activity 7

Lucille G. Yu BSEC 3

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Activity 7

Large Signal Amplifiers: Class AB Amplifier

Objectives:

1.) To observe Class AB amplifier characteristics.

2.) To measure voltage inputs and outputs Vi1, Vi2, Vo1 and Vo2.

3.) To compare the input and output phases of the circuit.

4.) To observe phase pattern between corrected and uncorrected circuit.

5.) To observe a crossover distortion in an uncorrected circuit.

Introduction:

Large-signal or power amplifiers primarily provide sufficient power to an output load to drive a

speaker or other power device, typically a few watts to tens of watts. The main features of a large-signal

amplifier are the circuits power efficiency, the maximum amount of power that the circuit is capable of

handling, and the impedance matching to the output device.

One method used to categorize amplifiers is by class. Basically, amplifier classes represent the

amount the output signal varies over one cycle of operation for a full cycle of input signal.

For Class AB amplifiers, the advantage of this small bias voltage shown by the figure above, provided by

series diodes or resistors, is that the crossover distortion created by the class B amplifier characteristics is

overcome, without the inefficiencies of the class A amplifier design. So the class AB amplifier is a good

compromise between class A and class B in terms of efficiency and linearity, with conversion efficiencies

reaching about 50% to 60%.


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Q1

PN2907A

Q2

R1

10k

R2

10k

1

V1

12 V

3

R3

10k

0

V2

12 V 4

0

C1

470F C2

470F

6

2

XFG1

7

0

5

0

Q1

PN2907A

Q2

R1

10k

R2

10k

1

V1

12 V

3

R3

10k

0

V2

12 V

4

0

C1

470F

C2

470F

6

XFG1

7

0

5

0

D1

1N4001GP

D2

1N4001GP

2

8

9

Materials:

Breadboard Connectors Resistors Capacitors

2 Diodes Function Generator 2 NPN transistors

Multitester Oscilloscope 2 DC Power Supply

Procedure:

1.) Construct the circuit below:

2.) Using an oscilloscope, observe the behavior of the signal produced and record the values

of Vi and Vo on the table below under uncorrected circuit.

3.) Graph the input and output signal.

4.) Construct the circuit below:


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5.) Observe the behavior of the signal using an oscilloscope and record the values of Vi and

Vo on the table below under corrected circuit.

6.) Graph the input and output signal.

Results:

Uncorrected Circuit

Vi 1.42 V

Vo 284 mV

Corrected Circuit

Vi 164 mV

Vo 148 mV

Graphs:

Uncorrected Circuit

Vi(yellow) and Vo(blue)


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Corrected Circuit

Vi(yellow) and Vo(blue)

Conclusion:

Since the two transistors are different ( an NPN transistor on the upper part while a PNP

transistor on the lower part), only one will let the current flow through each of the positive/negative

half cycle of the AC signal/ voltage source. An NPN is ON when there is a positive potential difference

measured from the emitter of an NPN transistor to its base while a PNP transistor behaves oppositely

since it is ON if and only if when its base is pulled high relative to the emitter ( negative potential

difference ).

The uncorrected circuit has cross over distortion since either of the transistor is off when the

supplied voltage is approximately less than the threshold value of 0.7 volts for it to function well. The

straight line part of the output signal shows that there is indeed a distorted signal.

To overcome this situation, two diodes were added to the circuit. On each half cycle, one is

forward biased while the other is backward biased and on the next half cycle, they exchange roles

hence, there is no time when both of them are forward neither backward biased at the time. The graph

of the corrected circuit shows a perfect sinusoidal wave since the distortion within the range of -0.7 to

0.7 volts of the supplied ac signal is aided by the presence of the two diodes.

activity 7

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