Download Lecture 1 Power Amplifiers

DMT 231 / 3
ELECTRONICS II
Lecture I
Power Amplifiers
(Class A & Class B)
POWER AMPLIFIER – Classification
They are grouped together based on their Q-points on the
DC load line.
POWER AMPLIFIER – Classification
In class-A; the transistor conducts during the
whole cycle of sinusoidal input signal
POWER AMPLIFIER – Classification
In class-B; the transistor conducts during
one-half cycle of input signal
POWER AMPLIFIER – Classification
In class-AB; the transistor conducts for slightly
more than half a cycle of input signal
POWER AMPLIFIER – Classification
In class-C; the transistor conducts for less
than half a cycle of input signal
POWER AMPLIFIER – Class-A Operation
For maximum swing ( +ve and –ve), transistor is biased such
that the Q point is at centre of the load line.
The transistor conducts for a full cycle of the input signal
POWER AMPLIFIER – Class-A Operation
Instantaneous power dissipation in transistor is;
pQ  vCE iC
For sinusoidal input signal;
iC  I CQ  I p sin t
And;
vCE
VCC

 V p sin t
2
POWER AMPLIFIER – Class-A Operation
For maximum possible
swing;
I p  I CQ
And;
VCC
Vp 
2
Therefore;
pQ 
VCC I CQ
2
1  sin
2
t 
POWER AMPLIFIER – Class-A Operation
When the input signal = 0, the transistor must be capable of
handling a continuous power of;
VCC I CQ
2
Efficiency;
PL

PS
PL = average ac power to the load
PS = average power supplied by the source (VCC)
POWER AMPLIFIER – Class-A Operation
For maximum possible swing;
1
1  VCC
PL  V p I p  
2
2 2
VCC I CQ

 I CQ 
4

Power supplied by the source;
PS  VCC I CQ
The efficiency;

VCC I CQ
4
Maximum theoretical
efficiency of class A
amplifier is therefore
25%
 VCC I CQ  0.25
POWER AMPLIFIER – Class-B Operation
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
Both devices are off when the input is zero
(See Figure)
POWER AMPLIFIER – Class-B Operation
When vI = 0, both A and B are
OFF and therefore vO = 0.
The complementary pair
POWER AMPLIFIER – Class-B Operation
When vI > 0, A is ON and B is
OFF and vO > 0.
POWER AMPLIFIER – Class-B Operation
When vI < 0, A is OFF and B is
ON and vO < 0.
POWER AMPLIFIER – Class-B Operation
The transfer characteristic of the
complementary pair
POWER AMPLIFIER – Class-B Operation
Approximate Class-B : Complementary push-pull circuit
Assuming ideal transistor;
when vI = 0;
both Qn & Qp are off and
vO =0
when vI > 0;
Qn conducts & Qp is off
and vO  vI
when vI < 0;
Qp conducts & Qn is off
and again vO  vI.
POWER AMPLIFIER – Class-B Operation
Crossover Distortion
Assuming cut-in
voltage of transistor is
0.6 V, vO = 0 for a
range 0.6 V < vI < 0.6
V.
The transfer
characteristic
becomes non-linear
(See Figure).
The range where both
transistors are
simultaneously off
known as the dead
band.
POWER AMPLIFIER – Class-B Operation
Crossover Distortion
The output will be distorted – crossover distortion (Figure)
POWER AMPLIFIER – Class-B Operation
Crossover Distortion
Crossover distortion can be eliminated by biasing the
transistor with small quiescent current – class-AB
POWER AMPLIFIER – Class-B Operation
Idealized Power Efficiency
VCC  V   V 
POWER AMPLIFIER – Class-B Operation
Idealized Power Efficiency
POWER AMPLIFIER – Class-B Operation
Idealized Power Efficiency
The Q-point is at
the cutoff point of
both transistors
(zero collector
current)
POWER AMPLIFIER – Class-B Operation
Idealized Power Efficiency
The output voltage of
the idealized class-B is;
vO  V p sin t
The maximum possible
value of Vp is VCC
The instantaneous
power dissipation in Qn
is;
pQn  vCEniCn
POWER AMPLIFIER – Class-B Operation
Idealized Power Efficiency
The collector current is;
iCn 
Vp
RL
sin t
for 0  t  
and iCn  0
for
  t  2
POWER AMPLIFIER – Class-B Operation
Idealized Power Efficiency
The collector-emitter
voltage is;
vCEn  VCC  V p sin t
Therefore the instantaneous
power dissipation in Qn is;
pQn  vCEniCn
 VCC
 Vp

 V p sin t  sin t 
 RL

for 0  t  
POWER AMPLIFIER – Class-B Operation
Idealized Power Efficiency
And;
pQn  0 for   t  2
Therefore, the average
power dissipation in Qn
is;
PQn 
VCCV p
RL

Vp
2
4 RL
POWER AMPLIFIER – Class-B Operation
Idealized Power Efficiency
Plotting PQn versus Vp as given by the equation;
PQn 
VCCV p
RL

Vp
2
4 RL
gives us the following curve:
POWER AMPLIFIER – Class-B Operation
Idealized Power Efficiency
Because of symmetry;
Differentiating
PQn 
PQn  PQn
VCCV p
RL

Vp
2
4 RL
with respect to Vp, for maximum PQn gives us;
2
VCC
PQn max   2
 RL
which occurs when
Vp 
2VCC

POWER AMPLIFIER – Class-B Operation
Idealized Power Efficiency
Since each power source supplies half sinewave of
current, the average value is;
IS 
Vp
RL
The total power supplied by the two sources is;
 Vp 

PS  2VCC I S  2VCC 
 RL 
POWER AMPLIFIER – Class-B Operation
Idealized Power Efficiency
The power delivered to the load is;
PL 
2
O  rms 
V
RL
The efficiency is;

V

p
/ 2
RL

2

V
2
p
2 RL
PL V p
 
PS 4VCC
Maximum efficiency occurs when
V p  VCC
POWER AMPLIFIER – Class-B Operation
Idealized Power Efficiency
Under this condition;


4
 0.785
Maximum theoretical efficiency of class B
amplifier is therefore 78.5%
The efficiency obtained in practice is less than the maximum
value of other circuit losses and because the peak output
voltage must remain less than VCC to avoid transistor
saturation which can cause distortion in the output signal.
POWER AMPLIFIER – Class-B Operation
Idealized Power Efficiency
The maximum transistor power dissipation occurs at;
Vp 
2VCC

Substituting in the expression for efficiency;

V p
4VCC
  2VCC  1



4VCC    2
50%