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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
ReferencesReferences: Floyd-Ch-3, 5, 6; Gao-Ch7;
Circuits and Analog ElectronicsCircuits and Analog Electronics
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
Key WordsKey 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
Base region(very narrow)
Emitter region
Collector region
Collector
Base
Emitter
Emitter-base junction
Collector-base junction
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
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
Construction of Bipolar junction transistors
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
Forward bias of EBJ injects electrons from emitter into base (small number of holes injected from base into emitter)
IE = IEN + IEP IEN
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
BJT in Active Mode
• 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
IB = IBN + IEP
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
BJT in Active Mode
Electrons that diffuse across the base to the CBJ junction are swept across the CBJ depletion region to the collector.
IC = ICN + ICBO
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
E
C
I
I ---common-base current gain
IC (1 - ) = IB + ICBO
IB = IBN +IEP
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
E
C
I
I IC (1 - )= IB+ICBO
IB = IBN +IEP
1
Let
EC
BCEOBC
BBCE
II
IIII
IIII
)1(
CBOBC III )1(
B
C
I
I ---common-emitter current gain Beta:
+ +
- -
vBE vCE
iB
iB iC
iE
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
BJT Equivalent Circuits
BJT DC model
+ +
- -
VBE=Von VCE
IB
IB IC
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
EC
BC
BBCE
II
II
IIII
)1(
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
C-E Circuits I-V Characteristics
Base-emitter Characteristic(Input characteristic)
CCEvBEB vfi
)(
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
C-E Circuits I-V Characteristics
Collector characteristic (output characteristic)
CiVC BCEfi )(
AμiB 40=
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
C-E Circuits I-V Characteristics
Collector characteristic (output characteristic) CiVC BCEfi )(
Saturation
Vsat
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
C-E Circuits I-V Characteristics
Collector characteristic
Saturation occurs when the supply voltage, VCC, is across the total resistance of the collector circuit, RC.
IC(sat) = VCC/RC
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
Cutoff
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.
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)
i i B
o L C
v R i
v R i
Discussion of an amplification effect
CEBEi L
B C
vvR R
i i
B Ci iWith i ov v
50 ~ 300ov
i
vA
v
E.g. for common-base configuration transistor:
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
DC Load Line and Quiescent Operation Point
DC load line
.Q
Q-point
ICQ
VCEQ
VCC
)(40 AR
V
R
VVI
b
CC
b
BECCB
Base-emitter loop:
kiRiVv CCCCCCE 410 Collector-emitter loop:
+ +
- -
vBE vCE
iB
iB iC
iE
Ch6 Basic BJT Amplifiers Circuits
6.1 Bipolar junction transistors (BJTs)
BJT AC Small-Signal Model
+ +
- -
vce ib
ib ic
ie
vbe rbe
• 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 controlledcurrent source ( ) or a current-controlled current source (ic = ib).• For small enough signals, approximate exponential curve with a linear line.
)(
)(26)1(300
mAI
mVr
Ebe
6.1 Bipolar junction transistors (BJTs)
1E C B B CI I I I I
0.7VBEV
C BI I
BJT fundamentals:
C-C C-E C-B
Input
Output
Functions
Summary for three types of diodes:
at inZ Z
BI BIBI
EI CI CIat inZ Z at inZ Z
at inV Vat inV V
Ch6 Basic BJT Amplifiers Circuits
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Key WordsKey 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.
oC
CER
cii
BBEC
i vviivv CBC 12
DC + small signal
OC vi Bi iv CB ii
coupling capacitor (only passes ac signals)
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
C-E Amplifiers
iV
Vi
+
iV
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
C-E Amplifiers
vBE=vi+VBE
bBB iIi
Apply a small signal input voltage and see ib
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
C-E Amplifiers
• vi = 0 IB 、 IC 、 VCE
ceCECE
CCC
bBBi
vVv
iIi
iIiv 0
)()( ioiMoM ffVV •
• vo out of phase with vi
iC=ic+IC
vCE=vce+VCE
See how ib translates into vce.
