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Chapter 6 Voltage Regulators - Part 2- - PowerPoint PPT Presentation

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Chapter 6 Voltage Regulators - Part 2-. Basic Linear Shunt Regulators. Shunt Regulator. Fig. 6.9 Block diagram of a basic components of a shunt regulator. Basic op-amp Shunt Regulator with load resistor. Fig. 6.10 Transistor shunt voltage regulator.

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Chapter 6

Voltage Regulators

- Part 2-


Shunt Regulator

Fig. 6.9 Block diagram of a basic components of a shunt regulator.


Basic op-amp Shunt Regulator with load resistor

Fig. 6.10 Transistor shunt voltage regulator.

  • In the basic shunt regulator, the control element is a transistor Q1, in parallel with the load.
  • A resistor R1, is in series with the load.

The operation of the circuit is similar to the series regulator, except that regulation is achieved by controlling the current through the parallel transistor Q1 When the output voltage tries to decrease due to a change in input voltage or load current caused by a change in load resistance, the decrease is sensed by R3 and R4.


A feedback voltage obtained from voltage divider R3 and R4 is applied to the op-amp’s non-inverting input and compared to the Zener voltage to control the drive current to the transistor.

The difference voltage reduces the op-amp’s output, but increasing the collector voltage. This keeps the load output nearly constant.



  • With IL andVOUT constant, a change in the input voltage produces a change in shunt current (Is)as follow:
  • Maximum Load current:



basic switching regulators1
Basic Switching Regulators
  • The control element (transistor) operates as a switch.
  • The transistor switches on and off and dissipates power only when it is on.
  • The transistor operates only on the ends of the load line except during the very short switching time.
  • As a result, efficiencies can be greater than 90%.
  • Three basic configurations of switching regulators are:
    • Step-down
    • Step-up
    • Voltage-inverter

Switching Regulator

Fig. 6.11 Block representation of three-terminal voltage regulator.


All switching regulators control the output voltage by rapidly switching the input voltage on and off with a duty cycle that depends on the load. Because they use high frequency switching, they tend to be electrically noisy.

on/off control



The output voltage is expressed as :

  • The period in the sum of the on-time and the off-time:
  • The ratio is called the duty cycle




Step-Down Configuration

A step-down switching regulator controls the output voltage by controlling the duty cycle to a series transistor. The duty cycle changes depending on the load requirement.

Because the transistor is either ON or OFF on all switching regulators, the power dissipated in the transistor is very small and the regulator is very efficient. The pulses are smoothed by an LC filter.


Step-Up Configuration

In a step-up switching regulator, the control element operates as a rapidly pulsing switch to ground. The switch on and off times are controlled by the output voltage.

Step-up action is due to the fact the inductor changes polarity during switching and adds to VIN. Thus, the output voltage is larger than the input voltage.


Voltage-Inverter Configuration

In a voltage-inverter switching regulator, the output is the opposite polarity of the input. It can be used in conjunction with a positive regulator from the same input source.

Inversion occurs because the inductor reverses polarity when the diode conducts, charging the capacitor with the opposite polarity of the input.



  • Several types of both linear (series and shunt) and switching regulators are available in integrated circuit (IC) form.
  • Single IC regulators contain the circuitry for:
  • reference source,
  • comparator amplifier,
  • control device, and;
  • overload protection.
  • Generally, the linear regulators are three-terminal devices that provides either positive or negative output voltages that can be either fixed or adjustable.

Fixed Voltage Regulator

The fixed voltage regulator has an unregulated dc input voltage Vi applied to one input terminal, a regulated output dc voltage Vo from a second terminal, and the third terminal connected to ground.

Fixed-Positive Voltage Regulator

The series 78XX regulators are the three-terminal devices that provide a fixed positive output voltage.



Fig. 6.12 (a) Standard configuration of the series 78XX regulator and

(b) typical packages


Fig. 6.12 Standard configuration of a 7812 voltage regulator.

The figure above shows:

An unregulated input voltage Vi is filtered by a capacitor C1 and connected to the IC’s IN terminal.

The IC’s OUT terminal provides a regulated +12 V, which is filtered by capacitor C2.

The third IC terminal is connected to ground (GND).


Fixed-Negative Voltage Regulator

The series 79XX regulators are the three-terminal IC regulators that provide a fixed negative output voltage.

This series has the same features and characteristics as the series 78XX regulators except the pin numbers are different.

Fig. 6.14 Standard configuration.


Adjustable-Voltage Regulator

Voltage regulators are also available in circuit configurations that allow to set the output voltage to a desired regulated value.

The LM317 is an example of an adjustable-voltage regulator, can be operated over the range of voltage from 1.2 to 37 V.

Fig. 6.15 Connection of LM317 adjustable-voltage regulator.