. In some circuit designs, the other transistor helps regulate
the bias of the collectors by shunting some voltage signal to a
resistor/capacitor bridge that goes to the base of the main current driver
and to ground, and has a control signal being sent to it from the ALC
circuit. This idea is designed to offer more control. It simply reduces
voltages at peak points on the SSB envelope to keep the collectors from
saturating and forming spectral impurities that aren't supposed to be at
the outputs.
The reduced voltage is controlled in the AM mode. [We're back to AM mode]
And the input signal from the RF amp stage[s] in AM mode is sent to the
base of the driver, with the audio gain controlled at the collector. The
bias for the bases' remains the same. However, the carrier and moduation
presented to the base has a greater level, and the voltage and current
needed at the collector is reduced, as a function of design and FCC
limitations of spectral purity and distortion componenets.
Because of collector, base, emitter junction flow, the Class
Characteristic of the amp stage changes - and the collector to base
junction is biased below Class AB and into Class C region in AM mode.
This is more important in the Final section. The amplified section from
the driver is presented to the Final - and again sees a Class C type of
input because of Collector to Base [or base to collector - electron flow]
potentials are lower. The carrier and audio envelope being presented, is
enough to properly drive the bases of the driver and final - with
limitations being imposed by the Mod amps. The envelope is regulated by
adjusting the voltage/current gains at the respective collectors. In a
common Emitter configuration, the efficiency design is from the power
gained by supplying extra current to the regulated voltage - and that is
supplied by the Mod Amp, Darlington - Pair configuration. When current
rises, voltage also can change, or drop - in porportion to the required
amount of gain as sensed by the AMC section which feeds back controls to
the AF attenuators, and the Mic amp driver to control the amplitude of the
envlope being generated.
On AM mode, and SSB - some extra control is added to keep the base
voltages stable in all modes of operation. The Thermistors are designed
to protect the transistors they're attached to, from excessive input
signal level and overheating by adjusting their own internal resistance
and acts like a limiter to remove the higher positive potentials through
rectification - and DC conductivity, into ground. The forward bias drop of
the diodes is used to help drive the transistor into full conduction, but
not saturation, by it's inherent voltage drop.
However, saturation would be prevented by keeping the base region from
exceeding the unwanted saturation level by preventing excessive current
and voltage from being applied to the base. This can be accomplished by
reducing the available collector voltage and current gain.
Another factor present in some amp designs is an actual limiter diode and
resistor and a small portion of the prior stanges RF is sent to the base
bias section of the Final from the collector output of the Driver [past
the DC blocking cap of the aforementioned stage] to help adjust the output
to input RF ratios into the next stage - and limits actual signal input to
prevent saturation from excessive base drive in both SSB and AM modes. It
also keeps forward voltage leakage at the base region from becoming an
unstable factor and casing instability problems.
To Steve...
We've had some correspondence in trying to figure out the type of Class
certain radios are. After doing some research, this is some of the things
I've found. If this helps, you're welcome.
Summary...
In SSB mode, the collectors are fully biased and because of electron flow
into the base region - the base voltage bias is higher and changes to a
Class AB type of amp. It's merely how you present the ratios of collector
to base voltages that can change the overall bias current in the
transistors and the class they operate in.
Question though...
CB radios fall into a Class D type designation - is this due to the type
of modulation drive used, even though the Driver/Final pair derive no true
operating voltage except for the collector regions? Or is it because the
amplifier design is made to straddle or transient between two classes of
amps due to the modified collector bias design? Just curious...
Continuing...
SSB needs no Modulation drive from the Mod amps and the Mod amps are
normally biased, to either fully on or partial control [depending upon the
manufactuer] - to create the extra overhead to make the base region more
sensitive to the inputs from the primary RF amp stages after mixing and
excited to the frequency selected.
]Remember, in SSB, the offset and filtered output of the balanced
modulator now contains audio mixed with IF and the selected frequency
offset above or below the centered expected frequency, and has no produced
carrier.[
This in turn makes the Final and Driver more linear in their operating
range. The base bias is normally taken from the TX switched side of the
regulated voltage that is used to power the respective RX or TX circuits
and the PLL and VCO sections. The entire modulation envelope generated, is
linearly [by adjusting collector bias] amplified to the extent of the
current and voltage made readily made available from the AM mod/SSB
switch. We are making the base region more sensitive to changes by
applying a greater voltage potential at the collector.
