Op Amp Voltage Regulators:

Voltage Follower Regulator – Refer once again to the Op Amp Voltage Regulators circuit in Fig. 17-11. The complete error amplifier has two input terminals at the bases of Q₅ and Q₆ and one output at the collector of Q₂, Transistor Q₆ base is an inverting input and Q₅ base is a noninverting input. The error amplifier circuit Is essentially an operational amplifier. Thus, IC operational amplifiers with their extremely high open-loop voltage gain are ideal for use as error amplifiers in dc voltage regulator circuits. Normally, an internally compensated op-amp (such as the 741) is quite suitable for most voltage regulator applications.

A simple voltage follower regulator circuit is illustrated in Fig. 17-­16. In this circuit, the op-amp output voltage always follows the voltage at the noninverting terminal, consequently, V_o remains constant at V_Z. The only design calculations are those required for design of the Zener diode voltage reference circuit (R₁ and D₁), and for the specification of Q₁.

Adjustable Output Regulator:

The circuit in Fig. 17-17 is that of a variable-output, highly stable de voltage regulator. As shown already in transistor circuit, the reference diode in Fig. 17-17 is connected at the amplifier noninverting input, and the output voltage is divided and applied to the inverting input. The operational amplifier positive supply terminal has to be connected to regulator supply voltage. If it were connected to the regulator output, the op-amp output voltage (at Q₂ base) would have to be approximately 0.7 V higher than its positive supply terminal, and this is impossible.

Design of the regulator circuit In Fig. 17-17 involves selection of R₁ and D₁, design of the voltage divider network (R₃, R₄, and R₅), and specification of transistors Q₁ and Q₂. Clearly, an op-amp voltage regulator is more easily designed than a purely transistor regulator circuit.

Current Limiting with an Op-amp Regulator:

When a large output current is to be supplied by an operational amplifier voltage regulator, one of the current limiting circuits described already may be used with one important modification. Figure 17-19 shows the modification. A resistor (R₁₃) must be connected between the op-amp output terminal and the Junction of Q_2B and Q_7C.

When an overload causes the regulator output voltage to go to zero, the op-amp output goes high (close to V_S) as it attempts to return V_o to its normal level. Consequently, because the op-amp normally has a very low output resistance, R₁₃ is necessary to allow I_C7 to drop the voltage at Q₂ base to near ground level. The additional resistor at the op-amp output is calculated as R₁₃ ≈ V_S/I_C7. Resistor R₁₃ must not be so large that an excessive voltage drop occurs across it when the regulator is supposed to be operating normally.