Transcript power amplifier

POWER AMPLIFIER
CHAPTER 4
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Outcome of Chapter 4
 Ability to perform simple DESIGN and
EVALUATE A, B and AB classes of BJT and
FET amplifiers, in terms of their frequency
response, equivalent circuit, termal
management (power dissipation) and gain.
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Outline
 Introduction
 Concept of Power Amplifier
 Power Amplifier Classification
 BJTs/ MOSFETs Power Amplifier
 Class A Power Amplifier
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Introduction
 Power amplifiers are used to deliver a relatively high amount of
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power, usually to a low resistance load.
Typical load values range from 300W (for transmission antennas)
to 8W (for audio speaker).
Although these load values do not cover every possibility, they do
illustrate the fact that power amplifiers usually drive lowresistance loads.
Typical output power rating of a power amplifier will be 1W or
higher.
Ideal power amplifier will deliver 100% of the power it draws
from the supply to load. In practice, this can never occur.
The reason for this is the fact that the components in the amplifier
will all dissipate some of the power that is being drawn form the
supply.
Concept of Power Amplifier
 Provide sufficient power to an output load to
drive other power device.
 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
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Power Amplifier Power Dissipation
VCC
The total amount of power
being dissipated by the
amplifier, Ptot , is
I CC
I1
Ptot = P1 + P2 + PC + PT + PE
I CQ
P1 = I12R1
The difference between this
total value and the total
power being drawn from the
supply is the power that
P2 = I22R2
actually goes to the load –
i.e. output power.
R1
PC = I2CQR C
PT = I2TQ R T
I EQ
R2
I2
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RC
RE
PE = I2EQ R E
Power Amplifier Efficiency 
 A figure of merit for the power amplifier is its efficiency,  .
 Efficiency (  ) of an amplifier is defined as the ratio of ac output
power (power delivered to load) to dc input power .
 By formula :  
ac output power
P (ac)
 100%  o
 100%
dc input power
Pi (dc)
 As we will see, certain amplifier configurations have much higher
efficiency ratings than others.
 This is primary consideration when deciding which type of power
amplifier to use for a specific application.
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Power Amplifiers Classification
Class A - The transistor
conducts during the whole
cycle of sinusoidal input
signal
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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
Efficiency Ratings
 The maximum theoretical efficiency ratings of class-A,
B, and C amplifiers are:
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Amplifier
Maximum Theoretical
Efficiency, max
Class A
25%
Class B
78.5%
Class C
99%
BJT Power Amplifier
 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)
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Typical dc beta characteristics (hFE
versus Ic) for 2N3055
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BJTs Power Amplifier

Current gain is smaller in power amplifier BJT.

The gain depends on IC and temperature may be related
to the following:
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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.
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
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 Instantaneous power dissipation
pQ  vCE iC  vBE iB  vCE iC
 The average power over one cycle
T
1
PQ 
T

0
vCE iC dt
 The maximum rated power,
PT  VCE I C
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MOSFETs Power Amplifier
 Characteristic of two power MOSFETs
Power
MOSFET
2N6757
Power
MOSFET
2N6792
150
400
ID (max) (A) (at T = 25C)
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2
PD (max) (W)
75
20
Parameter
VDS (max) (V)
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Performance Characteristic of MOSFETs
Power Amplifier
 Faster switching times
 no second breakdown.
 Stable gain and response over wide temperature range.
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Class A Amplifier
vin
Av
vout
 output waveform  same shape  input waveform +  phase
shift.
 The collector current is nonzero 100% of the time.
 inefficient, since even with zero input signal, ICQ is nonzero
(i.e. transistor dissipates power in the rest, or quiescent,
condition)
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Basic Operation
Common-emitter (voltage-divider) configuration (RC-coupled amplifier)
+VCC
I CC
I1
I CQ
R1
RC
RL
v in
R2
RE
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Typical Characteristic Curves for
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
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DC Input Power
+VCC
The total dc power, Pi(dc) , that an
amplifier draws from the power supply :
I CC
I1
I CQ
R1
RC
Pi (dc)  VCC I CC
RL
I CC  I CQ  I 1
I CC  I CQ
( I CQ  I 1 )
v in
R2
RE
Pi ( dc )  VCC I CQ
Note that this equation is valid for most amplifier power analyses. We can rewrite
for the above equation for the ideal amplifier as
Pi (dc)  2VCEQ I CQ
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AC Output Power
AC output (or load) power,
Po(ac)
2
Po ( ac)  ic ( rms) vo ( rms)
ic
vo
vo ( rms)

RL
Above equations can be used to
calculate the maximum
possible value of ac load power.
HOW??
vce
vin
rC
RC//RL
R1//R2
Disadvantage of using class-A amplifiers is the fact that
their efficiency ratings are so low, max  25% .
Why?? A majority of the power that is drawn from the supply
by a class-A amplifier is used up by the amplifier itself.
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IC(sat) = VCC/(RC+RE)
IC(sat) = ICQ + (VCEQ/rC)
DC Load Line
ac load line
IC
IC
(mA)
VCE(off) = VCC
VCE(off) = VCEQ + ICQrC
VCE
VPP2
 VCEQ  I CQ  1
Po ( ac)  

