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EXPT NO:- 7

Title:- Complimentary-symmetry power amplifier and its DC analysis. Measurement of efficiency.

Aim:- To determine the percentage efficiency of class AB complementary symmetry

power amplifier.

Instruments & Components:- Power Supply, Multimeter ,power transistors(TIP122 & TIP127),resistors, function generator, CRO, connecting wires, Bread Board.

Theory:- APPARATUS : -

1. Kit for above experiment,

2. DMM.

3. CRO

4. Function generator,

5. Power supply.

6. Connectors etc.

THEORY:

Small signal amplifiers operate in the linear portion of the transfer characteristics which is very close to the Q point of the amplifier. Small signal amplifiers can be analyzed with the help of small signal h-parameters. Small signal amplifiers are also known as "voltage amplifiers". This is because these amplifiers are used primarily for voltage amplification but they are not capable of supplying a large power to the loads such as loud speakers. Whenever the load demands a large power, we have to use specially designed amplifiers called "power amplifiers".

Fig1 Concept of power amplifiers

Important Features of a Power Amplifier:

Some of the important features of a large signal amplifiers are as follows :

• Impedance matching with the load is necessary.

• Power transistors are required to be used.

• Power amplifiers are bulky.

• Harmonic distortion is present in their output.

• They are capable of handling a large power.

Efficiency :

Efficiency of power amplifier is defined as the ratio of output power to the input power.

.∙. % efficiency (η) = (Pout/Pin)*100

But Pin= Pout + Plosses

.∙. % efficiency (η) = Pout *100

( Pout + Plosses)

Classification of Power Amplifiers :

Depending on the position of the Q-point or operating point on the load line, the power amplifiers are classified into following four categories:

This classification has been done on the basis of position of Q point on the load line.

Position of Q point for various power amplifiers

Sr. No.

Type of power amplifier

Position of Q-point

1.

Class - A

At the center of load line

2.

Class - B

In the cut off region

3.

Class - AB

Just above the cut off

4.

Class - C

Below the cut off

Distortions in Amplifiers :

Types of distortions :

The three basic types of distortions are :

• Amplitude distortion or non-linear distortion

• Frequency distortion

• Delay or phase shift distortion

In addition to these three, there are few more such as cross-over distortion.

CLASS B POWER AMPLIFIER :

• The Q point of a class B amplifier is adjusted to be on the X-axis, i.e. the transistor is biased at cut-off. Due to this, a transistor can conduct only during the positive half cycle of the ac input signal.

• That means the conduction takes place for only 180° or a half cycle period. Lot of distortion will therefore be present at the output.

• In order to overcome this problem, two transistors are used instead of one. These transistors conduct in the alternate half cycles of the input signal to produce the full ac signal. Types of class B power amplifier :

The class B amplifier using two transistors can be of two types :

• Class B - Push pull amplifier.

• Class B - Complementary symmetry amplifier. Let us discuss these circuits one by one.

Advantages of Class B Amplifier

1. Higher efficiency (78.5%)

2. Zero power dissipation under quiescent condition.

3. Impedance mulching with load is possible.

4. Second harmonic component gets automatically cancelled.

Disadvantages :

1. Cross over distortion is present in the output waveform.

2. Efficiency is not so high.

It is important to have a small collector current under no signal condition. Otherwise, we can have cross over distortion. Suppose there is no bias at all applied to emitter junction. Then incoming ac signal has to rise about 0.7V to overcome barrier potential. Because of this, no collector current exists when the signal is less than 0.7V. The action of the other half cycle is complementary; the other transistor does not turn on until the ac input signal is more negative than 0.7V. For this reason, if no bias at all is applied to the emitter junction the output of a class B push pull amplifier looks like Figure 4.14. a.

The output is distorted; it is no longer a sine wave because of the clipping action between half cycles. Since clipping occurs between the times one transistor goes off and the other comes on, we call it crossover distortion. To eliminate crossover distortion, we need to apply a small forward bias to each emitter diode. This means locating Q point slightly above cut off as shown. This is known as class AB operation.

Complementary n-p-n and p-n-p transistors are used instead of using two of the same type. The single input signal is applied to both base inputs. A simple circuit is as shown.

Since the transistors are of opposite type, they will conduct on opposite half cycle of the input. Each transistor acts as an emitter follower. R1 and R2 are so chosen that emitter potential VE with respect to ground ± . Also

During positive half cycle Q1 is ON, and Q 2 is OFF. During the negative half cycle Q2 is ON, and Q1 is OFF. The operation of the circuit is push-pull with each transistor sending current to RL during alternate half cycles.

Procedure:-

1) Connect the circuit as shown in fig.

2) Apply the input signal Vin

3) Measure Idc on multimeter and Vop-p on CRO.

4) Calculate Pac, Pdc, Pd (per transistor) and % efficiency.

5) Repeat the same for Vin = Vcc

Observations:-

Sr. No.

Quntity

Measured values

For Vin

For Vin=Vcc

1

Idc

2

Vop-p

Calculations:-

Take Idc= 2X Idc (measured)

1.Pdc = Vcc X Idc

2.Pac=Vo(rms)/RL

3. % Efficiency = (Pac/Pdc) X100

4. Pd=Pdc-Pac

5. Pd per transistor = Pd/2

Result :

Sr. No.

Quantity

Values

1.

% Efficiency (η)

2

Power Dissipation(Pd)

CONCLUSION :--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

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