Reply-To: "Kevin Aylward"
From: "Kevin Aylward"
Subject: Re: SPICE and amp stability
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X-Inktomi-Trace: public1-pete2-5-cust19.pete.broadband.ntl.com 1037780714 26962 188.8.131.52 (20 Nov 2002 08:25:14 GMT)
Date: Wed, 20 Nov 2002 08:25:11 -0000
NNTP-Posting-Date: Wed, 20 Nov 2002 08:25:14 GMT
Walter Harley wrote:
> 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.
Typically, you might need an output stabilizing network for inductive
loads (speakers). This is a series RC to ground at the output, and a
parallel LR in series with the very output to the speaker. This puts a
resistive load on the amp at HF, and disconnects the cables capacitance
> 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 guess?
You can guess the parasitic as a starter.
> 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?
No. The circuit input is the wrong place to measure loop gain
> is there some more correct way to determine the loop characteristics?
You have to look at the *loop* gain. This is the open loop gain around
the *feedback* path. This involves setting up a circuit such that the
opamp has a closed loop for dc (or very, very low frequencies), but open
loop for the main plot. This is so the amp will bias correctly. The
easist way to see how this is done is to run my SuperSpice
LoopCutter.sss example:-). It is all set up to do this already. You can
change the model attached to the opamp.
In PSpice you might have to use an large inductor to brake the loop at
> 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?
In generaly, they can be quite good.
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.