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Circuits and Analog Electronics
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
6.2 Single-Stage BJT Amplifiers
6.3 Frequency Response
6.4 Power Amplifiers
References: Floyd-Ch-3, 5, 6; Gao-Ch7;
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
Key Words:
Construction of BJT
BJT in Active Mode
BJT DC Model and DC Analysis
C-E Circuits I-V Characteristics
DC Load Line and Quiescent Operation Point
BJT AC Small-Signal Model
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
This lecture will spend some time on understanding
how the bipolar junction transistor (BJT) works based
on what we have known about PN junctions. One way
to look at a BJT transistor is two back-to-back diodes,
but it has very different characteristics.
Once we understand how the BJT device operates, we
will take a look at the different circuits (amplifiers)
which we can build.
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
Construction of Bipolar junction transistors
Emitter-base
junction
Base region
(very narrow)
Emitter region
Collector
Collector region
Emitter
Base
Collector-base
junction
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
Construction of Bipolar junction transistors
NPN BJT shown
• 3 terminals: emitter, base, and collector
• 2 junctions: emitter-base junction (EBJ) and collector-base
junction (CBJ)
– These junctions have capacitance (high-frequency model)
• BJTs are not symmetric devices
– doping and physical dimensions are different for emitter
and collector
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
Standard bipolar junction transistor symbols
Depending on the biasing across each of the junctions, different
modes of operation are obtained – cutoff, active and saturation
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
BJT in Active Mode
Two external voltage sources set the bias conditions for active
mode
– EBJ is forward biased and CBJ is reverse biased
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
BJT in Active Mode
IE＝IEN＋IEP IEN
Forward bias of EBJ injects electrons from emitter into base
(small number of holes injected from base into emitter)
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
BJT in Active Mode
IB ＝IBN＋ IEP
• Most electrons shoot through the base into the collector
across the reverse bias junction
• Some electrons recombine with majority carrier in (P-type)
base region
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
BJT in Active Mode
IC = ICN + ICBO
Electrons that diffuse across the base to the CBJ junction are
swept across the CBJ depletion region to the collector.
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
BJT in Active Mode
IE＝IEN＋IEP IEN
IC = ICN + ICBO
IE = IB + IC
Let ICN＝IE
IB＝IBN＋IEP
IC

---common-base current gain
IE
IC (1－) = IB + ICBO
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
BJT in Active Mode
IE＝IEN＋IEP IEN

IC
IE
IB＝IBN＋IEP
IC=ICN+ICBO
IE=IB+IC
IC (1－)= IB+ICBO
Let
Beta:  

IC
IB

1
I C   I B  (1   ) I CBO
---common-emitter current gain
 I E  I C  I B  (1   ) I B

 I C   I B  I CEO   I B
 I  I
E
 C
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
BJT Equivalent Circuits

＋
iB
iC
vBE
－
IB
IC
＋

iE
－

＋
VCE
VBE＝Von
IB
－

vCE

BJT DC model

 iB

＋


IE

－
•Use a simple constant-VBE
model
– Assume VBE = 0.7V
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
BJT DC Analysis
• Make sure the BJT current equations and
region of operation match
VBE > 0,
VBC < 0,  VE < VB <VC
• Utilize the relationships (β and α) between
collector, base, and emitter currents to solve
for all currents
 I E  I C  I B  (1   ) I B