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
C-E Amplifiers Considering (all the capaertors are replaced by open circuits)
CV
Considering (all the capaertors are replaced by short circuits)
iV
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
C-E AmplifiersConsidering (all the capaertors are replaced by open circuits)
CV
Considering (all the capaertors are replaced by short circuits)
iV
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Graphical Analysis
VCC
• 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
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
VCC
'' LCQCEQCC RIVV
')//( LcLCcce RiRRiv
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Graphical Analysis
Q-point is centered on the ac load line:
VCC
VCC
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Graphical Analysis
Clipped at cutoff(cutoff distortion)
Q-point closer to cutoff:
VCC
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Graphical Analysis
Clipped at cutoff(saturation distortion)
Q-point closer to saturation:
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 models1. Determine the DC operating point of the BJT - in particular, the collector current2. Calculate small-signal model parameters: rbe
3. Eliminate DC sources – replace voltage sources with shorts and current sources with open circuits4. Replace BJT with equivalent small-signal models5. Analysis
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Small-Signal Models Analysis
IC ≈ βIB,
IE = IC + IB = (1+β)IB
eEBEbBCBC RIVRIR)II(V
))(1( eb
BECB RRR
VVI
)( eECCCCE RRIRIVV
Example 1
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
vs
CCbb
bB V
RR
RV
21
2
eBe
BEBEC RV
R
VVII /
C
B
II
)RR(IVV eCCCCCE
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
VBB VCC
RcN
NP
c
e
b
(b)
VBB
VCC
Re
N
NP
c
e
b
(c)
E B CV V V E B CV V V E B CV V V
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Common-Collector Amplifier
eEBEbBCC RIVRIV eBBEbB RIVRI )1(
eb
CC
eb
BECCB RR
V
RR
VVI
1)1(
BC II
eCCEeECECC RIVRIVV
eCCCCE RIVV
Note : is slightly less than due to the voltage drop introduced by
oV iV BEV
1VA
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.
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Common-Collector Amplifier
)//()]//)(1([ LeebebLebebi RRIrIRRrIV
)//)(1()//( LebLeeo RReIRRIV
1)//)(1(
)//(
)//)(1(
)//)(1(
Lebe
Le
Lebe
Le
i
O
V RRr
RR
RRr
RR
V
VA
Let’s find Av ,
Ai :
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Common-Collector Amplifier
bbiLebeb RIIRRrI )())//)(1((
b
bLeb
b
bLebebi R
RRRI
R
RRRrII
)//)(1()//)(1(
L
Lebo R
RRII
)//)(1(
bLe
b
L
Lei RRR
R
R
RRA
)//)(1(
)//)(1(
L
Le
R
RR )//)(1(
)//)(1( Le RR << Rb
iAL
Le
R
RR )//)(1( >>1
Let’s find Av ,
Ai : )//()1()//( LebLeeLo RRIRRIRI
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Common-Collector Amplifier
)//)(1()//( LebebLeebebi RRriRRiriv
b
ii i
vR )//)(1( Lebe RRr
)//(////)]//)(1([// LebbLebebii RRRRRRrRRR
Let’s find Ri :
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Common-Collector Amplifier
Let’s find Ro :
bb IIII Re
)1()//(11
1
bsbee
o
RRrRi
vR
Re eI I I Re eI I I
1Re e Re bI I I I I
bsbee RRr
v
R
v
//)1(
1
)//(// bsbe
e
RRrR
IeI
ReI
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Common-Collector Amplifier
bLebei RRRrR //)]//)(1([
1
)//(// bsbe
eoRRr
RR
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Common-Collector Amplifier
bLebei RRRrR //)]//)(1([
1
)//(// bsbe
eoRRr
RR
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.
