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Measure input/output impedance with Spice

Discussion in 'Electronic Design' started by [email protected], Oct 14, 2012.

  1. Guest

    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.
     
  2. Tim Williams

    Tim Williams Guest

    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
     
  3. Fred Abse

    Fred Abse Guest

    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.
     
  4. Guest

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

    Version 4
    SHEET 1 880 680
    WIRE 112 160 48 160
    WIRE 240 160 192 160
    WIRE 304 160 240 160
    WIRE 416 160 368 160
    WIRE 448 160 416 160
    WIRE 544 160 528 160
    WIRE 544 192 544 160
    WIRE 240 208 240 160
    WIRE 416 224 416 160
    WIRE 48 304 48 240
    WIRE 240 304 240 288
    WIRE 240 304 48 304
    WIRE 304 304 240 304
    WIRE 416 304 416 288
    WIRE 416 304 304 304
    WIRE 544 304 544 272
    WIRE 544 304 416 304
    FLAG 304 304 0
    SYMBOL res 208 144 R90
    WINDOW 0 0 56 VBottom 2
    WINDOW 3 32 56 VTop 2
    SYMATTR InstName R1
    SYMATTR Value 10
    SYMBOL res 256 304 R180
    WINDOW 0 36 76 Left 2
    WINDOW 3 36 40 Left 2
    SYMATTR InstName R2
    SYMATTR Value 75
    SYMBOL cap 368 144 R90
    WINDOW 0 0 32 VBottom 2
    WINDOW 3 32 32 VTop 2
    SYMATTR InstName C1
    SYMATTR Value 33p
    SYMBOL cap 432 288 R180
    WINDOW 0 24 56 Left 2
    WINDOW 3 24 8 Left 2
    SYMATTR InstName C2
    SYMATTR Value 22p
    SYMBOL ind 544 144 R90
    WINDOW 0 5 56 VBottom 2
    WINDOW 3 32 56 VTop 2
    SYMATTR InstName L1
    SYMATTR Value 10µ
    SYMBOL res 528 176 R0
    SYMATTR InstName R3
    SYMATTR Value 100
    SYMATTR SpiceLine ""
    SYMBOL voltage 48 144 R0
    WINDOW 123 24 124 Left 2
    WINDOW 39 0 0 Left 2
    SYMATTR InstName V1
    SYMATTR Value2 AC 1
    SYMATTR SpiceLine ""
    TEXT 48 96 Left 2 !.ac dec 1000 10 100meg
    TEXT 48 72 Left 2 !.net I(R3) V1
     
  5. Fred Abse

    Fred Abse Guest

    What I said.
     
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