AC Explorations - Continued

published 2025-12-10

by Christopher Howard

The LM368 circuit I built recently does not handle quite as much current as I was imagining. I had misread figure 3 in the data sheet, which indicated a plateau of about 3.7 V output voltage with a 4 ohm load. The units for that are peak-to-peak, meaning the RMS value would be 3.7 / 2 / √2 or about 1.3 Vrms. With a 5.7 ohm resistor I was seeing about 1.65 Vrms on the output, before the peaks of the signal started getting chopped off. So, max RMS current would be around 300 mA with the 4 ohm resistor.

Of course, if you are just making measurements, and not trying to heat something up, then one could deal in max voltage and max current, i.e., the peak of the cycle. And 300 mA RMS current would be over 400 mA peak which is decent current for my purposes.

For experimentation, I connected this circuit to a LCR network with a 470 nF cap and a 1 mH inductor. Together, the cap and the inductor had 16.5 ohms internal resistance. By my calculations — which I have not double checked — that should be a resonant frequency of 7420 Hz and, and a peak current of 137 mA, assuming an output voltage of 1.6 Vrms or 2.26 Vp-p, which I picked so I wouldn't have to tweak my signal generator settings. The frequency 7420 gives a reactance of 46.6 ohms on the inductor (2 pi f L) and I assumed the same reactance on the capacitor since we are at resonant frequency. Assuming no mistakes so far, that should give us a peak voltage across the capacitor or inductor of 0.137 A × 46.6 Ω = 6.39 V.

Hooking things up, I found that the resonant frequency appeared to be a little lower, around 6.8 kHz, with a peak-to-peak voltage across the capacitor of about 10.5 V, which is 5.25 V peak. I think that was a reasonable close result, given I was still working on the breadboard, and all the inaccuracies that that might be introducing, from the poor connections and the extra conductor material. I could vary the result quite a lot by pressing the capacitor more firmly onto the breadboard with my finger.

I built a version of the amplifier circuit off the breadboard, in deadbug style, but it seems to be picking up a significant amount of HF interference, though not enough to cause instability.

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