Download 2# Power Amplifier

POWER AMPLIFIER
Class
B
Class AB
Class 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
CROSSOVER DISTORTION
DEAD BAND
The Ideal Class B
i Cn
iCn 
Vp
RL
sin t
iCn 
vo  V p sin t
i Cp
vo  VP sin t
Vp
RL
sin t
• Maximum possible value of Vp is VCC
vCEn  VCC  V p sin t
• The instantaneous power in Qn is;
pQn  vCEniCn
 Vp

pQn  VCC  V p sin t  sin t 
 RL



The average power in Qn is
PQn 
VCCV p
RL

Vp
2
4 RL
Differentiating for maximum PQn with respect to Vp equal to
zero gives us
dPQn
VCC 2V p


0
dV p RL 4 RL
then VP 

2VCC

Maximum average power dissipation;
2
VCC
PQn max   2
 RL


The average power delivered to the load is
PL 
2 RL
V
Power source supplies half sinewave of current, I S  p
RL
the average value is;
 Vp 

PS   Ps   VCC I S  VCC 
 RL 

2
V
1 p
The total power supplied by the two sources is
 Vp 

PS  2VCC I S  2VCC 
 RL 

The efficiency is
2
V
1 P
2 RL
VP
CC RL
PL


PS 2V
maximum


4
 

V p
4VCC
efficiency when VP  VCC 
 0.785  78.5%
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
iCn  iL  iCp
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
2
PL (max) 

1
1 VCC
2
I CQ RL 
2
2 RL
The possible average signal power supply by VCC
PS  VCC I CQ

VCC

RL
2
The maximum possible power conversion efficiency
PL (max)
 (max) 

PS
2
1 VCC
2 RL
VCC 2
RL
1
  0.5  50%
2
TRANSFORMER COUPLED AMPLIFIER