Kinda neat eh?
In AM, we reduce voltage at the collector to prevent excessive saturation
and change the class of the amp stages. Because, the collector to base
voltage region operating characteristics affect the bases' response level
sensitivity - lowering the bias of the bases' linear region to below it's
linear operation voltage knee.
In SSB, we increase the potential of the collector and make the Base
region more sensitive to changes in smaller signals and pushes the
transistor into more linear operating characteristics.
I hope this helps answer some questions...or clarify my thoughts on this
issue...
Thanks folks!
:+> Andy <+:
the...@aol.com
In article <5bptin$9...@bashir.peak.org>, bi...@PEAK.ORG (Bill Nelson)
writes:
>Not in any CB radio that I have seen. To have faithful reproduction of
the
>waveform with a Class C amp,
Not in any CB radio that I have seen."faithful reproduction" indicates a
linear response. Linear response and Class C ain't quite compaatabell.
Carl
The2x4
cod...@aol.com
I know that the big explanation in last message I sent, does contain some
errors. Not because of carelessness, or ignorance - it's from the lack of
clarity on some issues.
[Yes, I do drink V8...I used coffee as a substitute last night.] ;-)
I got a little confused, the outputs of the Colectors at the Mod amps are
going into the respective Collectors of the Final and Driver - this may
have appeared confusing to some and I do appologize...Audio gain is
controlled by the Collectors of the Mod Amp/SSB switch into the Collectors
of the Driver and Final. I may have explained it in transposed error.
Most CB radios [if not all] use Common Emitter configurations for the
amplifier section.
And I still have a lot of questions - and points I need to get some
clarification on.
Forgive me, but one topic is Thermistors versus Varistors, are these terms
used interchangeably? One reference here states Thermistors are a class
of devices that exhibit temperature to ohmic changes, while Varistor
devices change values in relation to temperature by varying their current
drop across their junctions. [Varistors and Thermistors - I once thought,
were two separate types. One exhibits changes in resistance, while
another type changes it's resistance to lower or raise current conduction
through the device.] Varistors - are they the silicon solid-state devices
and Thermistors simply resistive changes? The reference is old - and its
an older Sams book that doesn't seem to be too clear on this. There are
also two types of Varistors - one is voltage to temperature compensating,
while another is rated to drop voltage to a rated level [much like
Zeners?] when excessive current is applied.
It's not a big deal - I'll stand corrected if it's considered Varistors,
and I think it is.
Another one, the Balanced Modulator - from what I understand, will always
have an output frequency from the audio IF being applied to the mixing
input. Even though there is no audio present.
And I don't know if this may tie into what Steve asked, but I've stumbled
onto a problem that I only know of - but not a true answer to. Perhaps by
opening a discussion, I hope to find an answer. I'm also getting my
money's worth from AOL. ;-)
It deals with the output of the TX sections when running in AB mode versus
class C. There seems to be a lack of information. All I can find in my
intuition about this, if someone can review this with me, and it deals
with the very types of items, or diodes used to limit saturation - and the
Class. We are looking at the Driver and Final sections.
The diodes orientation is across the Base to Emitter with Common Emitter
to ground, so it's forward biased and it exhibits a voltage drop across
the junction - so looking through amplifier designs - I see that the diode
can also rectify the generated envelope being applied to the Base. So, is
this diode [Varistor - really] desgined to limit voltage and/or prevent
conduction of the RF wave into the transistors negative region? I'm
looking at the problem two ways, and how it's answered can possibly shed
some light and answer Steves' question in relation to the components
applied to the circuit.
How I can only answer your question Steve, is from knowing the transistor
parameters and subjecting intuition from there. I'm not really taking a
flying leap, but I do want to warn you that much of this - although based
on prior experience and knowledge - may omit something or be perceived
incorrectly. If you find something I've missed or sounds confusing - let
me know.
