Emitter Follower Regulator:

To get a better voltage regulation in shunt regulator, the zener diode can be connected to base circuit of a power transistor as shown in the figure 1. This amplifies zener current range. It is also called as emitter follower regulation.

Fig. 1

This configuration reduces current flow in the diode. The power transistor used in this configuration is called as pass transistor. The purpose of CL  is to ensure that the variations in one of the regulated power supply loads will not be fed to other loads. i.e. capacitor effectively shorts out high-frequency variations.

Due to the current amplifying property of transistor, the current in zenor dioide is small. Hence there is less voltage drop across the diode resistance, and zener approximates an ideal constant voltage source.

Operation of the circuit:

The current through resistor R is sum of the zener current I_Z and the transistor base current I_B ( = I_L / β ).

I_L = I_Z + I_B

The output voltage across R_L resistance is given by

V_O = V_Z - V_BE

Where V_BE ? 0.7 V

Therefore, V_O= constant.

The emitter current and load current are same. The current I_R is supposed to be constant for a given supply voltage. Thus, if I_L increases, it requires more base currents, to increase base current Iz decreases. The difference in this regulator with the zener regulator is that in later case zener current decreases by same amount by which the load current increases.Thus the current range is less, while in shunt regulators, if I_L increases by ΔI_L then IB must increase by ΔI_L / β or I_Z should decrease by ΔI_L / β. Thus the current range control is more for same rating zener.

The simplified circuit of shunt regulator is shown in the figure 2.

Fig.2

In the power supply power regulation is basically, because of its elevated internal impedance. In circuit discussed, the unregulated supply has resistance RS of the order of 100 ohm. The use of emitter follower is to reduce output resistance and it becomes nearly.

R_O = ( R_z + h_ie ) / (1 + h_fe)

Here R_Z represents dynamic zener resistance. The voltage stabilization ratio S_V is

S_V = ∂ V_o / ∂ V_I = R_z / (R_z + R)

The SV can be improved by increasing R. This increases V_CE and power dissipated in transistor. Other disadvantages of circuit are.

1.   No stipulation for varying the output voltage since it is almost equal to zener voltage.

2.   Change  in the V_BE and  Vz   because of  temperature  variations  appear  at  output  since  the transistor is connected in series with the load, it is called as series regulator and transistor is the allow series pass transistor.

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