Voltage Regulators:The function of a voltage regulator is to provide a stable dc voltage to electronic circuits and capable of providing substantial output current. If the element or component used for voltage regulation is connected series with the load, then it is called series voltage regulator and if it is connected across the load, it is called as shunt voltage regulator.

1. Zener diode voltage regulator2. Transistor voltage regulator

Zener diode voltage regulator:A Zener diode is a silicon P-N junction diode which is generally operated in

reverse break down region. Zener diode is a specially designed P-N junction diode which is heavily doped in reverse biased condition. It allows the current to keep the voltage constant across the Zener diode at the Zener break down voltage. For example, A zener diode Fz 9.1V will maintain a constant voltage of 9.1V across the diode if the input greater than 9.1V. The Zener diode is therefore ideal for applications such as generating reference voltage in an amplifier or a voltage stabilizer for low-current applications.

Generally the zener diode is connected in parallel with the load and hence it sis

called as shunt regulator. A resistance RS is connected in series with the zener to limit the current flows in the circuit. Hence the resistance is called as series current limiting resistor. The output voltage (Vo) is taken across the load resistance (RL). Since the reverse bias characteristics of zener diode are used in voltage regulation, the input voltage is always maintained greater than zener voltage (Vz) for proper regulation. The circuit diagram for zener voltage regulator is shown in the fig.

Fig – Zener voltage regulator

The input current passing through the current limiting resistor RS is obtained as follows:

Vs=IsRs+Vz

Is=Vs−VzRs

where Vs – the DC input voltage applied Vz – the voltage across zener

Ideally zener diode acts as a constant voltage source Vz. But practically there is a small resistance across zener causes a little voltage drop across it (Iz Rz). Therefore VL = Vz + Iz Rz

Assume Rz ≅ 0 then the above equation becomes VL ≅ Vz .

Operation:The output voltage varies due to two facts 1) variation in input voltage 2) variation in load current.

i. Assume the load resistance RL is fixed and the input voltage varies within the limit. As the input voltage increases, the input current IS also increases which will cause an increase in current through the zener without affecting the load current. The increase in input current cause increase in the voltage drop across series resistor Rs and hence the voltage across load is maintained constant.

ii. If the input voltage decreases, the input current IS also decreases which will cause a decrease in current through the zener. The decrease in input current cause decrease in the voltage drop across resistor and hence the voltage across load is maintained constant.

iii. Assume the input voltage is constant and load current is varied due to change in the load resistance. The input current is the sum of zener current and the load current. Hence the variation in the load current causes decrease in zener current and so the resistance across zener increases to maintain a constant voltage across it. Therefore the output voltage across load also becomes constant since it is connected parallel to zener.

Transistor Voltage regulators:The efficiency of Zener regulated power supply becomes very low when the load

current is high. Under such conditions a Zener controlled transistor is always used for maintaining output voltage constant. Basically there are two types of Zener controlled transistor voltage regulators. They are

Series Voltage Regulators Shunt Voltage Regulators.

Transistor Series Voltage Regulator (Emitter Follower Regulator)

Circuit description:A simple series voltage regulator using an NPN transistor and a Zener diode is

shown in the figure. This circuit is called a series regulator because collector and emitter terminals of the transistor are in series with the load, as illustrated in the figure. This circuit is also called an emitter follower voltage regulator because transistor Q is connected in emitter follower configuration. Here, the transistor Q is termed a series-pass

transistor. The unregulated dc supply (filtered output from the rectifier) is fed to the input terminals and regulated output voltage Vout is obtained across the load resistor RL.

Zener diode provides the reference voltage and the transistor acts as a variable resistor, whose resistance varies with the operating conditions (base current IB). The principle of operation of such a regulator is based on the fact that a large proportion of the change in supply (or input) voltage appears across the transistor and, therefore output voltage tends to remain constant.Keeping in mind the polarities of different voltages we have

Vout = Vz – VBE

The base voltage of the transistor remains almost constant being equal to that across the Zener diode, Vz.

Operationi. Let the supply (or input) voltage increase which will cause the output voltage

Vout to increase. An increase in output voltage Vout will result in decrease of VBE because Vz is fixed and decrease in VBE will reduce the conduction level. This will lead to increase in the collector-emitter resistance and hence the collector – emitter voltage and as a result the output voltage will be reduced. Thus output voltage will remain constant.

ii. Similarly if the input voltage decreases, the output voltage Vout also decreases. A decrease in output voltage Vout will cause an increase of VBE since Vz is fixed and so increase in VBE will increase the conduction level. This will lead to decrease in the collector-emitter resistance and hence the collector – emitter voltage. As a result of this, the output voltage will be increased and remains constant.

iii. Now let us consider the effect of change in load on the output voltage — say current is increased due to decrease in RL. Under such a situation the output voltage Vout tends to fall and, therefore, VBE tends to increase. As a result the conduction level of the transistor will increase which causes decrease in the collector-emitter resistance. The decrease in the collector-emitter resistance of the transistor will cause an increase in current IL to compensate for the decrease in RL. Thus the output voltage IL RL remains almost constant.

iv. Similarly the load current is decreased due to increase in RL. Therefore the output voltage Vout decreases which causes increase in VBE that allows more conduction. And so the collector-emitter resistance increases, therefore the load current IL

decreases. Thus the output voltage IL RL remains almost constant.