I C   I B
 I  I
E
 C
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
C-E Circuits I-V Characteristics
Base-emitter Characteristic(Input characteristic)
i B  f (v BE )
vCE  C
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
C-E Circuits I-V Characteristics
Collector characteristic (output characteristic)
iC  f (VCE )
iB C
iB = 40 μA
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
C-E Circuits I-V Characteristics
Collector characteristic (output characteristic)
iC  f (VCE )
iB C
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
C-E Circuits I-V Characteristics
Collector characteristic
Saturation
Saturation occurs when the
supply voltage, VCC, is
across the total resistance
of the collector circuit, RC.
IC(sat) = VCC/RC
Vsat
Once the base current is high enough to produce saturation, further increases in
base current have no effect on the collector current and the relationship IC = IB is
no longer valid. When VCE reaches its saturation value, VCE(sat), the base-collector
junction becomes forward-biased.
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
C-E Circuits I-V Characteristics
Collector characteristic
When IB = 0, the transistor is in
cutoff and there is essentially no
collector current except for a
very tiny amount of collector
leakage current, ICEO, which can
usually be neglected. IC  0.
Cutoff
In cutoff both the base-emitter
and the base-collector junctions
are reverse-biased.
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
C-E Circuits I-V Characteristics
Collector characteristic
linearity
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
Discussion of an amplification effect
vi  Ri  iB
vo  RL  iC
vCE
vBE
Ri 
 RL 
iB
iC
With iB  iC
vi  vo
E.g. for common-base configuration transistor:
vo
Av 
 50 ~ 300
vi
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
DC Load Line and Quiescent Operation Point
Q-point
VCC
ICQ
.
Q
VCEQ
Base-emitter loop: I B 
DC load line
Collector-emitter loop:
VCC  VBE VCC

 40( A)
Rb
Rb
vCE  VCC  iC RC  10  iC  4k
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
BJT AC Small-Signal Model

＋
iB
iC
vBE
－
 iB


iE


＋

＋
vCE
vbe
－
－
ib
rbe
ic
 ib

＋
vce
－

ie

rbe  300  (1   )
26(m V )
I E (m A)
• We can create an equivalent circuit to model the transistor for small signals
– Note that this only applies for small signals (vbe < VT)
• We can represent
 the small-signal model for the transistor as a voltage controlled
current source (i E  I S e vBE / VT ) or a current-controlled current source (ic = ib).
 signals, approximate exponential curve with a linear line.
• For small enough
v BE / VT
e
I


I


i
C
S
E
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
VBE  0.7V
BJT fundamentals:
I E  IC  I B  1   I B  IC
IC   I B
Summary for three types of diodes:
Input
Output
Functions
C-C
C-E
C-B
IB
IB
IB
IE
IC
IC
Zat  Zin
Zat  Zin Zat  Zin
Vat  Vin Vat  Vin
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Key Words:
Common-Emitter Amplifier
Graphical Analysis
Small-Signal Models Analysis
Common-Collector Amplifier
Common-Base Amplifier
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
C-E Amplifiers
To operate as an amplifier, the BJT must be biased to operate in active
mode and then superimpose a small voltage signal vbe to the base.
DC + small signal
coupling capacitor
(only passes ac signals)
C2
iC iB
RC
C1
vi 
 vBE   iB  

 ic 
vCE 
vo
vi  iB
iB  iC
iC  vO
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
C-E Amplifiers
+
Vi
Vi
Vi
Vi
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
C-E Amplifiers
Apply a small signal
input voltage and see ib
iB  I B  ib
vBE=vi+VBE
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
C-E Amplifiers
See how ib translates into vce.
• vi = 0  IB、IC、VCE
vi  0
iC=ic+IC
i B  I B  ib


iC  I C  iC 
vCE  VCE  vce 
• VoM  ViM
f ( o)  f (i )
• vo out of phase with vi
vCE=vce+VCE
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
C-E Amplifiers
Considering VC (all the capaertors are replaced
by open circuits)
Considering Vi (all the capaertors are replaced
by short circuits)
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
C-E Amplifiers
Considering VC (all the capaertors are replaced
by open circuits)
Considering Vi (all the capaertors are replaced
by short circuits)
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Graphical Analysis
• Can be useful to understand the operation of BJT
circuits.
• First, establish DC conditions by finding IB (or VBE)
• Second, figure out the DC operating point for IC
VCC
Can get a feel for whether the BJT will stay in active region of operation
– What happens if RC is larger or smaller?
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Graphical Analysis
vce  ic ( RC // RL )  ic RL'
VCC '  VCEQ  I CQ RL '
VCC
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Graphical Analysis
Q-point is centered on the ac load line:
VCC
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Graphical Analysis
Q-point closer to cutoff:
VCC
Clipped at cutoff
(cutoff distortion)
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Graphical Analysis
Q-point closer to saturation:
VCC
Clipped at cutoff
(saturation distortion)
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Graphical Analysis
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Small-Signal Models Analysis
Steps for using small-signal models
1. Determine the DC operating point of the BJT
－ in particular, the collector current
2. Calculate small-signal model parameters: rbe
3. Eliminate DC sources
– replace voltage sources with shorts and current
sources with open circuits
4. Replace BJT with equivalent small-signal models
5. Analysis
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Small-Signal Models Analysis
Example 1
VC  (I B  I C )R  I B R b  VBE  I E R e
VC  VBE
 IB 
Rb  (1   )(R  Re )
IC ≈ βIB,
IE = IC + IB = (1+β)IB
VCE  VC  I C RC  I E ( R  Re )
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Small-Signal Models Analysis
Example 1
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Small-Signal Models Analysis
Example 2
VB 
Rb 2
VCC
Rb1  Rb 2
IC  I E 
vs
IB 
VB  VBE
V B/ Re
Re
IC

