Measure input/output impedance with Spice

[daku...@gmail.com]

Could some electronics/Spice guru shed some
light on this. Suppose I want to measure the
input impedance of a test circuit. The impedance
can be AC or DC. Let us consider DC.

I set up the following circuit, with a DC
voltage source, connected with a voltage
source with zero DC and AC output, followed
by the circuit under test. I sweep the DC
voltage of the first DC voltage source. and
the input impedance is:

Zin = 1/(Iin(dI/dV)) where Iin is the
instantaneous current indicated by the
zero output voltage source, and dI and
dV are measured. Is this a sound scheme ?
All hints/suggestions would be of immense
help.

[Tim Williams]

On a DC sweep, any plot of voltage vs. current has coordinates in ohms (or
reciprocal in mhos).

There are two kinds of resistance to note: static (DC) resistance, and
incremental (dynamic, differential) resistance. Static is when you take
the coordinates of a point on a curve, and divide. Incremental is the
slope of the curve at that point.

All linear resistor networks have static = incremental.

Note that if the resistance is measured with respect to different parts of
the circuit, it's more properly transresistance (e.g., an amplifier with
voltage output and current input has a gain of Vo / Iin, a resistance).

For general purpose, you probably want the AC impedance, not the DC
impedance. Use an AC analysis for this. You can find the impedance from
the current drawn from an AC voltage source applied to the port in
question (setting all other sources to 0V AC).

For example, here's a grounded emitter, class A, tuned amplifier:
http://t3sl4.dnsdynamic.net/Images/ResonantStage2.png
(Ignore T1.)
The S parameters measured are:
http://t3sl4.dnsdynamic.net/Images/ResonantStage1.png

Return loss (gamma) is a matching parameter. When:
gamma = -1: Load is open circuit (infinite resistance)
-1 < gamma < 0: Impedance is higher than source impedance (R4 = 100 ohm)
gamma = 0: Impedance equals source
0 < gamma < 1: Impedance is lesser than source
gamma = 1: Load is a short circuit (0 ohms)
gamma > 1: Load is negative!

Note that this circuit actually has a negative input impedance from about
380k to 500k. This type of circuit can oscillate very easily, and
precautions have to be taken to ensure the input isn't overly reactive.

I could've just as well plotted input impedance in the graphs, using the
same parameters, and the AC definition of input impedance.

Hmm, this graph is showing gamma as low as -4. Might've got the formula
wrong...

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

<daku...@gmail.com> wrote in message
news:e511ff1d-bf65-4740...@googlegroups.com...

[Helmut Sennewald]

Hello,

You can get the impedance using the .TF analysis for DC while stepping your
DC-source.
.step V1 -1 1 0.1
.TF ....

It's also possible to use the AC-analysis while stepping your DC-source.
This greatly works with LTspice.
Then plot V(...)/I(...).

.step V1 -1 1 0.1
.AC list 1

All the instructions above are for LTspice.
Other SPICE programs may have a different syntax for ".step" or eventually
don't have this feature.

Best regards,
Helmut

[Jim Thompson]

For DC I'd sweep the input with a CURRENT source, the DC "impedance"
is simply the voltage. For incremental, just take the
derivative/slope.

For AC, with a DC bias, you'll have to do a .AC with a VAC source,
then do some complex (A + jB) math to get impedance, real and
imaginary parts.

In PSpice the DC part is handled by sweeping parameter.

Over the years I have made macros which, from a .AC/VAC analysis, spit
out Cin, and its serial resistance, to easily obtain CMOS OpAmp input
characteristics.

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

[robind...@gmail.com]

Use LTSpice.
Using the .net command. Works for AC.
Right click on plot window , select trace. Take your pic. Zin , Zout , S11 etc

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