Super Diodes and detectors
Graeme Zimmer
This post was prompted by the recent discussion about Diode-Connected
Transistors.
The Crystal Set nutters are always seeking the holy grail of a perfect diode
detector
(it must be passive of course).
The problem is that existing diodes are lousy rectifiers at low currents
(three steps forward and two steps back).
What is needed is a diode which has a steep forward/reverse curve at very
low currents
(e.g. even with bias, a Silicon diode is worse than the old fashioned
Germanium).
The favorite at present is a zero-bias FET, using the high Q of a tuned
circuit to develop a voltage swing to drive the Gate, which switches
(varies) conduction in the (low impedance) Drain/Source path.
Any comments please?
Thanks ............ Zim
John Larkin
I think the best low-level detector diode is a germanium back diode,
essentially a tunnel diode with very low peak point current. People
still make them.
John
Graeme Zimmer
> essentially a tunnel diode with very low peak point current.
Thanks John,
Yes, Tunnel diodes have a sharp knee and can work well with apropriate bias.
There are many other tricks, such as using as high impedance as possible (to
get higher voltage swing) and a Schottky (eg HP 5082-2835).
The best "normal" diode so far seems to be the rather obscure ITT FO-215
some info on the FO-215 at
http://www.theradioboard.com/rb/viewtopic.php?p=16891&sid=12ca5c44ec9c2b7b97a6af75bc6770e7
The FET idea can be seen at:
http://www.arrl.org/files/file/Technology/tis/info/pdf/culter.pdf
........... Zim
Robert Baer
m II
> I think the best low-level detector diode is a germanium back diode,
> essentially a tunnel diode with very low peak point current. People
> still make them.
..as visions of a straw covered alpine shack floor, with a germanium
spinning wheel in the corner come to mind. Outside, the dark and
foreboding clouds of the impending Silicon invasion are gathering. A
silence has filled the Valley.
mike
Graeme Zimmer
Yes, they are often used.
No nearly as efficient as good dynamic headsets however.
The best are the old Navy "Sound Powered" Headsets.
If you can find them
................... Zim
maitchy
valley, it never really had the same potential.
Back to the original point though, JFETs with source & drain connected
together are one thought. The silicon Schottky CDSH270 (replacement for the
1N270, etc) is quite good compared with old timers like the OA47..OA95,
1N34A, etc. Original germanium diodes have low Vf only at low currents
(possibly if people applied some of what has been learnt since the 1950's
they could make much better modern germanium devices?). I am interested in
back-diodes though, to try... I thought their supply had dried up...
possibly they have advantages at high speed (well, compared with "precision
rectifier" circuits anyway). So where do I get back-diodes from now?
Using a JFET (or pentode?) as an infinite impedance detector (which I think
was what was meant in the first post) is pretty good (tends to be a bit
fussy about the resistor value, but only a bit fussy). I have used them at
68MHz (with an MPF102, extracting sound FM carrier from VHF PAL TV
signals)... no RF amplification, so a lot like a crystal set in finding the
right rabbit to use for its ears.
I recall "super diode" being used to describe transistors with base to
collector, used as a diode (e.g. thermal compensation in output stages),
but they aren't great at low signal levels... pretty good at higher
currents though. Now "super diode" is used only to mean "precision
rectifier circuit" in many web pages.
---------------------------------------
Posted through http://www.Electronics-Related.com
John Larkin
<M.Aitchison@n_o_s_p_a_m.chch.planet.org.nz> wrote:
>Ah... Germanium Valley was never as popular and successful as Silicon
>valley, it never really had the same potential.
>
>Back to the original point though, JFETs with source & drain connected
>together are one thought. The silicon Schottky CDSH270 (replacement for the
>1N270, etc) is quite good compared with old timers like the OA47..OA95,
>1N34A, etc. Original germanium diodes have low Vf only at low currents
>(possibly if people applied some of what has been learnt since the 1950's
>they could make much better modern germanium devices?). I am interested in
>back-diodes though, to try... I thought their supply had dried up...
>possibly they have advantages at high speed (well, compared with "precision
>rectifier" circuits anyway). So where do I get back-diodes from now?
Areoflex, among others
http://www.aeroflex.com/ams/metelics/micro-metelics-prods-tunnel-diodes.cfm
A back diode is the only germanium IC-like (planar, passivated, wire
bonded) process I've heard of. Too bad they don't make serious
switching-type TDs this way.
>
>Using a JFET (or pentode?) as an infinite impedance detector (which I think
>was what was meant in the first post) is pretty good (tends to be a bit
>fussy about the resistor value, but only a bit fussy). I have used them at
>68MHz (with an MPF102, extracting sound FM carrier from VHF PAL TV
>signals)... no RF amplification, so a lot like a crystal set in finding the
>right rabbit to use for its ears.
>
>I recall "super diode" being used to describe transistors with base to
>collector, used as a diode (e.g. thermal compensation in output stages),
>but they aren't great at low signal levels... pretty good at higher
>currents though. Now "super diode" is used only to mean "precision
>rectifier circuit" in many web pages.
The best silicon RF detectors are low-barrier schottky diodes. SMS7621
is nice... 2 volts reverse rating, 0.26 pF junction capacitance.
Jfets used as diodes are pretty horrible. Lots of capacitance, lots of
series resistance. Even the leakage is mediocre compared with what you
can do with the c-b junction of a good transistor.
John
tm
"maitchy" <M.Aitchison@n_o_s_p_a_m.chch.planet.org.nz> wrote in message
news:09ednUQR1K7...@giganews.com...
Here are some Russian Back Diodes. I've had good luck with their TDs.