There may be a little crossover distortion from not having a little too much separation between the bias voltages of the nFET and pFET, but I plan to stick in a trimpot and see whether trimming the bias resistor helps.

The mic is definitely staying in saturation, but I’ll have to discuss that with the students more than I had originally planned, since the linear region is something to avoid here. I kind of like the idea of having them plot the i-vs-v curve, though, since it is not what they would expect (not having had any nonlinear models at the point in the course where they’ll be doing the plot).

I also found out that the power amp based on a single op amp plus cMOS push-pull output stage was oscillating at around 4kHz,and I had to add a compensating capacitor between the op amp output and the negative input of the op amp. I believe that the problem is that the FETs and the bias network acted as a large capacitive (or RC) load on the op amp,resulting in enough phase change that the feedback signal was shifted. Now I’m wondering whether the power amp stage is too complicated for a first circuits class. I hope not, since I’ve already bought the FETs for the students and scheduled both threshold voltage measurement and the power amp lab.

]]>What range of signal amplitude are you getting from the electret mic? If it’s biased at 3V, I can’t imagine that you’re in any danger of going into the linear region. You mention a load resistor — do you necessarily need one? If the signal will eventually be fed into a non-inverting op-amp (that presumably has megaohms of input impedance), then for the purpose of characterizing the mic, could you use the voltmeter (or scope) as the “load”?

I know what you mean about not expecting students to come up with the two-part model. I don’t know of a model that accurately represents the slope of the “saturation” region either. I’ve always seen it described as “approximately constant current” but drawn with a slight positive slope, in that way that textbooks often gloss over their choices of abstractions. Would it make sense for students to characterize the mic in the frequency domain instead? My students often find it interesting to discover that “everything is a band-pass filter”, so that when we sweep an entire circuit, the loss of (say) high-frequency signals consists of contributions from every stage. A way to extend their sense-making is to ask, “If we wanted to use this circuit at higher frequencies, which stage would have to change first?”

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