8-1

İzmir University of Economics

ETE 331 Analog Electronics - Lab

EXPERIMENT 8 Power Amplifiers

A. Background

Class A Stage

The amplifier given in Fig. 8.1 is of type called Class A amplifier.

If the input is a sinusoidal, then the output is also sinusoidal over the DC biasing voltage (Fig.

8.2).

= + sin volts

Fig. 8.2

The useful AC power is then

= 2⁄

vOt,volts

VCQ

T t

VCQ+VCP

VCQ-VCP

vS

+

vO

-

R2

R1 RL

VCC

C1

Fig. 8. 1. Class A Amplifier

8-2

For the power delivered to the circuit, the power spent over the base circuitry is neglected

and the average power delivered from the power supply is obtained as:

=

The power efficiency is defined as

= ⁄

or

= 2 !

The maximum efficiency is reached if

= 2⁄ and = 2⁄ and = 2⁄

Under these conditions, the efficiency becomes:

= 0.25 = 25%

i.e.., the 25 percent of the power delivered to the circuit is obtained as useful load power.

Class B Stage

The efficiency of a power amplifier may be increased using the following (Fig. 8.3) Class B

amplifier called push-pull amplifier.

Fig. 8.3

During the positive cycles, Q1 conducts, during the negative cycles Q2 conducts; but the

voltage over the load is a continuous sinusoidal (Fig. 8.4). The signal over the load then does

not contain any DC component.

Fig. 8.4

vOt,volts

T t

VCP

-VCP

vS

+

vO

-

RL

VCC

- VCC

Q1

npn

Q2

pnp

8-3

For the Class B amplifier then

= 2⁄

and the power from the above power supply proportional to VCC and the average current

through the collector of Q1.

& = &'

The collector current of Q1 is a half-wave rectified sine wave (Fig. 8.5).

Fig. 8.5

The average (DC) value of the collector current is then

&' = (⁄ or &' = (⁄

and

& = ( ⁄

The total power from the two sources is therefore

= 2& =2(

Hence the efficiency is

=

= 2⁄

2 (⁄ = 4⁄

(⁄ =

(4

As it can be seen from the above result, the efficiency is linearly proportional to VCP. The

maximum efficiency is reached when VCP = VCC, then

=(

4= 78.5%

Class AB Amplifier

Because of the turn-on voltages of base-emitter junctions of the transistors, the output

voltage is distorted and not a pure sine wave (Fig. 8.6). This distortion is called cross-over

distortion.

Fig. 8.6

vOt,volts

T t

VCP – VBE1(on)

-VCP + VEB1(on)

iC1t,volts

T t

ICP

ICP= VCP/RL

8-4

The cross-over distortion may be avoided by using an additional circuitry at the input side

(Fig. 8.7).

(a) (b)

Fig. 8.7

The turn-on voltages of the diodes (Fig. 8.7a) or the DC voltage drop over R2’s (R2 << RL) are

set very close to the base emitter drops of the transistors to cancel out the cross-over

distortion at the output.

B. Preliminary Work

a) Consider the Class A stage given in Fig. 8.8. Assume β = 120 and VBE(on) = 0.6 V for the

transistor. Determine the values of R1 and R2 to set VCQ = 6 V and VBB = 3.5 V.

Fig. 8.8

vS

+

vO

-

R2

R1

RL

1K

VCC = 12 V

C1

RE

100ΩCE

vS

+

vO

-

RL

VCC

- VCC

Q1

npn

Q2

pnp

R1

R1

R2

R2 vS

+

vO

-

RL

VCC

- VCC

Q1

npn

Q2

pnp

R1

R1

C. Experimental Work

1. Set up the circuit given in Fig.

Work. (CE = 100 µF, C1 = 10

(a) Measure VCQ. If VCQ ≠

(b) Apply a sinusoidal signal to the inp

(c) Measure IS and calculate P

IS = ……. mA => P

(d) Now adjust VCP = 3 V.

VCP(max) = …………

vS

Experimental Work

Set up the circuit given in Fig. 8.9 with the resistors you have determined in Preliminary

= 10 µF. Be careful with the polarities of the capacitors)

Fig. 8.9

≠ 6V, make necessary modifications to set VCQ = 6 V.

Apply a sinusoidal signal to the input at f = 1 kHz.

and calculate PS.

mA => PS = VCC IS = …………… mW

= 3 V. Plot vO below. What is the maximum VCP that you can get?

…. Volts

+

vO

-

R2

R1

RL

1K

VCC = 12 V

C1

BD 135

A

IS

CE RE

100Ω

8-5

8.9 with the resistors you have determined in Preliminary

F. Be careful with the polarities of the capacitors)

= 6 V.

that you can get?

(e) Calculate the efficiencies

VCP

3 V

VCP(max)(……..V)

2. Now set up the push-pull amplifier given in Fig. 8.

(a) Apply a sinusoidal signal

VCP(max) = …………

ciencies for the following VCP values.

PL PS

pull amplifier given in Fig. 8.10. (VCC = 12 V)

Fig. 8.10

Apply a sinusoidal signal at 1 kHz to the input. Plot vO(t) for VCP = 6 V.

…. Volts

vS

+

vO

-

RL

1 K

- VCC

BD

135

BD

136

100 Ω

100 Ω

VCC = 12 V

A

IS

8-6

η

(b) Calculate the efficiencies

VCP

6 V

VCP(max)(……..V)

(c) Set up the following Class AB Amplifier.

input. Plot vO(t) for V

vS

ciencies for the following VCP values.

PL PS

Set up the following Class AB Amplifier. Apply a sinusoidal signal at 1 kHz

for VCP = 6 V.

Fig. 8.11

S

+

vO

-

RL

1 K

- VCC

BD

135

BD

136

100 Ω

100 Ω

VCC = 12 V

R1

8-7

η

at 1 kHz to the