VCE  VCC  I C ( R C  R e )
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Small-Signal Models Analysis
There are three basic configurations for single-stage
BJT amplifiers:
– Common-Emitter
– Common-Base
– Common-Collector
e
N P N
b
VBB
e
c
c
b
RC
VCC
(a)
VE  VB  VC
VBB
N
P
N
e
Rc
b
VCC
(b)
VE  VB  VC
VBB
N
P
N
Re
c
VCC
(c)
VE  VB  VC
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Common-Collector Amplifier
VCC  I B Rb  VBE  I E Re  I B Rb  VBE  (1   ) I B Re
IB 
VCC  VBE
VCC

Rb  (1   ) Re Rb  1   Re
I C  I B
VCC  VCE  I E Re  VCE  I C Re
VCE  VCC  I C Re
(a) 共集电极电路
Note : Vo is slightly less than Vi due to the voltage drop introduced by VBE
AV  1
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Common-Collector Amplifier
The last basic configuration is to tie the collector to a fixed voltage, drive
an input signal into the base and observe the output at the emitter.
(a) 共集电极电路
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Common-Collector Amplifier
Let’s find Av， Ai：


Vo  I e ( Re // RL )  I b (e1   )(Re // RL )


Vi  I b [rbe  (1   )(Re // RL )]  I b rbe  I e ( Re // RL )

(1   )(Re // RL )
 ( Re // RL )
 AV   

1
Vi rbe  (1   )(Re // RL ) rbe  (1   )(Re // RL )
VO
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Common-Collector Amplifier
Let’s find Av， Ai：
I o RL  I e ( Re // RL )  (1   ) I b ( Re // RL )
(1   )(Re // RL )
Io  Ib
RL
Ib (rbe  (1   )(Re // RL ))  ( I i  Ib ) Rb
rbe  (1   )(Re // RL )  Rb
(1   )(Re // RL )  Rb
Ii  Ib
 Ib
Rb
Rb
Ai 
(1   )(Re // RL )
Rb
(1   )(Re // RL )


RL
(1   )(Re // RL )  Rb
RL
Ai 
(1   )(Re // RL )
>>1
RL
(1   )(Re // RL ) << Rb
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Common-Collector Amplifier
Let’s find Ri：
Ri
vi  ib rbe  ie ( Re // R L )  ib rbe  (1   )( Re // R L )
R i 
vi
 rbe  (1   )( Re // R L )
ib
Ri  Ri // Rb  [rbe  (1   )( Re // RL )] // Rb  Rb //  ( Re // RL )
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Common-Collector Amplifier
Let’s find Ro：
I Re  I  I e
I  I Re  I e
I  I Re  Ie  I Re  1   Ib
I
I
Re 

Ro
(b)
Ie
I Re
I
I  I Re  Ib  Ib
v
v

 (1   )
Re
rbe  Rs // Rb
v
1
Ro  
1
i 1 
Re (rbe  Rs // Rb ) (1   )
(rbe  Rs // Rb )
 Re //
1 
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Common-Collector Amplifier
I
I
Ri
(a)
Ri  [rbe  (1   )( Re // RL )] // Rb
Re 

