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Jun 14th, 2024, 5:43am
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 Measuring input impedance (Read 7888 times)
 rajath New Member Offline Posts: 9 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. Back to top IP Logged
 Ken Kundert Global Moderator Offline Posts: 2384 Silicon Valley 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 Back to top IP Logged
 rajath New Member Offline Posts: 9 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 . 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. Back to top IP Logged
 Ken Kundert Global Moderator Offline Posts: 2384 Silicon Valley 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. 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 Back to top IP Logged
 CaoThong New Member Offline Posts: 2 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 Back to top IP Logged
 Thomas Hansson Guest Re: Measuring input impedance Reply #5 - Apr 17th, 2003, 5:54am   HiOr why not measure the input impedance using the zm as part of the s-parameters. Magnitude, real or imaginary part could be plotted.Thomas Back to top IP Logged
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