  VCEQ I CQ 
8 RL
 2  2  2
ac load line
IC
VCE
Q - point
dc load line

VCE
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Po ( ac )
Pi ( dc )
1
VCEQ I CQ
2
 100% 
 100%  25%
2VCEQ I CQ
Limitation
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Example
+VCC = 20V
Calculate the input power [Pi(dc)], output power
[Po(ac)], and efficiency [] of the amplifier circuit for
an input voltage that results in a base current of
10mA peak.
VCC  VBE 20V  0.7V

 19.3mA
RB
1k
ICQ  I B  25(19.3mA)  482.5mA  0.48 A
IBQ 
VCEQ  VCC  ICRC  20V  (0.48 A)( 20)  10.4V
V
20V
I c ( sat)  CC 
 1000mA  1A
RC
20
VCE ( cutoff )  VCC  20V
IC ( peak)  Ib ( peak)  25(10mA peak )  250mA peak
Po ( ac) 
Pi ( dc)

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I C2 ( peak )
250  10 A)

3
2
RC
(20)  0.625W
2
2
 VCC I CQ  (20V )(0.48 A)  9.6W
Po ( ac)
Pi ( dc)
 100%  6.5%
Vi
RB
1k
IC
RC
20
Vo
  25
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.
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Exercise
The Q-point of common source circuit
is VDSQ=4V
a) Find IDQ
b) 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.
c) Calculate the power conversion
efficiency where the signal power
is the power delivered to RL.
Ans:
60mA,
5V,
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31.25mW, 5.2%
Design of Class A C-E Amplifier
 To find R1, R2, RE, RC use the DC analysis and design
formula;
VCC
VCC
RE
VEQ 
; VCQ 
; R2 
10
2
10
I C  I CQ  given in data sheet.
If have R L ; I C  10 I L
 To find Zi, Zo, Av, Ai use AC analysis (without loading effect)
 To find Zi, Zo, Avs, Ai use AC analysis (with loading effect if
have Ri and RL
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Example & Exercise
 Will be given in our class.
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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
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 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
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Complimentary Push-Pull Circuit
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CROSSOVER DISTORTION
DEAD BAND
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The Ideal Class B
i Cn
iCn 
Vp
RL
sin t
iCn 
Vp
RL
sin t
vo  V p sin t
i Cp
vo  VP sin t
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• 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

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 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
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 The average power delivered to the load is
PL 
 Power source supplies half sinewave of current,
the average value is;
2
V
1 p
2 RL
IS 
Vp
RL
 Vp 

PS   Ps   VCC I S  VCC 
 RL 
 The total power supplied by the two sources is
 Vp 

PS  2VCC I S  2VCC 
 RL 
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 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%
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Class AB Power Amplifier
Small quiescent bias
on each output
transistor to
eliminate crossover
distortion
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Class C Power Amplifier
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Class AB Voltage Transfer Curve
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Collector Currents & Output Current
iCn  iL  iCp
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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.
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Inductively Coupled Amplifier
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 The maximum possible average signal power delivered to the
load
PL (max) 
1
1 VCC
2
I CQ RL 
2
2 RL
2
 The possible average signal power supply by VCC
PS  VCC I CQ
VCC

RL
2
 The maximum possible power conversion efficiency
2
1 VCC
2 RL
L
P (max)
 (max) 

PS
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VCC 2
RL
1
  0.5  50%
2
Transformer Coupled Amplifier
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•
The theoretical maximum efficiency of a basic RCcoupled class-A amplifier is limited to 25%.
•
In practical circuit, the efficiency is less than 25%.
•
Used for output power of about 1 W only.
•
Transformer coupling can increase the maximum
efficiency to 50%
•
Disadvantage of transformer coupling – expensive &
bulky.
Neglecting transformer resistance and assuming RE is small;
VCEQ  VCC
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For ideal transformer;
iL  aiC
and
v2 
v1
a

N1 
 a  turn ratio 

N2 

iC
v2 v1 / a
RL  
iL
aiC
v1 1
RL   2
iC a
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v1
RL '
iC
RL ' a 2 RL
Turn ratio is designed for
maximum symmetrical
swing, hence;
2VCC VCC
RL ' 

 a 2 RL
2I CQ I CQ
The maximum average power delivered to load equals
maximum average power delivered to the primary of the
transformer
1
PL max )  VCC I CQ
2
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(VCC and ICQ are maximum
possible amplitudes of signal)
The average power supplied by the VCC source is;
PS  VCC I CQ
The maximum possible efficiency is;
PL max )

 0.5  50%
PS
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