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Measuring input impedance (Read 7888 times)
rajath
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Measuring input impedance
Dec 26th, 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 27th, 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 27th, 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 .
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 27th, 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
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Re: Measuring input impedance
Reply #4 - Jan 17th, 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
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Re: Measuring input impedance
Reply #5 - Apr 17th, 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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