Ro
(b)
(rbe  Rs // Rb )
Ro  Re //
1 
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Common-Collector Amplifier

AV 

VO

Vi
Ai 

 ( Re // RL )
1
rbe  (1   )(Re // RL )
(1   )(Re // RL )
>>1
RL
Ri  [rbe  (1   )( Re // RL )] // Rb
(a) 共集电极电路
(rbe  Rs // Rb )
Ro  Re //
1 
C-C amp characteristics:
• Gain is less than unity, but close (to unity) since β is large and rbe is small.
• Also called an emitter follower since the emitter follows the input signal.
• Input resistance is higher, output resistance is lower.
- Used for connecting a source with a large Rs to a load with low
resistance.
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Common-Base Amplifier
Ground the base and drive the input signal into the emitter
Rc
(a) 共基极电路
VB  VBE  I E R e
VB 
VCC
R b2
R b1  R b 2
VCE  VCC  I C RC  I E Re  VCC  I C ( RC  Re )
IC  I E 
IB 
IC

VB  VBE VB

Re
Re
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Common-Base Amplifier
Ro
Ri
(a) 共基极电路
i ( R // RL )  ( Rc // RL )
 Av  c c

 ib rbe
rbe
R
R
 C
 C (R  R ) I

(
R

R
)
C
L
C
L
  Io 

 E 1
A
i
(1   )
IC
Ii  rbe r
be
rbe
// Re
R i=
(1   )
Ro≈RC
(1   )
For RL<<RC, Ai 
// Re

(1   )
 1sinceI E  I C 
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Common-Base Amplifier
 RC ( R  R )
I
RC
C
L
A i  o 

(1   )
RC  RL
I
i
For RL<<RC, Ai 
Av 
 ( Rc // RL )

(1   )
1
rbe
rbe
rbe
//
R

R i=
e
(1   )
(1   )
Ro≈RC
(a) 共基极电路
CB amp characteristics:
• current gain has little dependence on β
• is non-inverting
• most commonly used as a unity-gain current amplifier or current buffer and not
as a voltage amplifier: accepts an input signal current with low input resistance
and delivers a nearly equal current with high output impedance
• most significant advantage is its excellent frequency response
Ch6 Basic BJT Amplifiers Circuits
6.3 Frequency Response
Key Words:
Basic Concepts
High-Frequency BJT Model
Frequency Response of the CE Amplifier
Ch6 Basic BJT Amplifiers Circuits
6.3 Frequency Response
Basic Concepts
1.0V
0.5V
0V
-0.5V
-1.0V
0.5ms
V(1)
1.0ms
1.5ms
2.0ms
2.5ms
V(2)
Time
3.0ms
3.5ms
4.0ms
Ch6 Basic BJT Amplifiers Circuits
6.3 Frequency Response
Basic Concepts
VO (t )
1.0V
0.5V
0V
-0.5V
-1.0V
0.5ms
V(1)
1.0ms
1.5ms
2.0ms
2.5ms
V(2)
Time
3.0ms
3.5ms
4.0ms
Ch6 Basic BJT Amplifiers Circuits
6.3 Frequency Response
Basic Concepts
800mV
600mV
400mV
200mV
0V
0Hz
V(2)
2KHz
V(1)
4KHz
6KHz
8KHz
10KHz
Frequency
12KHz
14KHz
16KHz
18KHz
1.0V
0.5V
0V
10Hz
V(2)
100Hz
1.0KHz
10KHz
100KHz
1.0MHz
20KHz
Ch6 Basic BJT Amplifiers Circuits
6.3 Frequency Response
Basic Concepts
Av  Av ( f ) ( f )
Lower cut off frequency
or
A  Av ( ) ( )
Upper cut off frequency
The drops of voltage gain (output/input) is mainly due to:
1、Increasing reactance of
Cs , Cc , Ce (at low f)
2、Porasitic capacetine elements of the net work (at high f)
3、Dissappearance of changing current(for trasformer coupled amp)
Ch6 Basic BJT Amplifiers Circuits
6.3 Frequency Response
High-Frequency BJT Model
In BJTs, the PN junctions (EBJ and CBJ) also have capacitances associated
with them
C
rbe
C
C
C
rbe
C'
C'
Ch6 Basic BJT Amplifiers Circuits
6.3 Frequency Response
Frequency Response of the CE Amplifier
rbe
C'
vs
There are three capacitors in the circuit.
At the mid frequency band, these are considered to be short circuits
and internal capacitors C',and C'are considered to be open circuits.
C'
Ch6 Basic BJT Amplifiers Circuits
6.3 Frequency Response
Frequency Response of the CE Amplifier
At low frequencies, C1, C2 are an
open circuit and the gain is zero.
Thus C1 has a high pass effect on the
gain, i.e. it affects the lower cutoff
frequency of the amplifier.
vs
1  C1 ( Rs  Rb1 // Rb2 // rbe )
f L1 
2 is the time constant for C2.
 2   1
1
2 1
---is neglected
Ch6 Basic BJT Amplifiers Circuits
6.3 Frequency Response
Frequency Response of the CE Amplifier
1  C1 ( Rs  Rb1 // Rb2 // rbe )
 2   1
---is neglected
Capacitor Ce is an open circuit. The
pole time constant is given by the
resistance multiplied by Ce.
vs
2
2
2
f L  1.1 f L1  f L 2   f Le
 ( Rb // Rs  rbe )

