Audio Engineering

Negative Feedback 371

Note that this is strictly a linear model, so that the slew-rate limiting associated with
Miller compensation is not modeled here. It would be done by placing limits on the
amount of current that can fl ow in and out of the input stage.

Figure 12.2 shows the response to a 1-V step input, with the dominant pole the only time
element in the circuit. (The other poles are disabled by making C 1 , C 2 0.00001 pF, because
this is quicker than changing the actual circuit.) The output is an exponential rise to an
asymptote of 23 V, which is exactly what elementary theory predicts. The exponential
shape comes from the way that the error signal that drives the integrator becomes less as
the output approaches the desired level. The error, in the shape of the output current from
G, is the smaller signal shown; it has been multiplied by 1000 to get mA onto the same
scale as volts. The speed of response is inversely proportional to the size of Cdom and is
shown here for values of 50 and 220 pF as well as the standard 100 pF. This simulation
technique works well in the frequency domain, as well as the time domain. Simply
tell SPICE to run an AC simulation instead of a TRANS (transient) simulation. The
frequency response in Figure 12.3 exploits this to show how the closed-loop gain in an

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0 s 1.0μs 2.0μs 3.0μs 4.0μs 5.0μs

v(7)

v(3)

Time

50p 100p 220p

v(3) v(7)^1000 * i (g1)

Figure 12.2: SPICE results in the time domain. As Cdom increases, the response V(7)

becomes slower and the error (g1) declines more slowly. The input is the step-function V(3)

at the bottom.