Re: Amp output Z

qrk wrote:

> On Tue, 12 Apr 2005 02:30:32 GMT, "Harry Dellamano"
> <harryd@xxxxxxxxxxxxx> wrote:
>
> >
> >"qrk" <SpamTrap@xxxxxxxxx> wrote in message
> >news:fgam51dbabvmr86kfrnfcpie1rev9bsrqm@xxxxxxxxxx
> >> On Mon, 11 Apr 2005 22:00:13 GMT, "Harry Dellamano"
> >> <harryd@xxxxxxxxxxxxx> wrote:
> >>
> >>> Ok, so I got this amplifier with an internal output resistance of 10
> >>> ohms
> >>>and we are outside of all feedback loops. It is coupled to the 600 ohm
> >>>resistive load thru a 5.6uF capacitor. The output voltage of the amp is
> >>>10.0Vrms at 400 Hz with no loads connected. If I measure the Zo with SPICE
> >>>using a swept 1 amp current source at the output, I get about 71 ohms
> >>>which
> >>>is the Xc of the coupling cap. If I use a DVM on the real circuit and
> >>>measure the output voltage at two different loads, 600 ohms and 300 ohms,
> >>>I
> >>>get about 13.0 ohms using Ro=((E1-E2)/(E1/600)-(E2/300)). I need to
> >>>measure
> >>>the Zo using a DVM and not Ro. By observation it must be 71 ohms. What
> >>>must
> >>>I do??
> >>
> >> Harry,
> >> Few things are suspect.
> >> Your equation has some sign and parentheses problems.
> >> It might be Ro=(E1-E2)/((E2/300)-(E1/600))
> >>
> >> Your equation is for pure resistance. This is a complex number
> >> problem. Use Spice to measure the output amplitude like you would with
> >> a DMM. I'll bet you end up with the same wrong answer.
> >>
> >> You need to deal with complex numbers since your series capacitor
> >> introduces a significant phase angle at 400 Hz. At 400 Hz your output
> >> impedance is 10-j71 Ohms.
> >>
> >> Mark
> >Hi Mark,
> > If I can only measure and vary the 600 ohm resistive load then all voltages
> >and currents that I can measure are in phase and complex math is useless.
> >This yields Zo=13 ohms but it sounds wrong. I like your 70 ohms above but
> >don't know how to go about measuring it.
> > OT Got to get a new SPICE puter. You were a big help the last time so I
> >may bug you again.
> > Thanks,
> > Harry
> >
>
> Harry,
> If you can change the output capacitor to a large value, like
> 10,000uF, then you can treat this like a pure resistance voltage
> divider. Or, connect up a network analyzer and measure directly.

Even making the cap 100uF would reduce the term to practical insignificance ! I's
certainly be tempted to do that as a quick and easy solution to any measurement
issues.


Graham

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