ZENER STABILISATION
The simplest form of voltage stabiliser is shown in Fig. 11.1(a), and consists of a series resistor and zener diode, which device has a sharp knee in its reverse voltage/current characteristic. Output voltage remains at this zener point for a wide range of diode currents. If the load is so great that the diode runs out of current, stabilisation breaks down. The most common application of this simple principle is in the provision of a stabilised tuning voltage for varicap tuners. Here the zener diode takes the form of a two-terminal IC (example ZTK33B) with built-in thermal compensation; unlike a simple zener, these devices have virtually zero temperature coefficient.
SERIES- AND SHUNT-REGULATORS
In situations of varying load and/or supply voltage it is easy to arrange an absorption circuit to fully compensate for variations in
supply and demand. The shunt regulator (Fig. 11.1(b)) is connected across the load and this combination is fed from the unstabilised sup- ply; the series resistor R may be a physical component, but more usu- ally is formed by the source impedance of the supply. The current flowing in the shunt transistor is regulated at its base to maintain constant voltage across its c−e terminals.
Less wasteful of energy is the series regulator, outlined in Fig. 11.1(c). Here the control transistor is in series with the load and alters its resistance to match the demand of the load, again maintaining constant output voltage.
A practical circuit for a series regulator, as typically used in a port- able TV, is given in Fig. 11.2. The regulator element is pnp power transistor VT201. Its voltage source is the dotted box, representing a mains transformer/rectifier/capacitor ensemble, or a 12 V car bat- tery. At switch-on conduction in VT201 is established by c−e current via VT202, itself turned on by base current from VT203 and ‘kick- start’ components C201 and W201. Voltage appears on the collector of VT201 then, and is potted down in R204/205/206 for application to the base of comparator transistor VT203. This transistor’s emitter is held at a steady voltage of 4.7 by zener W202 and resistor R207. VT203 collector provides an error voltage to drive a proportional current into VT202 base. This latter device behaves as a current
amplifier, steering the base current in regulator VT201. Output volt- age is trimmed by preset R205.
Imagine a bright picture tending to pull down the 11.3 V supply line. As it falls, VT203 (pnp) base voltage falls to increase its collector current, which increases VT202 base current. VT202 collector current increases as a result, and since all of it flows through the base of regulator VT201, the latter is turned harder on to supply the extra current demand and restore the correct 11.3 V line voltage. Many such d.c. regulator circuits can respond up to and beyond 100 Hz rate, and the regulating mechanism described above can work on a ‘sawtooth’ basis to eliminate hum-ripple from a ‘coarse’ mains- derived supply. By the same token, it can absorb ripple induced on the supply line by timebase or audio output stages.