The Varistor being used, has a voltage region that makes the transistor
operate close to the lower knee of it's linear range. We'll choose an
aribtrary value of 1.00 volts at 30 mA ratings for the diode. We don't
really need to know these figures - but if you remember the tune up
sections, you note the bias adjustments require a certain current level in
mAs' for Collector current to drain through - into the Base and Emitter
regions and this drain can't exceed the mA ratings of the diode. The Base
to Emitter junction has it's own internal resistance, so some flow will
appear at the Base and remain a factor unless it's removed - and the Diode
does part of this removal job - we also have voltage appearing as part of
the Base bias from the switched TX line. We also need room for the base
current and voltage drive needed, and being delivered to it, from the
prior amp stage. The 1.00 figure is also needed to help explain the
reasoning I have when looking at the "Window" [it's a pretty good term -
because in a graphical sense - that's what I see too.] the transistor
operates in.
[In real life terms, a typical transistor has a forward voltage drop of
approximately 0.7 volts - and the Varistor, usually an MV1Y - has a drop
of about 0.68 volts. Because of the characteristics of Silicon and heat -
the voltage drop of a typical transistor can rise to over 2 volts when
running hot - increasing the potential of saturation problems. Due to the
type of doping substrate used in a Varistor, it's designed to increase the
drop potential and allow for more current sinking - shunting away
excessive current drive being built up from the transistors internal
resistance changing from the rise in it's internal voltage drop.
Preventing Thermal runaway.]
The AM signal being applied to the transistor can easily exceed the
voltage and current being supplied by the prior stage.
By adjusting the Collector current, the level of voltage needed at the
Base to operate in the linear region is also affected. However, because
of the output of the prior stage has a carrier and modulation envelope -
and is being applied to the base in the next stage - this signal can cause
the Base region to saturate. And there may not be enough voltage and
current present at the Collector to prevent it without some form of
regulation at the Collector and or Base voltage control.
In the amplifier design of a typical CB SSB radio, the Base region is
current limited by the bias adjustment pot and the Varistor. Both are
conducting current and voltage at DC levels and the diode serves a
function of preventing the RF signal from making the transistor conduct
into the negative region and too far forward into the positive region by
it's own Diode characteristics or shunting current and voltage dropping,
in relation to temperature and capacity. If the transistors' forward
voltage drop at the Emitter and Collector regions is less than the
Varistors' expected drop - the Transistor will conduct into the negative
region and the base will have a negative bias potential and shutoff -
causing distortion. The conduction region in the Base area is voltage
dependent and if the Varistor cannot respond to the signal present in the
Base, the Base will force the shutoff and cause harmonics to appear at the
Collector output from the "spike" or capacitive discharge from reverse
electron flow attempting to go into the now reverse biased boundary. The
"spike" is short lived and the remainder of the conduction into the
negative region is flat.
In the amplifier design, the Varistor is designed to prevent the Base from
going above it's linear range in potential than it's own internal voltage
drop to the Emitter - and also present a way for electrons to pass by the
base region into ground - limiting current and voltage at the Base - much
like a parallel resistor that conducts only one way. At higher
temperatures the Varistor increases it's forward drop current potential.
The varistor conducts first on the positive peaks then shuts off - and
allows the Collector current and it's voltage - carry the Base region into
the bottom of the conduction region, and if the input from the prior amp
stage is designed properly - not into the cutoff or negative region.
This affects the class of the amp stages. When using SSB, Collector
voltages are higher - and the Base sees less of a barrier at the junction
to the Emitter/Collector. Emitter current flow is reduced because of the
current needed at idle is much less. There is no signal present at the
Base to cause conduction unless mixed RF with audio is present. The bias
setting in this area mainitans the proper "pinch" off state yet provides
voltage to conduct when RF is present. In AM, the Base sees a signal, and
Collector flow is much greater because the Base is attempting to conduct,
not only from the Collector, it's also flowing into the Emitter. Because
the Collector voltage is higher, and the Emitter potential is less, the
Base region sees less of a barrier and allows more voltage and current
potentials to overcome the barrier and allow currents to flow past the
Base into the Emitter from the Collector region. In both AM and SSB the
results are the same, only the Collector to Base voltage ratios change the
barrier regions' sensitivity. Higher Collector voltages reduce the
barrier region transistion state in the Base, yet allows the Base to still
remain off - only more sensitive to the impedance changes that occur in
the prior stage when RF is present in low levels that would normally not
cause the Base region to conduct.
