From: "Walter Harley"
Subject: SPICE and amp stability
Date: Tue, 19 Nov 2002 15:30:30 -0800
Organization: Posted via Supernews, http://www.supernews.com
Reply-To: "Walter Harley"
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
I have a simple 100mW audio amp circuit that, naively implemented, is
unstable at RF. The amp consists of an LM833 inverting stage with Av = -4,
followed by a push-pull emitter follower biased as class AB. Feedback is
taken from the output of the emitter follower.
I can stabilize the amp by putting a small-value capacitor between the opamp
output and the inverting input - basically taking the emitter follower out
of the loop at high frequencies. However, unless I use a very tiny value,
this causes substantial intermodulation distortion as the emitter follower
is not all that linear without feedback. If I do use a tiny value, I'm
concerned that it won't be stable in real life.
Rather than just try cap values at random, I would like to really understand
what's going on. So, I'd like to model the phase response and gain
bandwidth of the system, and find a way to compensate the circuit so that
gain drops below unity by the frequency where there is 180 degree phase
shift. But I'm not sure how to do this, either analytically or in PSpice,
for four reasons that maybe y'all can help me with:
1) How do I figure out the phase response of the feedback network? The only
"capacitors" in the circuit are the input capacitance of the opamp, which is
not specified in the datasheets, and the various parasitic capacitances, and
whatever appears across the load (probably cable capacitance). Do I just
2) To simulate the system and calculate a Bode plot, should I model the
parasitic capacitances in PSpice and then look at phase shift between the
circuit input and the inverting input of the opamp? Or is there some more
correct way to determine the loop characteristics?
3) Do the SPICE models for opamps accurately depict the bandwidth, phase
shift, and input capacitance in general? If not, which of these parameters
do I need to externally model?
4) Does anyone know where to find a reasonable SPICE model for the LM833? I
can't find one anywhere, except for one on OnSemi's site that shows JFET
inputs, which doesn't seem correct for an LM833.