POWER AMPLIFIER

Class BClass ABClass C

CLASS B POWER AMPLIFIER

Consists of complementary pair electronic devices

One conducts for one half cycle of the input signal and the other conducts for another half of the input signal

When the input is zero, both devices are off, the bias currents are zero and the output is zero.

Ideal voltage gain is unity

For input larger than zero, A turn ON and supplies current to the load.

For input less than zero, B turn ON and sinks current from the load

Complimentary Push-Pull Circuit

DEAD BAND

CROSSOVER DISTORTION

The Ideal Class B

tVv po sin

tR

Vi

L

pCn sin

i Cn

i Cp

tVv Po sin

sin tR

Vi

L

pCn

• Maximum possible value of Vp is VCC

• The instantaneous power in Qn is;

tVVv pCCCEn sin

CnCEnQn ivp

t

R

VtVVp

L

ppCCQn sinsin

The average power in Qn is

Differentiating for maximum PQn with respect to Vp equal to zero gives us

Maximum average power dissipation;

L

p

L

pCCQn R

V

R

VVP

4

2

L

CCQn R

VP

2

2

max

CCP

L

p

L

CC

p

Qn

VVthen

R

V

R

V

dV

dP

2

04

2

The average power delivered to the load is

Power source supplies half sinewave of current, the average value is;

The total power supplied by the two sources is

L

pS R

VI

L

pL R

VP

2

2

1

L

pCCSCCsS R

VVIVPP

L

pCCSCCS R

VVIVP

22

The efficiency is

%5.78785.04

when efficiency maximum

42

2

21

CCP

CC

p

RV

CC

RV

S

L

VV

V

V

VP

P

L

P

L

P

Class AB Power Amplifier

Small quiescent bias on each output transistor to eliminate crossover distortion

Class C Power Amplifier

Class AB Voltage Transfer Curve

Collector Currents & Output Current

CpLCn iii

Example

The parameters are VDD=10V, RL=20Ω. The transistor are matched and K=0.2A/V2, VT=1V, IDQ=0.05 when vo=5V.

Determine the required biasing in a MOSFET class AB output stage.

Inductively Coupled Amplifier

The maximum possible average signal power delivered to the load

The possible average signal power supply by VCC

The maximum possible power conversion efficiency

L

CCLCQL R

VRIP

22

2

1

2

1(max)

L

CCCQCCS R

VIVP

2

%505.02

1(max)(max) 2

2

21

L

CC

L

CC

RV

RV

S

L

P

P

TRANSFORMER COUPLED AMPLIFIER