The extra push being supplied by the prior stage in the AM mode, without
the Varistors or the reduction of junction currents - will make the next
stages appear as Class A - but unable to conduct into the negative portion
unless the Collector voltage at the next stage is greater than it's own
expected Base regions potentials. If the Base swings into the negative
region, cutoff and distortion occurs. And the output appears to indicate
Class C operation. To prevent this from happening - the Varistors'
characteristics helps prevent the switching crossover distortion from
occuring by preventing the Base to conduct into the negative region by the
Diodes' own conduction before the DC voltage bias present from the TX
power line keeps the Base from complete cutoff.
The output in the prior stage needs to have it's own controls and
therefore has limited gain - to a point that the linearity of it's output
does not exceed the input "window" levels of the next stage. Depending
upon amplification - the prior stage can have a Class A operation into the
base of the next stage. Class A operation into the Driver/Final stages
helps in reducing output distortion.
If you note, there is a small value Capacitor used on the Collector in AB
amps - to help in producing a low end curve from the discharge and slow
rise time from charging [it's frequency dependent] of currents and voltage
as the flow into the Emitter of the Driver and Final. This produces a
smoother waveform and helps reduce transient crossover distortion products
when the Base pinches off flow.
Well, flame away if you must - but I'm trying to find answers...
:+> Andy <+:
ko4qc
questions,buy another book that does.
Ko4qC
William Hughes
Codys PC
>
>Class C is fine for CW and FM. It is NOT good for AM, although many amps
>seem to use such biasing.
>
>--
>Bill Nelson (bi...@peak.org)
Which is why ask the Question...
What is Class D?
Considering the explanations I've recollected from memory and little
reference left of numerous moves in my past life...;-) All I can think of
is that the Amp design in the radios is a new classification, or the
Type-Certification stamp classifying the radio as a "Certifiably" Class D
emmission device.
Let's open up a little more on this...
A SSB/AM CB radio seems [IMHO] a combination of AB and C type of
classification - just by looking and deducting ideas from it. It's
doesn't truly bias like Class C and it doesn't operate truly like a Class
AB either.
OR...
The FCC's classification of an operating device...
Which is it?
Now, you may want to slam me for this. But, I haven't seen an answer to
the question.
Are we looking for a new type of Hybird amplifier...?
Rethink the development of the different classes of Amps?
Or, just nobody cares - it's the FCC's problem...
*sigh*
:+> Andy <+:
Bill Nelson
Codys PC (cod...@aol.com) wrote:
: What is Class D?
: Considering the explanations I've recollected from memory and little
: reference left of numerous moves in my past life...;-) All I can think of
: is that the Amp design in the radios is a new classification, or the
: Type-Certification stamp classifying the radio as a "Certifiably" Class D
: emmission device.
I suspect the stamp simply means that the transceiver is certified for
Class D Citizen's Band usage, and has nothing at all to do with the class
of operation of the final amp.
: Let's open up a little more on this...
: A SSB/AM CB radio seems [IMHO] a combination of AB and C type of
: classification - just by looking and deducting ideas from it. It's
: doesn't truly bias like Class C and it doesn't operate truly like a Class
: AB either.
: OR...
: The FCC's classification of an operating device...
: Which is it?
See above.
--
Bill Nelson (bi...@peak.org)
bill harris
bi...@PEAK.ORG (Bill Nelson) wrote:
>Brian Ellsworth (ka...@ziplink.net) wrote:
>
>: Class C is not suitable for AM or SSB when used as a 'linear' amplifier. The
>: simple reason class C is used in the final of CB radios is that the modulation
>: occurs IN the final not before it. When the radio is in the SSB mode the
>: modulation occurs prior to the final stage requiring the final to maintain a
>: more 'linear' response.
>
>output of the final amp. This would require very high modulation power, and
>is actually used by some AM radio stations.
>
>--
>Bill Nelson (bi...@peak.org)
I would say most if not all standard AM CB radios use class C in the
final with modulation applied to the final stage. To obtain 100%
modulation of a class C stage the modulator must be able to supply audio
at 1/2 the input power of the RF final, thus for a 5 watt final the
modulator must supply up to 2.5 watts audio.
Brian Ellsworth
Hmm, Bill's response never made it to my server... Anyway comments on the class
C thing. I was talking about modulation to the 5watt final. Seems to me the
modulation power required would be about 5 watts. What's all this 'very high
power' talk? I haven't looked at a CB schematic in years but on older AM only
sets class C is used with the modulation transformer coupled into the collector
or plate circuit.
-be