RE: [SI-LIST] : Output Impedance

Andrew Ingraham (Andrew.Ingraham@digital.com)
Thu, 2 Apr 1998 08:37:43 -0500

Kevin Weldon wrote:

> =A0 What I do is connect the input node of the output buffer=20
> to Vcc.=A0 Then I=A0connect the output node to Vcc through=20
> a resistor.=A0 Then I simply change the value of the resistor=20
> until I measure Vcc/2 at the output node of the buffer. The=20
> value of the resistor at this point is equal to the output impedance=20
> of the buffer.=A0 HSpice's Goal Optimization works well for this.=20
> =A0=A0=A0 Is there a simpler way to do this?=20

Just connect an ideal DC voltage source to the output node, set to
Vcc/2. Then measure the current through that voltage source.

That will tell you the DC output resistance at Vcc/2 (or whatever
voltage you choose).

But it is certainly true that there are many possible answers for
"output impedance" because it is nonlinear.

Re: the original question. It is essentially not possible to tell the
output impedance by just examining a SPICE netlist. You must simulate
it.

Regards,
Andy