 e  
// Re Ce
1 


1
f Le 
2 e
Ch6 Basic BJT Amplifiers Circuits
6.3 Frequency Response
Frequency Response of the CE Amplifier
At high frequencies, C1, C2 Ce are all
short circuit.
The frequency that dominates is the
lowest pole frequency.
vs
The time constant is neglected for C'
( RL  1 jC' )
rbe
 C  ( Rb // Rs // rbe )C
C'
C'
fH 
1
2 C
In summary:the lower cut off frequency is determined by network capacitence.
e.g.C1 C2 , Ce  The higher cut off frequency is determined by the parasitic
ferquency of the BJT. e.g. C
Ch6 Basic BJT Amplifiers Circuits
6.3 Frequency Response
Frequency Response of the CE Amplifier
rbe
C'
j

A v  Avm 
vs
C'
(1  j
f
fL
f
f
)(1  j
)
fL
fH

f
For f L  f  f H ,
 0  Av  Avm — mid - frequency
fH
f
j

f
fL
For f  f L ( f  f H ),
 0,  Av  Avm
— low - frequency
f
fH
1 j
fL

fL
1
For f  f H ( f  f L )  0,  Av  Avm
— High  frequency
f
f
1 j
fH
f
 ,
fL
Ch6 Basic BJT Amplifiers Circuits
6.3 Frequency Response
Frequency Response of the CE Amplifier
rbe
vs
j

A v  Avm 
fL 
(1  j
L
1

2 2 L
f
fL
f
f
)(1  j
)
fL
fH
fH 
H
1

2 2 H
C'
C'
Ch6 Basic BJT Amplifiers Circuits
6.3 Frequency Response
Frequency Response of the CE Amplifier
decade
decade
0
Ch6 Basic BJT Amplifiers Circuits
6.4 Power Amplifiers
Key Words:
Power Calculation
Class-A, B, AB Amplifiers
Complementary Symmetry(Push-Pull) Amplifier
Biasing the Push-Pull Amplifier (OCL)
Single-Supply Push-Pull Amplifier (OTL)
Ch6 Basic BJT Amplifiers Circuits
6.4 Power Amplifiers
An Analog Electronics System Block
Sensor
Energy
conversion
Voltage
Amplifiers
Power
Amplifiers
Signal
Amplifiers
Load
Energy
conversion
Ch6 Basic BJT Amplifiers Circuits
6.4 Power Amplifiers
The output power delivered to the load RL:
Po 
Vom I om
2
1
 Vom I om
2 2
The average power delivered by the supply:
1
PS 
T
T

