Transistor Series Voltage Regulator:

When a low power zener diode is used in the simple Transistor Series Voltage Regulator, the load current is limited by the maximum diode current. A high-power Zener used in such a circuit can supply higher levels of load current, but much power is wasted when the load is light. The emitter follower regulator shown in Fig. 17-1 is an improvement on the simple regulator circuit because it draws a large current from the supply only when required by the load. In Fig. 17-1(a), the circuit is drawn in the form of the common collector amplifier (emitter follower). In Fig. 17-1(b), the circuit is shown in the form usually referred to as a series regulator. Transistor Q₁ is termed a series pass transistor voltage regulator.

The output voltage (V_o) from the series regulator is (V_Z – V_BE), and the maximum load current (I_L(max)) can be the maximum emitter current that Q₁ is capable of passing. For a 2N3055 transistor, I_L could approach 15 A. When I_L is zero, the current drawn from the supply is approximately (I_Z + I_C(min)), where I_C(min) is the minimum collector current to keep Q₁ operational. The Zener diode circuit (R₁ and D₁) has to supply only the base current of the transistor. The Transistor Series Voltage Regulator is, therefore, much more efficient than a simple Zener diode regulator.

Series Voltage Regulator with Error Amplifier:

A series regulator using an additional transistor as an Error Amplifier is shown in Fig. 17-2. The error amplifier improves the line and load regulation of the circuit. The amplifier also makes it possible to have an output voltage greater than the Zener diode voltage. Resistor R₂ and diode D₁ are the Zener diode reference source. Transistor Q₂ and its associated components constitute the error amplifier, that controls the series-pass transistor (Q₁). The output voltage is divided by resistors R₃ and R₄, and compared to the Zener voltage level (V₂). C₁ is a large-value capacitor, usually 50 μF to 100 μF, connected at the output to suppress any tendency of the regulator to oscillate.

When the circuit output voltage changes, the change is amplified by transistor Q₂ and fed back to the base of Q₁ to correct the output voltage level. Suppose that the circuit is designed for V_o = 12 V, and that the supply voltage is V_S = 18 V. A suitable Zener diode voltage in this case might be V_Z = 6 V. For this V_Z level, the base voltage of Q₂ must be, V_B2 = V_Z + V_BE2 = 6.7 V. So, resistors R₃ and R₄ are selected to give V_B2 = 6.7 V and V_o = 12 V. The voltage at the base of Q₁ is, V_B1 = V_o + V_BE1 = 12.7 V. Also, V_R1 = V_S – V_B1 = 5.3 V. The current through R₁ is largely the collector current of Q₂.

Now suppose the output voltage drops slightly for some reason. When V_o decreases, V_B2 decreases. Because the emitter voltage of Q₂ is held at V_Z, any decrease in V_B2 appears across the base-emitter of Q₂. A reduction in V_BE2 causes I_C2 to be reduced. When I_C2 falls, V_R1 is reduced, and the voltage at the base of Q₁ rises (V_B1 = V_S – V_R1) causing the output voltage to increase. Thus, a decrease in V_o produces a feedback effect which causes V_o to increase back toward its normal level. Taking the same approach, a rise in V_o above its normal level produces a feedback effect which pushes V_o down again toward its normal level.

When the input voltage changes, the voltage across resistor R₁ changes in order to keep the output constant. This change in V_R1 is produced by a change in I_C2, which itself is produced by a small change in V_o. Therefore, a supply voltage change (ΔV_S) produces a small output voltage change (ΔV_o). The relationship between ΔV_S and ΔV_o depends upon the amplification of the error amplifier. Similarly, when the load current (I_L) changes, I_B1 alters as necessary to increase or decrease I_E1. The I_B1 variation is produced by a change in I_C2 which, once again, is the result of an output voltage variation ΔV_o.

Series Voltage Regulator without Error Amplifier:

The performance of a series regulator without an error amplifier (Fig. 17-1) is similar to that of a Zener diode regulator, except in the case of the load effect. The series-pass transistor tends to improve the regulator load effect by a factor equal to the transistor h_FE.

The error amplifier in the regulator in Fig 17-3 (reproduced from Fig. 17-2) improves all aspects of the circuit performance by an an amount directly related to the amplifier voltage gain (A_v). When V_S changes by ΔV_S, the output change is,

If ΔV_S is produced by a variation in the ac supply voltage, the power supply source effect is reduced by a factor of A_v. ΔV_S might also be the result of an increase or decrease in load current that causes a change in the average level of the dc supply voltage. Thus, the load effect of the power supply is reduced by a factor of A_v.

Now consider the effect of supply voltage ripple on the circuit in Fig. 17-3. The ripple waveform appears at the collector of transistor Q_1. If there was no negative feedback, it would also be present at Q₁ base and at the regulator output. However, like supply voltage changes, the input ripple is reduced by a factor of A_v when it appears at the output. The ripple rejection ratio is calculated as the decibel ratio of the input and output ripple voltages.

Regulator Design:

To design a Transistor Series Regulator Circuit Diagram (as in Fig. 17-3), the Zener diode is selected to have V_Z less than the output voltage. A Zener diode voltage approximately equal to 0.75 V_o is usually suitable. Appropriate current levels are chosen for each resistor, and the resistor values are calculated using Ohm’s law. Transistor Q₁ is selected to pass the required load current and to survive the necessary power dissipation. A heat sink (see Section 8-8) is normally required for the series-pass transistor in a regulator that supplies large load currents. As discussed, a large capacitor is usually connected across the output to ensure amplifier ac stability, (C₁ in Fig. 17-3).

The difference between the regulator input and output voltages is the collector emitter voltage of the series-pass transistor (Q₁), and this voltage must be large enough to keep the transistor operational. The minimum level of V_CE1 (known as the dropout voltage) occurs at the lowest point in the ripple waveform of (rectified and filtered) raw dc input. If V_CE1 is too small for correct operation at this point, a large-amplitude ripple waveform appears at the regulator output.