Decoupling

Author: N.H. Crowhurst

The filters that we have discussed will reduce the ripple to well below 1 volt in a 250-volt supply. This would seem to be quite good enough, until we consider that this plate supply may be needed for the first stage of a high-gain amplifier. The audio voltage at the plate of the first stage may not be more than, say, 10 millivolts. If there should be as much as 10 millivolts of hum in the plate supply voltage fed to the top end of the plate load resistor, the ripple would be equal to the audio voltage at this point. The hum, of course, has to be kept well below the audio voltage to avoid its becoming audible. This means that extra smoothing is needed to cut the hum down to a much lower value. (This attention is not necessary at the output stage, where there may be 100 volts or more audio, so we can use the simple smoothing circuit there.) The additional smoothing required in the supply for the early stages may be provided by additional resistors and capacitors.

Decoupling filters are used to reduce the ripple voltages

Fortunately, high-gain low-level stages do not take much plate current. This means that relatively large-value resistors and capacitors that provide a high degree of ripple reduction can be used without dropping the plate potential appreciably. These additional components are necessary for another reason. We discussed feedback as something desirable that we introduce intentionally. Feedback can also be undesirable and be introduced unintentionally. This is one place where this occurs if care is not taken.

Undesirable feedback can cause oscillation

The normal high-voltage supply has an impedance due to the reactance of the final smoothing capacitor and resistors in the circuit that varies from a few ohms to perhaps several hundred ohms, according to design. Even if this impedance is only a few ohms, the output-stage audio current will probably be a fluctuation of 50 milliamperes or more and the power-supply impedance will produce half a volt or more audio across it. This half-volt of audio superimposed on the high voltage supply, will be injected into the front end stage, unless we provide some means of getting rid of it. The further resistor and capacitor, used to reduce the ripple or hum voltages as well as smoothing, bypass (decouple) this audio voltage. For this reason these extra resistors and capacitors are called decoupling elements.