Class B Power Amplifier

Class B Amplifier Operation

Class B

The amplifier conducts through 180 of the input.

The Q-point is set at the cutoff point.

Output only conducts for 180 or one-half of the AC input signal.

The Q-point is at 0V on the load line, so that the AC signal can only swing for one-half cycle.

To obtain output for the full cycle of signal, it is necessary to use

two transistors.

Since one part of the circuit pushes the signal high during one half-

cycle and the other part pulls the signal low during the other half-

cycle. The circuit is referred to as a push-pull circuit.

A pnp transistor that provides the positive half.

An npn transistor that provides the negative half of the AC cycle

Class B Amplifier Efficiency

The maximum efficiency of class B amplifier is 78.5%.

The power supplied to the load is drawn from the power supply,

then

where Idc is the average current expressed as

The power delivered to the load can be calculated by either one of

these equations

( ) dcdci CCP V I=

Peak value of output current

( )( )

2 rmsac L

o

L

VP

R= ( )

( ) ( )2 2p-p pac

8 2

L Lo

L L

V VP

R R= =

)(

2pIV LCC

)(2

pII Ldc

Efficiency

Power dissipated by output resistors

( ) ( )2 dc acQ i oP P P= -

Power dissipated by the two output power transistors

2

2

Q

Q

PP =

Power dissipated by each output power transistor

%100)(

4%100

)(

)(% 0

CC

L

i V

pV

dcP

acP

Example

For a class B amplifier providing a 20V peak signal to a 16 load

and a power supply of VCC = 30V, determine the input power,

output power, and circuit efficiency.

A peak of load current is

The dc value of the current drawn from the power supply is

( )pLV LR

( )( )p

p LL

L

VI

R= = A

V25.1

16

20

AApII Ldc 796.0)25.1(2

)(2

The input power delivered by the supply voltage is

The output power delivered to the load is

Hence, the efficiency of this circuit is

( ) dcdci CCP V I= ( )( )30V 0.796A 23.9W= =

( )( )2 p

ac2

Lo

L

VP

R= =

%100)(

)(% 0

dcP

acP

i

WV

5.12)16(2

)20( 2

%3.52%1009.23

5.12

W

W

Maximum power consideration

The maximum output power is when VL(p) = VCC

The corresponding peak ac current IL(p) is then

( )( )2 p

ac2

Lo

L

VP

R= ( )

2

maximum ac2

CCo

L

VP

R=

( )p CCL

L

VI

R=

( ) dcdci CCP V I=

L

CC

L

CCCCi

R

V

R

VVdcP

222)( maximum

L

CCL

R

VpII

2)(

2 maximum dc

The maximum circuit efficiency for class B is

The maximum power dissipation of class B is found to be

%100/2

2/

%100)(

)(%

2

2

0

LCC

LCC

i

RV

RV

dcP

acP

%54.78%1004

L

CCQ

R

VP

2

2

2

2 maximum

The maximum efficiency of a class B amplifier can also be

expressed as

Example VCC = 24V

VL(p) = 22V

VL(p) = 6V

( )( )2 p

ac2

Lo

L

VP

R=

)(

2)( pIVdcP LCCi

CC

L

i V

pV

dcP

acP )(%)54.78(%100

)(

)(% 0

%7224

22%)54.78(%

V

V

%6.1924

6%)54.78(%

V

V

Example

For a class B amplifier using a power supply of VCC = 30V and

driving a load of 16 load, determine the maximum input power,

output power and transistor dissipation.

The maximum output power is

The maximum input power is

( )

2

maximum ac2

CCo

L

VP

R= W

V125.28

)16(2

)30( 2

L

CCi

R

VdcP

22)( maximum W

V81.35

)16(

)30(2 2

The maximum circuit efficiency is

The maximum power dissipation of class B is found to be

%100)(

)(% 0

dcP

acP

i

%54.78%10081.35

125.28

W

W

L

CCQ

QR

VPP

2

22 2

5.02

maximum maximum

WV

7.5)16(

)30(25.0

2

2

Transformer-Coupled Push-Pull Circuit

The center-tapped transformer on the input produces opposite

polarity signals to the two transistor inputs.

The center-tapped transformer on the output combines the two

halves of the AC waveform together.

Complementary-Symmetry Push-Pull Circuit

During the positive half-cycle of the AC input, transistor Q1 (npn)

is conducting and Q2 (pnp) is off.

During the negative half-cycle of the AC input, transistor Q2 (pnp)

is conducting and Q1 (npn) is off.

Crossover distortion

If the transistors Q1 and Q2 do not turn on and off at exactly the

same time, then there is a gap in the output voltage.

Quasi-Complementary

Push-Pull Amplifier

A Darlington pair and a feedback pair combination perform the

push-pull operation. This increases the output power capability.

Example

Calculate the input power, output power, power handled by each

transistor, and circuit efficiency for an input of 12 Vrms.

For 12V rms input signal, the peak input voltage thus is

By assuming that the amplifier has ideally a voltage gain of unity,

then

and the output power across load is

( )p 17 VLV =

( )( )2 p

ac2

Lo

L

VP

R=

VVrmsVpV ii 17)12(2)(2)(

WV

125.36)4(2

)17( 2

In addition, the peak load current is

from which the dc current of the supply is

Hence, the input power is

AV

R

pVpI

L

LL 25.4

4

17)()(

AApII Ldc 71.225.42

)(2

WAVpIVIVdcP LCCdcCCi 75.67)71.2)(25()(2

)(

The power dissipated by each transistor thus is

The circuit efficiency for the 12 V rms input voltage is then

( ) ( )2 dc ac

2 2

Q i oQ

P P PP

-= =

( )67.75 36.125 W15.8 W

2

-= =

%100)(

)(% 0

dcP

acP

i

%3.53%10075.67

125.36

W

W

Homework

Determine (a) Maximum Po(ac), (b) maximum Pi(dc), (c)

Maximum %, (d) Maximum power dissipated by both transistors

Quiz

Calculate (a) Po(ac), (b) Pi(dc), (c) %, (d) power dissipated by both transistors

Quiz

(a)

(b)

(c)

(d)

( ) dcdci CCP V I=

( ) ( )2 dc ac 40.5WQ i oP P P= - =

WVrms

R

rmsVacP

L

Lo 5.40

8

)18()()(

22

WV

VR

pVV

L

LCC 81

8

2182)40(

)(2

%100)(

)(% 0

dcP

acP

i

%50%10081

5.40

W

W

Homework

(a)

(b)

(c)

(d)

( )

2

maximum ac2

CCo

L

VP

R= W

V25.56

)8(2

)30( 2

L

CCi

R

VdcP

22)( maximum W

V62.71

)8(

)30(2 2

%54.78%10062.71

25.56%

W

W

WV

R

VP

L

CCQ 8.22

)8(

)30(22 maximum

2

2

2

2

2