Semiconductor Diodes as Rectifiers

Rectification, or the conversion of alternating current into unidirectional or direct current, is one of the principal processes used in electro­nics.

Because semiconductor diodes have the pro­perty of unidirectional conduction (they con­duct well in the forward direction and poorly, if at all, in the reverse direction), most of them are quite logically used for a. c. rectification. An elementary rectifier circuit is shown in Fig. 3.6a.

Fig. 3.7. Rectifier circuit using a diode

Let us agree that the generator (source) supplies a sinusoidal emf:

E = U_m sin ωt.

During one half-cycle the diode is forward-biased, a current flows through it and produces a voltage drop U_L across the load resistor R_L. During the next half-cycle, the diode is reverse-biased, there is practically no current flowing, and U_L = 0. Thus, a pulsating current passes through the diode. This is the rectified current. It produces a rectified voltage across the load resistor R_L (Fig. 3.8).

Fig. 3.8. Explaining the operation of a simple rectifier circuit

The useful part of such a voltage is its direct (constant) or average compo­nent:

U_av = U_m / π = 0.318U_m . (3.6)

A smoothing filter uses high-value capacitors (Fig. 3.6b) which provide a bypass for the a. c. current so that as little of it could flow into the load as possible (Fig. 3.9).

Fig. 3.9. Ripple reduction by a capacitor

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