Controlled Transistor Series Regulator

A controlled transistor series regula­tor is shown in the figure. The circuit is quite similar to that of a simple transistor series voltage regulator except that an additional transistor Q₂ is inserted in the circuit. The emitter terminal of this transistor Q₂ is connected to the negative terminal of input supply through a Zener diode. The base of this additional transistor is connected to the variable tap of a potentiometer.

This voltage regulator employs the principle of negative feedback to hold the output voltage almost constant despite variations in supply voltage and /or load current. That is why this regulator is also called a negative feedback regulator.

Transistor Q₁ the control ele­ment, is called the pass transistor because all the load current flows through it. Zener diode and resistor R₄ act as a reference element. The voltage divider (or potentiometer) consisting of resistors R₁ and R₂ samples the output voltage and de-livers a negative feedback voltage to the base of transistor Q₂ and this feedback voltage (V_F = V_BE + V_Z) controls the collector current of tran­sistor Q₂.

Operation

Suppose the output voltage increases (due to any rea­son), the voltage across R₂ is also increased as it is part of the output circuit. This causes an increase in voltage (V_BE2 + V_z ). As a result I_B2 and also I_c2, increases. Assuming I₃ relatively constant, I_B decreases. Decrease in base current of transistor Q₁ causes the increase in collector-emitter resist­ance of transistor Q_1. This causes an increase in V^ thereby off-setting the increase in output voltage. Thus output voltage remains constant. Reverse happens should the output voltage decrease.

The voltage V₂ provided by the potential divider R₁-R₂ must be equal to the sum of the base-emitter voltage of transistor Q₂ and the Zener diode. That is

V_BE2 + V_z = V₂ = R₂ / (R₁ + R₂) * V_out

V_out = (1 + R₁/R₂) (V_BE2 + V_z) = A_f (V_BE2 + V_z)

Thus the regulated output voltage is equal to the closed-loop gain multiplied by the sum of Zener voltage and base-emitter voltage of transistor Q₂ (the sensing element).

Like emitter follower regulator this regulator also has the drawback of excessive power dissipation. Due to this high power dissipation output of the power supply is limited to 30-40 V, as safe value of V_CE is 50 V.

Another drawback of this regulator is that it has no overload/short-circuit protection.