Capacitive Load Resonance Effect.

There is one 'problem' which will occur to some extent with any amplifier using an output inductor and overall negative feedback. In the next diagram the upper trace shows the gain at the output before the inductor with a 2uF load. The small peak at 180kHz is no problem, but the lower trace is the gain at the input of the output stage. (This is a linear approximation with unlimited signal levels, so it is not what would happen in reality where clipping and other sources of nonlinearity will limit driver stage output.) The 75V peak here is rather more worrying, and with a 60V supply there will be severe clipping with a 1V input. In reality we would hope never to encounter a 1V input at 180kHz with a 2uF load, so there is no need to worry too much. The cause of the problem is the series resonance of the inductor and the capacitor load which load the output stage with about 0.1 ohms, and then the driver tries to apply 75V to the output stage in an attempt to produce 3V across this 0R1. The output current would then be 30A, so there would still be a problem even if there was no driver clipping. There are ways to reduce the peak, for example reducing the 2R2 damping resistor, but this may cause other problems if reduced too far. More recent versions of the MJR7 do have lower damping resistors, so the problem should now be less than suggested by this simulation.

Note that the MJR-7 has a second-order low-pass response because of the 470p input filter and the 10p across the feedback resistor (the 470p at the input base does not affect the closed-loop response, but it is an essential part of the loop stabilisation). This, giving -0.5dB at 20kHz, helps by keeping the output down to 3V at 180kHz. Some published designs have less input filtering, often starting at a much higher frequency, and in some cases using small output inductors with 8R damping resistors. The above problem, if it ever really is a problem, could in this case be far more serious. The inductor values are also commonly greater than my 0.4uH, so the effect will be lower down the frequency range where there is less attenuation.


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