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rajath New Member Offline Posts: 9	Measuring input impedance 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.
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Ken Kundert

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Re: Measuring input impedance
Reply #1 - 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

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rajath

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Re: Measuring input impedance
Reply #2 - 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 Sad

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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.

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Ken Kundert

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Re: Measuring input impedance
Reply #3 - 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. Wink

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

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CaoThong New Member Offline Posts: 2	Re: Measuring input impedance Reply #4 - 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
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Thomas Hansson Guest	Re: Measuring input impedance Reply #5 - 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
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