Power Amplifiers
Class B
Class AB
Class B
The circuit
each transistor
conducts for a half of
every signal period
complementary pair
push-pull arrangement
Operation
vI  (0.6V;  0.6V) T1 – (off), T2 – (off)
vI  0.6V T1 – (on), T2 – (off)
vO  vI  vBE1  vI  0.6V
iO  0
iC1  iO
iC 2  0
vo max  VCC  VCEsat  VCC
vO  0V
vI  0.6V T1 –(off), T2 –(on)
vO  vI  vBE 2  vI  0.6V
iO  0
iC 2  iO iC1  0
vo min  VCC  VCE1sat  VCC
Voltage
transfer
characteristic
crossover
distortions
Waveforms
Assume the signal magnitude sufficiently high to
neglect the crossover distortions.
vo (t )  Vˆo sin t
Vˆo  VCC
Powers. Efficiency
T 2
T
Vˆo sin t
1
1

PPS   VCC iPS (t )dt   VCC
dt
T0
T 0
RL
T 2
ˆ
ˆ
V
V
1
V
V
1
2


CC
o
CC o
PPS 
PPS 
sin
t
dt
 RL
T RL 0
T
PO

PPS
PPS 
PPS

PPS
PO  VOrms I Orms
2 VCCVˆo

 RL
VˆO2

2 RL
Average efficiency:
For Vˆo  VCC
For VˆO  VCC
PPS max
PO max
2
2 VCC

 RL
2
VCC

2 RL
PO
VˆO2  RL
 VˆO



ˆ
PPS
2 RL 2 VCCVO
4 VCC
Maximum average efficiency: for VˆO  VCC
 max 

4
 78.5%
The amplifying transistor
PT 
OPTIONAL
1
1 VCCVˆo Vˆo2
PT  PPS  PO  

2
 RL
4 RL
2
ˆ2
ˆ2
V
V
V
1
CC
VˆO  VCC PT 
 CC  0.068 CC
 RL 4 RL
RL
1
PPS  PO 
2
Maximum efficiency:
What is the maximum average power dissipated by a
transistor depending on the output voltage magnitude?
dPT
0
ˆ
dV

V̂o 
O
Maximum average power:
Maximum instantaneous
power:
iC max
2

VCC  0.64VCC
V̂o  0.64VCC
Vˆo  VCC
VCC

RL
PT max
2
2
VCC
1 VCC
 2
 0.1
 RL
RL
pT max
2
2
VCC
1 VCC

 0.25
4 RL
RL
vCE max  2VCC
Class AB
- Crossover distortion, specific to class B amplifier can be
virtually eliminated
Class AB Amplifier. Basic Circuit
• biasing the complementary output
transistors at a small output current
VBias≈ 0.6V
I  ISe
vI - positive
vO t   vI t   VBias  VBEn  vI t 
vI - negative
vO t   vI t   VBias  VBEp  vI t 
VBias
VT
Voltage Transfer
Characteristic (VTC)
Solutions to
generate VBias
???
Biasing using Diodes
R should allow the currents in the diodes
and in the bases of the transistors, even
for maximum output current
Vˆ  8V β=50
V = +15V
CC
O
RL=100Ω, ID1=1.5mA
R=?, IR=? (each R)
T is the driver
transistor from previous
amplifier stage
Biasing using VBE Multiplier
VBB

R1 
VBE1
 1 
 R2 
Short-Circuit Protection
Protection against the effect of
short-circuiting the output
• Effective in ensuring device
safety
• Disadvantage: under normal
operation up to 0.6V appears
across each protection resistor.
The voltage swing will be
reduced by that much, in each
direction
• Negative feedback, protection
of the output transistor against
thermal runaway
iOmax
VBE 2 0,6V


REn
REn
Illustration
I Bias
0.6  0.6  0.6

 6mA
0.1
P - adjusts the biasing
voltage between VBE and
3.5VBE
C - in the bases of Tn
and Tp it is the same
variable input voltage
iOmax=0.6A
Use of Compound Transistors
with Higher Current Gain
• Necessary for high output currents
The Darlington npn configuration
The compound pnp configuration preferred in IC
Exercise
± VCC = ± 15V
RL1  500Ω
RL 2  50Ω
a) What is the expression vO(vi) for vi[-10V;10V]? Plot the vO(vi) VTC.
What is the operating class of this aplifier? What is the maximum
theoretical average efficiency?
b) Plot vO(t) for: (1) vi(t)= 0.2sint [V]; (2) vi(t)= 10sint [V].
c) For vi(t)= 10sint [V], vO(t) can be well approximated by a sinewave
(negleting crossover distorsions). Compute: the average output power,
the average power consumption, and the average efficiency of the stage.
Plot the collector-emiter voltage and colector current for T1 and T2.
d) Propose a solution to eliminate crossover distorsion