POWER AMPLIFIERS
Part I
Classification of power
amplifiers
1
POWER AMPLIFIERS
Required
•
To deliver a large current to a small load resistance e.g.
audio speaker; or to deliver a large voltage to a large
load resistance e.g. switching power supply;
•
To be of low output resistance in order to avoid loss of
gain and to maintain linearity (to minimize harmonic
distortion)
•
To deliver power to the load efficiently
2
POWER TRANSISTORS - BJT
Transistor limitations
1
maximum rated current,
2
maximum rated voltage,
3
maximum rated power, and
4
maximum allowed temperature.
3
Parameter
Small-signal BJT
(2N2222A)
Power BJT
(2N3055)
Power BJT
(2N6078)
VCE (max) (V)
40
60
250
IC (max) (A)
0.8
15
7
PD (max) (W)
1.2
115
45

35 – 100
5 – 20
12 – 70
fT (MHz)
300
0.8
1
4
5
Current gain is smaller in power BJT. The gain depends
on IC and temperature may be related to the following:
1. maximum current that connecting wires can handle
2. at which current gain falls below a stated value
3. current which leads to maximum power dissipation.
4. maximum voltage limitation associated with avalanche
breakdown in reverse-biased collector-base junction.
5. second breakdown in BJT operating at high voltage and
current.
6
Instantaneous power dissipation
pQ  vCE iC  vBE iB
The second term is small, hence
pQ  vCE iC
The average power over one cycle
1
PQ 
T

T
0
vCE iC dt
7
If collector current and collector-emitter voltage are dc
quantities, the maximum rated power, PT
PT  VCE I C
The power handling ability of a BJT is limited by two factors,
i.e. junction temperature and second breakdown. SOA must
be observed, i.e. do not exceed BJT power dissipation.
The safe operating area (SOA) is bounded by IC(max); VCE(sus)
and PT (Figure)
8
SOA of BJT (linear scale)
9
SOA of BJT (logarithmic scale)
10
11
Top view
Cross-section view
Physical structur of BJT
12
POWER TRANSISTORS - MOSFETs
MOSFET is superior over BJT:
•
Faster switching times
•
There is no second breakdown.
•
Stable gain and response over wide
temperature range.
•
May be driven directly by TTL.
•
Immune to thermal runaway
13
For MOSFET 2N6757, the maximum drain current, ID, is 8A
and breakdown voltage, VDS (max) is 150V.
Parameter
Power
MOSFET
2N6757
Power
MOSFET
2N6792
VDS (max) (V)
150
400
ID (max) (A) (at T = 25C)
8
2
PD (max) (W)
75
20
MOSFET’s must also be operated within SOA
14
The SOA for 2N6757 is shown in the following figure
15
Cross section of
VMOS device
Cross section of
DMOS device
HEXFET
structure
Structure of MOSFET
16
Classes of Amplifiers
They are grouped together based on their Q-points on the DC
load line.
17
In class-A; the transistor conducts during the
whole cycle of sinusoidal input signal
18
In class-B; the transistor conducts during
one-half cycle of input signal
19
In class-AB; the transistor conducts for slightly
more than half a cycle of input signal
20
In class-C; the transistor conducts for less
than half a cycle of input signal
21
Cass–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
22
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
For maximum possible swing;
I p  I CQ
and
VCC
Vp 
2
23
Therefore;
pQ 
VCC I CQ
2
1  sin
2
t 
(See graphical
representation)
24
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)
25
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
26
Cass–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)
27
28
Complementary push-pull circuit
Assuming ideal
transistor;
when vI = 0;
both Qn & Qp are off;
An approximate class-B
circuit comprising
complementary BJT pair
working in push-pull
configuration.
when vI > 0;
Qn conducts & Qp is off;
when vI < 0;
Qp conducts & Qn is off
29
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
The output will be distorted – crossover distortion (See
Figure)
Crossover distortion can be eliminated by biasing the
transistor with small quiescent current – class-AB
30
Dead band
31
Theoretical maximum efficiency of class-B amplifiers
vCEn  VCC  V p sin t
vO  V p sin t
32
Q n : on
Q p : off
Q n : off
Q p : on
i Cn
i Cn
i Cp
i Cp
iL
33
vO  V p sin t
i Cn
0


2
Maximum possible value of Vp
is VCC.
vCEn  VCC  V p sin t
iCn 
Vp
RL
sin t for 0  t  
and
i Cp
iCn  0 for   t  2
34
The instantaneous power in Qn is;
pQn  vCEniCn
 VCC
 Vp

 V p sin t  sin t 
 RL

for 0 < t < 
and
pQn  0
for  < t < 2
The average power in Qn is;
PQn 
VCCV p
RL

Vp
2
4 RL
35
PQn  PQn
(symmetry)
Differentiating for maximum PQn with respect to Vp
gives us;
2
VCC
PQn max   2
 RL
2VCC 

 when V p 

 

Since each power source supplies half sinewave of
current, the average value is;
IS 
Vp
RL
36
The total power supplied by the
two sources is;
 Vp 

PS  2VCC I S  2VCC 
 RL 
The power delivered to the load is;
PL 
2
O  rms 
V
RL
The efficiency is;

V

p / 2
RL

2

V p2
2 RL
PL V p
 
PS 4VCC
37
Maximum efficiency occures when
V p  VCC
Under this condition;


4
 0.785
Maximum theoretical efficiency
of class B amplifier is therefore
78.5%
38
Cass–AB operation
Small quiescent bias on each
output transistor to eliminate
crossover distortion
Small ICQ flows through
each transistor in the
absent of input signal
39
40
Cass–C operation
Transistor conducts
for less than half a
cycle of input signal
•
•
•
Tuned circuit is required.
Used for RF amplifier.
Efficiency > 78.5%
B – E junction is reverse-biased to
obtain Q-point beyond cut-off.
41