1)//)(1(
)//(
Lebe
Le
i
O
V RRr
RR
V
VA
iAL
Le
R
RR )//)(1( >>1
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Common-Base Amplifier
2b2b1b
CCB R
RR
VV
eEBEB RIVV
e
B
e
BEBEC R
V
R
VVII
C
B
II )( eCCCCeECCCCCE RRIVRIRIVV
Ground the base and drive the input signal into the emitter
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Common-Base Amplifier
Ri Ro
be
Lc
beb
Lccv r
RR
ri
RRiA
)//()//(
i
oi I
IA
1
)1(
)(
//)1(
)(
C
ELC
C
ebe
be
LC
C
I
IRRR
Rr
rRR
R
For RL<<RC, CEi IIA
since1)1(
e
be Rr
//)1(
Ri=
Ro≈RC
Ch6 Basic BJT Amplifiers Circuits
6.2 Single-Stage BJT Amplifiers
Common-Base Amplifier
be
Lcv r
RRA
)//(
i
oi I
IA
LC
CLC
C
RR
RRRR
)1(
)(
For RL<<RC, 1)1(
iA
)1(//
)1(
be
ebe r
Rr
Ri=
Ro≈R
C 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 WordsKey Words:
Basic Concepts
High-Frequency BJT Model
Frequency Response of the CE Amplifier
Ch6 Basic BJT Amplifiers Circuits
6.3 Frequency Response
Basic Concepts
Time
0.5ms 1.0ms 1.5ms 2.0ms 2.5ms 3.0ms 3.5ms 4.0msV(1) V(2)
-1.0V
-0.5V
0V
0.5V
1.0V
Ch6 Basic BJT Amplifiers Circuits
6.3 Frequency Response
Basic Concepts
)(tVO
Time
0.5ms 1.0ms 1.5ms 2.0ms 2.5ms 3.0ms 3.5ms 4.0msV(1) V(2)
-1.0V
-0.5V
0V
0.5V
1.0V
Ch6 Basic BJT Amplifiers Circuits
6.3 Frequency Response
Basic Concepts
Frequency0Hz 2KHz 4KHz 6KHz 8KHz 10KHz 12KHz 14KHz 16KHz 18KHz 20KHz
V(2) V(1)0V
200mV
400mV
600mV
800mV
Frequency
10Hz 100Hz 1.0KHz 10KHz 100KHz 1.0MHzV(2)
0V
0.5V
1.0V
Ch6 Basic BJT Amplifiers Circuits
6.3 Frequency Response
Basic Concepts
Lower cut off frequency Upper cut off frequency
)()()()( vvv AAorffAA
The drops of voltage gain (output/input) is mainly due to:
1 、 Increasing reactance of (at low f)
2 、 Porasitic capacetine elements of the net work (at high f)
3 、 Dissappearance of changing current(for trasformer coupled amp)
ecs CCC ,,
Ch6 Basic BJT Amplifiers Circuits
6.3 Frequency Response
C
C
In BJTs, the PN junctions (EBJ and CBJ) also have capacitances associated with them
rbe
C
C
C'rbe C'
High-Frequency BJT Model
Ch6 Basic BJT Amplifiers Circuits
vs
6.3 Frequency Response
Frequency Response of the CE Amplifier
C'rbe C'
There are three capacitors in the circuit.
At the mid frequency band, these are considered to be short circuits and internal capacitors and are considered to be open circuits.
C',C'
Ch6 Basic BJT Amplifiers Circuits
vs
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 thegain, i.e. it affects the lower cutoff frequency of the amplifier.
)////( 2111 bebbs rRRRC
2 is the time constant for C2. 12 ---is neglected
11 2
1
Lf
Ch6 Basic BJT Amplifiers Circuits
vs
6.3 Frequency Response
Frequency Response of the CE Amplifier
)////( 2111 bebbs rRRRC
12 ---is neglected
Capacitor Ce is an open circuit. The pole time constant is given by the resistance multiplied by Ce.
eebesb
e CRrRR
//
1
)//(
222
211.1 LeLLL ffff
eLef
2
1
C'rbe C'
Ch6 Basic BJT Amplifiers Circuits
6.3 Frequency Response
Frequency Response of the CE Amplifier
vs
At high frequencies, C1, C2 Ce are all short circuit. The frequency that dominates is thelowest pole frequency.
The time constant is neglected for)1( '
CjRL C'
CrRR besbC )////(
C
Hf
2
1
In summary:the lower cut off frequency is determined by network capacitence.
