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POWER
AMPLIFIER
Concept
of Power Amplifier
Power BJTs
Power MOSFETs
Classes of Power Amplifier
1
CONCEPT OF POWER AMPLIFIER

To deliver a large current to a small load resistance
e.g. audio speaker;

To deliver a large voltage to a large load resistance
e.g. switching power supply;

To provide a 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
TRANSISTOR LIMITATIONS

maximum rated current,

maximum rated voltage,

maximum rated power, and

maximum allowed temperature.
3
POWER BJTS

Comparison of the characteristics and maximum ratings of a small-signal
and power BJT
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
300
0.8
1
Parameter

fT (MHz)
4
Typical dc beta characteristics (hFE
versus lc) for 2N3055
5
POWER BJTS

Current gain is smaller in power BJT.

The gain depends on IC and temperature may be
related to the following:





maximum current that connecting wires can handle
at which current gain falls below a stated value
current which leads to maximum power dissipation.
maximum voltage limitation associated with avalanche
breakdown in reverse-biased collector-base junction.
second breakdown in BJT operating at high voltage and
current.
6
VCE(sus) =115 volt at which these
curve merge and the minimum
voltage necessary to sustain the
transistor in breakdown.
The breakdown voltage,
VCE0 ~130 volt when the
base terminal is open
circuited, IB=0
7

Instantaneous power dissipation
pQ  vCE iC  vBE iB  vCE iC

The average power over one cycle
1
PQ 
T


T
0
vCE iC dt
The maximum rated power,
PT  VCE I C
8
•The Safe Operating Area (SOA) is bounded by IC(max); VCE(sus)
and PT
• The SOA of a bipolar transistor plotted on: (a) linear
scales and (b) logarithmic scales
9
Example

The parameters of the common emitter circuit are RL=8Ω,
VCC=24 volt. Determine the required current, voltage and
power ratings of the a power BJT.
10
Power MOSFETs

Characteristic of two power MOSFETs
Power
MOSFET
2N6757
Power
MOSFET
2N6792
150
400
ID (max) (A) (at T = 25C)
8
2
PD (max) (W)
75
20
Parameter
VDS (max) (V)
11
PERFORMANCE CHARACTERISTIC OF
POWER MOSFETs

Faster switching times

no second breakdown.

Stable gain and response over wide temperature range.
12
Classes of Power Amplifiers
Class A - The transistor
conducts during the whole
cycle of sinusoidal input
signal
Class AB - The transistor
conducts for slightly more
than half a cycle of input
signal
Class B - The transistor
conducts during one-half
cycle of input signal
Class C - The transistor
conducts for less than half a
cycle of input signal
13
Class A Operation
Configuration : No inductor @ transformer are used
(a) Common-emitter amplifier,
(b)dc load line (the Q point is at centre of the load line)
(c) instantaneous power dissipation versus time in the transistor
14

Instantaneous power dissipation in transistor is
pQ  vCE iC

For sinusoidal input signal, the collector current;
iC  I CQ  I p sin t

And the collector emitter voltage;
vCE 

For maximum possible swing;
I p  I CQ

VCC
 V p sin t
2
VCC
Vp 
2
and
Therefore, the instantaneous power dissipation in transistor
pQ 
VCC I CQ
2
1  sin
2
t 
15

The power conversion efficiency


signalload power (PL )
supplypower (PS )
The average ac power delivered to the load
VCC I CQ
1
 1  VCC 
PL (max)   VP I P   
I CQ  
4
2
 2  2 

The average power supply by the VCC source
PS  VCC I CQ

The maximum attainable conversion efficiency
 (max) 
1
4
VCC I CQ
VCC I CQ
1
  0.25  25%
4
16
Example



The common source circuit parameters are VDD=10V, RD=5kΩ and the
transistor parameters are Kn=1mA/V2, VTN=1V and =0.
Assume the output voltage swing is limited to the range between the
transition point and vDS=9V to minimize nonlinear distortion.
Calculate the actual efficiency of a class A output stage.
17
Exercise




The Q-point of common source circuit is VDSQ=4V
Find IDQ
Determine the max peak to peak amplitude of a symmetrical sinusoidal
output voltage if the min value of instantaneous drain current must be
no less than 0.1IDQ and the min value of instantaneous drain source
voltage must be no less than vDS=1.5V.
Calculate the power conversion efficiency where the signal power is the
power delivered to RL.
Ans:
60mA,
5V,
31.25mW, 5.2%
18