0
VCC iC (t )dt  VCC
1
T
T

iC (t )dt  VCC I C
0
The efficiency in converting supply power to useful output power is
defined as
P
  OM  100%
PS
Ch6 Basic BJT Amplifiers Circuits
6.4 Power Amplifiers
Power Calculation
The DC power by
the supply
PC  VCEQ I CQ  (VCC  I CQ RC ) I CQ
2
 PS  I CQ
RC
The DC power delivered to BJT by the supply
Ch6 Basic BJT Amplifiers Circuits
6.4 Power Amplifiers
Power Calculation
The average power dissipated as heat in the BJT:
1 T
vCE iC dt

0
T
1 T
  ( I CQ  I m cos t )(VCEQ  Vm cos t )
T 0
1
 I CQVCEQ  I mVm  PC  PCL
2
PT 
Ch6 Basic BJT Amplifiers Circuits
6.4 Power Amplifiers
Class-A Amplifiers
Class-B Amplifiers
Ch6 Basic BJT Amplifiers Circuits
6.4 Power Amplifiers
Class-AB Amplifiers
Ch6 Basic BJT Amplifiers Circuits
6.4 Power Amplifiers
Complementary Symmetry Power Amplifier (Class-B)
Ch6 Basic BJT Amplifiers Circuits
6.4 Power Amplifiers
Complementary Symmetry Power Amplifier (Class-B)
2
2
V

sin
t
1   VCCVon
on
PT 1 
sin tdt  
d t 
 0
0
2 
RL
RL


Von 2  2
1 VCCVon 

sin tdt 
sin tdt 



0
0
2  RL
RL


Von 2 1 
1 VCCVon 
 
sin tdt 
1  cos 2tdt 


0
0
2  RL
RL 2

Von 2  1 VCCVon
Von 2  1 VCCVon Von 2 
1 VCCVon


2

  cos t  0    

 

2  RL
2RL  2  RL
2RL  RL  
4 
Ch6 Basic BJT Amplifiers Circuits
6.4 Power Amplifiers
Complementary Symmetry Power Amplifier (Class-B)
PO 
I om

2
1 Vom

2 2 RL
Vom
2
POM
Assuming vo  Vom sin  t
1
PT 1 
2


0
1
vCE iC d  t  
2
2
1 VCC

2 RL
 vCE  VCC  vo

vO


V

v
0 CC O d  t 
RL
2

1  VCCVOm VOm




RL  
4 
PT 1  0 for VOm  0
VOm  VCC
2
4   VCC
PT  PT 1  PT 2 

2
RL
2
VCC
4 
PT 1 

RL
4
Note: PT represents the amount of
power dissipated by the BJT as heat
Ch6 Basic BJT Amplifiers Circuits
6.4 Power Amplifiers
Complementary Symmetry Power Amplifier (Class-B)
I om
2
1 Vom

2 2 RL
Vom
2
1 VCC
POM 
2
2 RL
2
V
4 
VOm  VCC
PT 1  CC 
RL
4
2
4   VCC
PT  PT 1  PT 2 

2
RL
PO 

2
2VCC
PE  PT  PO 
RL
2
1 VCC
P

  O  2 RL

2
2 VCC
PE
4
Note that for class A: η﹦25﹪ ~ 50﹪
 RL
class B: η﹦78.5﹪
class AB: η=25﹪ ~ 78.5﹪
=78.5%
Ch6 Basic BJT Amplifiers Circuits
6.4 Power Amplifiers
Complementary Symmetry Power Amplifier (Class-B)
Crossover
distortion
Ch6 Basic BJT Amplifiers Circuits
6.4 Power Amplifiers
Biasing the Push-Pull Amplifier (Class-AB) (OCL)
To overcome crossover distortion, the biasing is adjusted to just overcome the
VBE of the transistors; this results in a modified form of operation called class
AB. In class AB operation, the push-pull stages are biased into slight
conduction, even when no input signal is present.
}VCC
}VCC
Power Calculation is the same as class-B
Ch6 Basic BJT Amplifiers Circuits
6.4 Power Amplifiers
Single-Supply Push-Pull Amplifier (OTL)
The circuit operation is the same as that described previously, except the
bias is set to force the output emitter voltage to be VCC/2 instead of zero
volts used with two supplies. Because the output is not biased at zero
volts, capacitive coupling for the input and output is necessary to block
the bias voltage from the source and the load resistor.