# Linear Regulator

The output voltage of the full-wave rectifier above varies with fluctuations in the input AC supply. Ideally we would like to have a fixed output voltage which we can use downstream. This is accomplished by a voltage regulator. Let's see how a linear voltage regulator works.

The simplest voltage regulator is a zener diode(1). This is simply a diode that is operated in its (reverse) avalanche breakdown region (the diode is manufactured for a specific breakdown voltage). Below is the current to voltage characteristic of a 1N750 zener with a breakdwn voltage of 4.7v. The current through the zener increases rapidly beyond this point.

To limit the current through the zener (so we don't destroy it), we add a resistor as below.

To increase output current capability of our regulator, we add a series pass transistor(2).

However, we know transistor characteristics vary with temperature and supply level. So we need a feedback mechanism to correct for the changes in supply and temperature. Consider the addition of a resistor divider and feedback transistor below:

Thus we have a regulator that works from 12v-30v input keeping the output constant at 10.6v. The feedback transistor helps in keeping the regulator output constant over changes in temperature and input supply. Let's analyze this circuit:

• We have a 4.7V zener and $$\ R1 = R2 = 1K \Omega$$
• Since the base-emitter drop of Q2 is 0.6V (diode drop), the base of Q2 is at 4.7 + 0.6 = 5.3V
• Hence, Vo = 10.6v (R1/R2 form a div/2 resistor divider) and this stays constant even with change in supply or temperature because:
1. If Vo goes below 10.6V, then the base of Q goes below 5.3v and Q2 stops conducting (base-emitter diode/junction less than 0.6v), raising the collector of Q1, which in-turn raises the emitter of Q1, i.e. Vo
2. If Vo goes above 10.6V, then the base of Q2 goes above 5.6v and Q2 conducts heavily (base-emitter diode/junction less than 0.6v), there is a large drop through R4, which lowers the base of Q1, and thereby the emitter of Q1, which is Vo

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