Rectification

Author: N.H. Crowhurst

Two types of rectifiers: the tube rectifier (left) and the barrier layer rectifer (right)

We can get the desired d-c voltage for plate supplies by transforming a-c and rectifying it. Two kinds of rectifier are used for audio amplifier circuits: the thermionic rectifier, which employs one or possibly two electronic diodes in one envelope; and the barrier-layer rectifier, which uses the rectifying properties of copper oxide, selenium, germanium, or silicon. (These materials are listed in the order of improving efficiency and also their sequence of development. Choice of which kind of rectifiers to use usually depends on the relative cost or the particular space requirements, whichever happens to be the most important.

The half-wave rectifier circuit

We can use either half-wave or full-wave rectification. Half-wave rectification saves one rectifier component; we only need one diode or one rectifier element. For small plate current supplies, where the current drain is very limited, this circuit is quite convenient. It has, however, the disadvantage that the whole of the current drain has to be passed through the rectifier in a small fraction of one half of the cycle. It also needs more elaborate attention when smoothing out the voltage. Where larger plate currents are needed, above say 50 milliamperes, full-wave rectification is almost always used. This enables two current pulses to be taken in every cycle instead of only one, and makes it much easier to smooth out the ripple.

The exact way in which the rectifier works to give the required d-c output voltage depends on the kind of circuit used to smooth out the ripples. If the rectifier is fed straight into a resistance load, the output waveform will be either a succession of half-waves, with a gap for each alternate half-wave, or if full-wave rectification is used, a succession of half-waves end to end. This kind of voltage supply is not suitable for audio amplifiers, because it would result in very considerable hum. For this reason we need to do something to smooth out the ripple component.

The full-wave rectifier circuit