Controlled Transistor Series Regulator or Negative Feedback Regulator:

A controlled transistor series regulator is shown in Fig. 43.6. This Controlled Transistor Series Regulator 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 delivers a negative feedback voltage to the base of transistor Q₂ and this feedback voltage (V_F = V_BE2 + 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_B1 decreases. Decrease in base current of transistor Q₁ causes the increase in collector-emitter resistance of transistor Q₁. This causes an increase in V_C1E1 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 i.e.,

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).

Limitations of Controlled Transistor Series Regulator:

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.