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Simulators >> Circuit Simulators >> Measuring input impedance
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Message started by rajath on Dec 26^th, 2002, 8:44pm

Title: Measuring input impedance
Post by rajath on Dec 26^th, 2002, 8:44pm

Hi,
I'm a newbie here, so pardon me if this has already been asked before. I've designed a CFB amplifer, and would like to measure it's ac input impedance ( the -ve terminal). Can anyone please suggest me a method of doing this with the spectre simulator?

thanx.

Title: Re: Measuring input impedance
Post by Ken Kundert on Dec 27^th, 2002, 12:10am

Rajath,
In general one measures input impedance by first applying an input using small-signal voltage source and then calculating the input impedance by dividing the applied voltage by the resulting current that flows in the input source. Having said this, I suspect you are asking about something more subtle. However, I cannot give you any more information because I am not sure what a CFB amplifier is, or what a -ve terminal is. I assume that CFB stands for current feedback, but I don't yet see how measuring the input impedance of a current feedback amplifier would differ from doing the same for voltage feedback amplifier. Perhaps you can describe the situation in more detail?

-Ken

Title: Re: Measuring input impedance
Post by rajath on Dec 27^th, 2002, 12:36am

I'm sorry for not explaining my problem fully. Actually I use the affirma environment with the simulator being Hspice. Strangely, this simulator does not allow me to plot the terminal current when running the AC analysis. I just realised that this problem doesnt arise when i use the spectre simulator, but I don't have the model files for it :( .
Of course I can always put a resistor in series and find the current, but is there a "classier" way ?!
As you rightly presumed, I was talking about the current feedback amplifier - which really shouldn't make a difference. But does it make sense to find the input impedance at DC quiescent levels, when the amplifier input actually sinks/sources large-signal currents during operation?

thanx.

Title: Re: Measuring input impedance
Post by Ken Kundert on Dec 27^th, 2002, 12:50pm

Rajath,
I'm afraid I will not be of much help with HSpice as I have never used it. My recommendation, of course, is to demand Spectre models. ;)

Concerning your question as to whether small-signal input impedance is sufficient, it will be if the amplifier is acting in a near-linear region. Since this is a feedback amplifier, assuming linearity is probably very appropriate, as long as you are not interested in the effective input impedance while the amplifier is being driven into clipping.

-Ken

Title: Re: Measuring input impedance
Post by CaoThong on Jan 17^th, 2003, 12:15am

Hi,

Do you have any independant current source
available ? In that case, why don't you rather
inject an AC current into the circuit and plot the
voltage output instead it would give you the
impedance.

CaoThong

Title: Re: Measuring input impedance
Post by Thomas Hansson on Apr 17^th, 2003, 5:54am

Hi
Or why not measure the input impedance using the zm as part of the s-parameters. Magnitude, real or imaginary part could be plotted.

Thomas