e.g. The higher cut off frequency is determined by the parasitic ferquency of the BJT. e.g. C
eCCC ,21
C'rbe C'
Ch6 Basic BJT Amplifiers Circuits
6.3 Frequency Response
Frequency Response of the CE Amplifier
vs )1)(1(HL
Lvmv
f
fj
f
fj
f
fj
AA
frequency-mid0,, —vmv
HLHL AA
f
f
f
ffffFor
frequency-low1
,0),( —
L
Lvmv
HHL
ff
j
ff
jAA
f
fffffFor
frequencyHigh
ff
jAA
f
fffffFor
H
vmvL
LH
—1
1,0)(
Ch6 Basic BJT Amplifiers Circuits
6.3 Frequency Response
vs
)1)(1(HL
Lvmv
f
fj
f
fj
f
fj
AA
H
HH
L
LL ff
2
1
22
1
2
C'rbe C'
Frequency Response of the CE Amplifier
Ch6 Basic BJT Amplifiers Circuits
6.3 Frequency Response
decadedecade
0
Frequency Response of the CE Amplifier
Ch6 Basic BJT Amplifiers Circuits
6.4 Power Amplifiers
Key WordsKey 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
Power Amplifiers
Voltage Amplifiers
Sensor Load
An Analog Electronics System Block
Energy conversion
Signal Amplifiers
Energy conversion
Ch6 Basic BJT Amplifiers Circuits
6.4 Power Amplifiers
CCCC
T
CCCCC
T
S IVdttiT
VdttiVT
P )(1
)(1
00
The average power delivered by the supply:
omomomom
o IVIV
P2
1
22The output power delivered to the load RL:
The efficiency in converting supply power to useful output power is defined as
%100S
OM
P
P
Ch6 Basic BJT Amplifiers Circuits
6.4 Power AmplifiersPower Calculation
The DC power by the supply
CCQS
CQCCQCCCQCEQC
RIP
IRIVIVP2
)(
The DC power delivered to BJT by the supply
Ch6 Basic BJT Amplifiers Circuits
6.4 Power AmplifiersPower Calculation
The average power dissipated as heat in the BJT:
CLCmmCEQCQ
mCEQm
T
CQ
T
CCET
PPVIVI
tVVtIIT
dtivT
P
2
1
)cos)(cos(1
1
0
0
Ch6 Basic BJT Amplifiers Circuits
6.4 Power AmplifiersClass-A Amplifiers
Class-B Amplifiers
Ch6 Basic BJT Amplifiers Circuits
6.4 Power AmplifiersClass-AB Amplifiers
Ch6 Basic BJT Amplifiers Circuits
6.4 Power AmplifiersComplementary Symmetry Power Amplifier (Class-B)
Ch6 Basic BJT Amplifiers Circuits
6.4 Power AmplifiersComplementary Symmetry Power Amplifier (Class-B)
2 2
1 0 0
sin1sin
2CC on on
TL L
V V V tP td t d t
R R
22
0 0
1sin sin
2CC on on
L L
V V Vtd t td t
R R
2
0 0
1 1sin 1 cos 2
2 2CC on on
L L
V V Vtd t td t
R R
2 2 2
0
1 1 1cos 2
2 2 2 2 4CC on on CC on on CC on on
L L L L L
V V V V V V V V Vt
R R R R R
Ch6 Basic BJT Amplifiers Circuits
6.4 Power Amplifiers
Assuming tVv omo sinoCCCE vVv
4
1
2
1
2
1
2
001
OmOmCC
L
L
OOCCCCET
VVV
R
tdR
vvVtdivP
for 0OmV01 TP
L
CCTTT R
VPPP
2
21 2
4
L
omomomO R
VVIP
2
2
1
22
L
CCOM R
VP
2
2
1
CCOm VV
4
42
1
L
CCT R
VP
Complementary Symmetry Power Amplifier (Class-B)
Note: represents the amount of power dissipated by the BJT as heat
TP
Ch6 Basic BJT Amplifiers Circuits
6.4 Power Amplifiers
L
omomomO R
VVIP
2
2
1
22
L
CCOM R
VP
2
2
1
L
CCTTT R
VPPP
2
21 2
4
CCOm VV
4
42
1
L
CCT R
VP
L
CCOTE R
VPPP
22
422
1
2
2
L
CC
L
CC
E
O
R
VR
V
P
P=78.5%
Complementary Symmetry Power Amplifier (Class-B)
Note that for class A: η 25 ~ 50﹦ ﹪ ﹪
class B: η 78.5﹦ ﹪
class AB: η=25 ~ 78.5﹪ ﹪
Ch6 Basic BJT Amplifiers Circuits
6.4 Power Amplifiers
Crossover distortion
Complementary Symmetry Power Amplifier (Class-B)
Ch6 Basic BJT Amplifiers Circuits
6.4 Power AmplifiersBiasing 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.
Power Calculation is the same as class-B
}VCC
}VCC
Ch6 Basic BJT Amplifiers Circuits
6.4 Power AmplifiersSingle-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.