In fact, the rail voltage fluctuations are a type of noise that is induced from dynamic operation of the amplifier. Unfortunately, most amplifier tests only evaluate the output at idle and under steady state output signal conditions, which does not reveal the dynamic failings of an amplifier. For example, the sagging of the power supply rail voltages under a heavy steady current draw will only initially causes a blocking distortion, as the bias voltages will eventually catch up with sagging by on the time constants of the coupling circuits. Constantly shifting in frequency and in volume, very little music contains a steady RMS signal. Thus if the amplifier is always seeing a sagging and expanding rail voltage and if not designed to compensate for this fact, it will always be faltering when faced with music.   

Differential Gain Circuit
    The output stage can be configured to provide gain. This is accomplished by bootstrapping the triode that drives the top output device. This allows the top output device to realize gain at the cost of a high output impedance.

    Replacing the cathode resistor with a constant current source, once again, does not eliminate the noise, but interestingly it does not invert the phase of the noise at the output.

    Keeping the current source and shifting the power supply filtering capacitor's termination from ground to the negative power supply rail (as shown below) is the trick to eliminating the noise. This is another example of how the whole circuit, the driver circuit and the output stage and the power supply, must be analyzed to fruitfully design a power amplifier.

    Eliminating the noise is a bit different in this circuit compared to the no-gain version. As shown below, the power supply noise grossly amplified by the top output device. Here the cathode resistor relays the negative rail noise to top device's input where it is greatly amplified.

< PREVIOUS

NEXT >