Advantages:1. The advantage of the above circuit is that the changes in Zener current are reduced by a factor β and thus the effect of Zener effect is greatly reduced and much more stabilized output is obtained.2. Efficiency is more than zener voltage regulator.

Limitations The output voltage cannot be maintained absolutely constant because both

VBE and Vz decrease with the increase in room temperature. There is no provision in the circuit for changing the output voltage.

It cannot provide good regulation at high currents because of small amplification provided by one transistor.

It has poor regulation and ripple suppression with respect to input variations as compared to other regulators.

The power dissipation of a pass transistor is large and therefore the transistor becoming hot.

Because of above limitations application of this regulator is limited to low output voltages.

Transistor Shunt Voltage Regulator

Circuit description:A shunt voltage regulator using an NPN transistor and a Zener diode is shown in the figure. A series resistance RSE is connected in series with the unregulated (or input), supply. Zener diode is connected across the base and collector terminals of the NPN transistor and the transistor is connected across the output, as shown in the figure. Unregulated voltage is reduced, due to voltage drop in series resistance RSE, by an amount that depends on the current supplied to the load RL. The voltage across the load is fixed by the Zener diode and transistor base-emitter voltage VBE

Output voltage is given as Vout = Vz + VBE = Vin – ILRSE

Operation:Since both Vz and VBE remain nearly constant so output voltage Vout remains nearly constant. This is explained below:

i. If the input (or supply) voltage increases, it causes increase in Vout and VBE resulting in increase in base current IB and therefore, increase in collector current Ic (Ic = β IB). Thus with the increase in supply voltage, current IL increases causing more voltage drop in series resistance RSE and thereby reducing the output voltage. This decrease in output voltage is enough to compensate the initial increase in output voltage. Thus output voltage remains almost constant. Reverse happens when the input voltage decreases.

ii. If the load resistance RL decreases, output current IL increases and this increase in output current leads to decrease in base and collector currents IB and Ic. Thus the input current I remain almost constant causing no change in voltage drop across series resistance RSE. Thus output voltage Vout being the difference of supply

voltage (fixed) and the voltage across series resistor Rsc (VR fixed) remains constant. Reverse happens should the load resistance increase.

Limitations There is considerable power loss in series resistor RSE. A large portion of supply current I flows through the transistor rather than to load. There are problems of over-voltage protection in the circuit.

For the above reasons, a series voltage regulator is preferred over the shunt voltage regulator.

Theory:One transistor circuit configuration that can be used to very good effect in many instances is the Darlington Pair. The darlington pair offers a number of advantages. The primarily use of darlington amplifier is, it offers a particularly high current gain and a high input impedance. Darlington pair transistor circuits can be bought as individual electronic components, i.e. two transistors, or it is also possible to obtain them as a single electronic component with the two transistors integrated onto one chip.

D.C. power supply, the resistance R1 provides feedback biasing. Capacitor Cin = 10 µF isolates the d.c. component and the internal resistance of the signal generator and couples the a.c. signal voltage to the base of the transistor. The coupling capacitor C2 =

10µF is the output capacitor which is used to connect further amplifiers in cascade. The output voltage is measured between the emitter and collector terminals.

The darlington pair circuit configuration is quite distinctive. It normally consists of two transistors and the emitter of the first transistor is connected directly to the base of the second one. Both collectors are connected together with supply. In this way the output current from the first transistor enters into the base of the second. This results in a very high level of current gain. The overall current gain of the darlington pair is the product of the two individual transistors:

Current gain=G1 ×G 2

where G1 and G2 are the current gains of the two individual amplifiers.

Fig – Darlington Pair Amplifier

Because of its high input impedance and low output impedance, an darlington amplifier is capable of giving power to a load connected to its output without requiring much power at the input. It thus operates as a buffer/ driver amplifier. The circuit has current gain instead of voltage gain. A small change to the input current results in much larger change in the output current supplied to the output load. The voltage gain of

darlington amplifier is similar to common collector circuit which is close to unity practically.

Frequency response curve:The curve representing the variation of gain of an amplifier with frequency is known as frequency response curve. It is shown in Fig. 2. The voltage gain of the darlington amplifier becomes almost constant and always very close to unity. At high frequency it decreases little and afterwards the gain starts decreasing with the increase of the frequency.

Fig – Frequency response