Measure input/output impedance with Spice

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.

Reply to
dakupoto
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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:

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(Ignore T1.) The S parameters measured are:
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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

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Reply to
Tim Williams

----- Original Message ----- From: Newsgroups: sci.electronics.design Sent: Sunday, October 14, 2012 8:05 AM Subject: Measure input/output impedance with Spice

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

Reply to
Helmut Sennewald

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

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| James E.Thompson, CTO                            |    mens     | 
| Analog Innovations, Inc.                         |     et      | 
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Reply to
Jim Thompson

In the case of DC, just do some plot math, or a .meas directive (RTFM), on the appropriate node.

In the AC case, add a .net directive (RTFM again), then do a normal .ac analysis. Under " visible traces", you'll find that you now get offered Zin, Zout, Yin, Yout, and all S parameters.

/examples/Educational/S-param.asc explains this quite well.

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Reply to
Fred Abse

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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Reply to
robindavis001

What I said.

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Reply to
Fred Abse

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