Op-Amp Series Voltage Regulator Design

Op-Amp Series Voltage Regulator Design

Hello Guys,

In This Blog i am going to explain about the Opamp Series voltage regulator design. The complete explanation is given in the video-

Circuit Diagram :

Working Explanation : 

There is basically three elements in design-

1. Reference Source(Vz) - e.g. Zener Diode or any reference ICs

2. Operational Amplifier

3. Series Pass Transistor(T1)

Lets assume 

Vin = input Voltage, 

Vo' = Output of Opamp , 

Vo = Output Voltage

V- = Voltage at inverting terminal

V+ = Voltage and non inverting Terminal

It can be seen in the above circuit V- is connected with Vo via resistor divider. So op-amp in connected in negative feedback configuration and works as

a closed loop configuration. Opamp tries to maintain V- voltage as same as V+ because of closed loop configuration and virtual short. So voltage at inverting terminal is nearly equal to voltage at Non inverting terminal.

i.e. V- = V+ = Vz                      (1)

Voltage at Inverting terminal is given by :

V- = Vo * {R2/(R1+R2)}          (2)

From equation (1)

Vz = Vo * {R2/(R1+R2)}

Vo = Vz * {(R1+R2)/R2}         (3)

From equation (3) it can be seen that output voltage of Voltage regulator is dependent on Vz, R1 and R2. So by changing the value of Vz, R1 and R2 we can control the regulator output.

Now if you apply KVL in another loop

Vo = Vo' - Vbe                           (4)

From equation 4 it can be seen that op-amp output Vo' is always greater than Vo.

Let us understand how this Voltage regulator works there is change in input voltage Vin.

Case 1: If Input voltage Vin increasing then how the output voltage Vo remained stable

As soon as input voltage Vin start increasing the output voltage Vo will also increase this causes the voltage at V- terminal will also increase. Since due to virtual short voltage at V- is nearly equal to Voltage at V+. So if voltage V- is increased that means V-  > V+ and hence op-amp output will become more negative that causes reduction in Vo'. Reduced value of Vo' inject less drive current to transistor (T1) base. Transistor T1 works in linear region so if base current is reduces, the collector current of transistor reduce and that produces high voltage drop across transistor T1 Vce. If Vce is increased the regulator maintains the required output voltage. That is how op-amp regulates the regulator output voltage and maintains constant output voltage.

Case 2: If Input voltage Vin decreasing then how the output voltage Vo remained stable

As soon as input voltage Vin start decreasing the output voltage Vo will also decrease this causes the voltage at V- terminal will also decrease. Since due to virtual short voltage at V- is nearly equal to Voltage at V+. So if voltage V- is decreased that means V-  < V+ and hence op-amp output will become more positive that causes voltage increment in Vo'. Increased value of Vo' inject more drive current to transistor (T1) base. Transistor T1 works in linear region so if base current is increases, the collector current of transistor increases and that reduces voltage drop across transistor T1 Vce. If Vce is reduced then Vo in increased and the regulator maintains the required output voltage. That is how op-amp regulates the regulator output voltage and maintains constant output voltage.

Advantage : High Efficiency

Disadvantage : External Short circuit protection required

Note : This circuit is not tested in production and it is only for educational purpose.

Please leave you comments and suggestion.

Thanks !!