Improving Regulator Performance:

Error Amplifier Gain – The performance of a regulator is dependent on the voltage gain of the error amplifier. A higher gain amplifier gives better line and load regulation. So, anything that improves the amplifier voltage gain will improve the regulator performance. Two possibilities to increase A_v are: using as transistor with a high h_FE value for Q₂, and using the highest possible resistance value for R₁.

Output Voltage Adjustment:

Regulator circuits such as the one designed are unlikely to have V_o exactly as specified. This is because of component tolerances, as well as, perhaps, not finding standard values close to the calculated values. Hence, some form of adjustment is required to enable the output voltage to be set to the desired level. In Fig. 17-5 the potentiometer (R₅) connected between resistors R₃ and R₄ provides output adjustment. The maximum output voltage level is produced when the potentiometer moving contact is at the bottom of R₅:

Minimum V_o occurs when the moving contact is at the top of R₅:

High Output Current Circuit:

For the circuit in Fig. 17-6 to function correctly, the collector current of Q₂ must be larger than the maximum base current flowing into Q₁. In regulators that supply a large output current, I_B1 can be too large for I_C2 to control. In this case, an additional transistor (Q₃) should be connected at the base of Q₁ to form a Darlington Circuit (or Darlington Pair), as in Fig. 17-7. Transistor Q₁ is usually a high power BJT requiring a heat sink and Q₃ is a low power device. The Q₃ base current is,

The current gain for the Darlington is (h_FE1 X h_FE3), and the level of I_B3 is low enough that I_C2 may easily be made much greater than I_B3. Note resistor R₆ in Fig. 17-7, which is included to provide a suitable minimum Q₃ operating current when I_L is very low.

Darlington transistors consisting of a pair of (low-power and high-power) BJTs fabricated together and packaged as a single device are available, These are usually referred to as power Darlington. The 2N6039 is an npn power Darlington with an h_FE specified as 750 minimum, 18 000 maximum. Resistor R₆ in Fig. 17-7 is not required when a power Darlington is used.

Preregulation:

Refer to Fig. 17-8, which is a partially reproduction of Fig. 17-2. Note that resistor R₁ is supplied from the regulator input, and consider what happens when, the supply voltage drops by 1 V. If I_C2 does not change, the voltage across R₁ remains constant, and the 1 V drop also occurs at the base of Q₁ and at the output of the regulator. This does not happen, of course. Instead, I_C2 changes to reduce the voltage across R₁ and thus keep the output voltage close to its normal level. The change in I_C2 is produced by a small change in the output voltage. If R₁ is connected to a constant-voltage source instead of the input, the change in I_C2 would not be required when V_S changes, and consequently, the output voltage change would not occur.

Now look at Fig. 17-9(a) where R₁ is shown connected to another Zener diode voltage source (R₇ and D₂). This arrangement is called a preregulator. When V_S changes, the change in V_Z2 that occurs is negligible compared to ΔV_S. Thus, virtually no change is required in I_C2 and V_o. A preregulator substantially improves the line and the load regulation of a regulator circuit.

The minimum voltage drop across R₁ should typically be 3 V. (Small values of R₁ give low amplifier voltage gain.) Also, a minimum of perhaps 6 V is required across R₇ to keep a reasonably constant current level through D₂. The The voltage (V_Z2) for D₂ is usually a relatively high voltage for a Zener diode. It may be necessary to use two diodes in series to give the desired voltage.

Figure 17-9(b) shows another preregulator circuit that has R₇ and D₂ connected across transistor Q₁. This gives an R₁ supply voltage of V₁ = (V_o + V_Z2).

Constant Current Source:

The constant-current source shown in Fig. 17-10 may be used in place of resistor R₁ as an alternative to a preregulator circuit. This arrangement passes all the required current to Q₃ base and Q₂ collector, but behaves as a very high resistance (1/ h_oe4) at the collector of transistor Q₂. Consequently, it increases the error amplifier voltage gain and results in a substantial improvement in the regulator performance. To design the constant current source V_CE4 should typically be a minimum of 3 V. The various voltage drops and current levels are then easily determined for component selection.

Differential Amplifier:

Figure 17-11 shows a regulator that uses a differential amplifier, or difference amplifier. In this circuit, Zener diode D₂ is the voltage reference source, and D₁ is used only to provide an appropriate voltage level at the emitter of transistor Q₂. V_o is divided to provide V_B6, and V_B6 is compared to V_Z2. Any difference in these two voltages is amplified by Q₅, Q₆, Q₂, and the associated components, and then applied to the base of Q₃ to change the output in the required direction.

The performance of the regulator is improved by the increased voltage gain of the error amplifier. However, the performance is also improved in another way. In the regulator circuit in Figs. 17-2 and 17-7, when I_C1 changes, the current through D₁ is also changed, (I_Z1 = I_R2 + I_E2). This causes the Zener voltage to change by a small amount, and this change in V_Z1 produces a change in output voltage. In the circuit in Fig. 17-11, the current through the reference diode (D₂) remains substantially constant because it is supplied from the regulator output. Therefore, there is no change in output voltage due to a change in the reference voltage.