basic bjt amplifiers circuits
TRANSCRIPT
Basic BJT Amplifiers Circuits
1 Bipolar junction transistors (BJTs)2 Single-Stage BJT Amplifiers3 Frequency Response4 Power Amplifiers
Saturday, April 18, 2020 1Anil K. Singh, Transistor ;ECC, ALD.
The transistor was probably the most important invention of the 20th Century, and the story behind the invention is one of clashing egos and top secret research.
First - BJTs
Reference:Bell Labs Museum
B. G. Streetman & S. Banerjee ‘Solid State Electronic Devices’, Prentice Hall 1999.Saturday, April 18, 2020 2Anil K. Singh, Transistor ;ECC, ALD.
Picture shows the workbench of John Bardeen (Stocker Professor at OU) and Walter Brattain at Bell Laboratories. They were supposed to be doing fundamental research about crystal surfaces.
The experimental results hadn't been very good, though, and there's a rumor that their boss, William Shockley, came near to canceling the project. But in 1947, working alone, they switched to using tremendously pure materials.
It dawned on them that they could build the circuit in the picture. It was a working amplifier! John and Walter submitted a patent for the first working point contact transistor.
Interesting story…
Saturday, April 18, 2020 3Anil K. Singh, Transistor ;ECC, ALD.
Shockley was furious and took theirwork and invented the junction transistorand submitted a patent for it 9 days later.The three shared a Nobel Prize in 1955.Bardeen and Brattain continued inresearch (and Bardeen later won anotherNobel).
Shockley quit to start a semiconductorcompany in Palo Alto. It folded, but itsstaff went on to invent the integratedcircuit (the "chip") and to found IntelCorporation.
By 1960, all important computers usedtransistors for logic, and ferrite cores formemory.
Interesting story…
Saturday, April 18, 2020 4Anil K. Singh, Transistor ;ECC, ALD.
Point-Contact Transistor –first transistor ever made
Saturday, April 18, 2020 5Anil K. Singh, Transistor ;ECC, ALD.
Saturday, April 18, 2020 6Anil K. Singh, Transistor ;ECC, ALD.
Qualitative basic operation of point-contact transistor
Problems with first transistor…
Saturday, April 18, 2020 7Anil K. Singh, Transistor ;ECC, ALD.
First Bipolar Junction TransistorsW. Shockley invented the p-n junction transistorThe physically relevant region is moved to the bulk of the material
Saturday, April 18, 2020 8Anil K. Singh, Transistor ;ECC, ALD.
Understanding of BJT
force – voltage/currentwater flow – current
- amplification
Saturday, April 18, 2020 9Anil K. Singh, Transistor ;ECC, ALD.
Basic models of BJT
Diode
Diode
Diode
Diode
npn transistor
pnp transistor
Saturday, April 18, 2020 10Anil K. Singh, Transistor ;ECC, ALD.
Nomenclature
• NPN
• Roughly read as
• NOT POINT IN
• It means points are outward i.e.
• Arrows of the two diodes (P-type) are in outward directions.
Saturday, April 18, 2020 11Anil K. Singh, Transistor ;ECC, ALD.
Qualitative basic operation of BJTs
Saturday, April 18, 2020 12Anil K. Singh, Transistor ;ECC, ALD.
Basic models of BJT
Saturday, April 18, 2020 13Anil K. Singh, Transistor ;ECC, ALD.
BJTs – Basic ConfigurationsFluid Flow AnalogyDifference between FET (field effect transistor) and BJTTechnology of BJTs
pnp BJT npn BJT
Saturday, April 18, 2020 14Anil K. Singh, Transistor ;ECC, ALD.
BJTs – Practical Aspects
Heat sink
Saturday, April 18, 2020 15Anil K. Singh, Transistor ;ECC, ALD.
BJTs – Testing
Saturday, April 18, 2020 16Anil K. Singh, Transistor ;ECC, ALD.
BJTs – Testing
Saturday, April 18, 2020 17Anil K. Singh, Transistor ;ECC, ALD.
Gain medium
Incoherent Light
Coherent Light
Transistor/switch/amplifier – a 3 terminal device
Source
Drain
Gate
Valve
ArteryVein
Emitter Collector
Base
Ion Channel
Dam Laser Heart
Axonal conductionMOSFETBJT
Saturday, April 18, 2020 18Anil K. Singh, Transistor ;ECC, ALD.
All of these share a feature with…
• Output current can toggle between large and small
(Switching Digital logic; create 0s and 1s)
• Small change in ‘valve’ (3rd terminal) creates Largechange in output between 1st and 2nd terminal
(Amplification Analog applications; Turn 0.5 50)
Saturday, April 18, 2020 19Anil K. Singh, Transistor ;ECC, ALD.
Basic BJT Amplifiers Circuits1 Bipolar junction transistors (BJTs)
Saturday, April 18, 2020 20Anil K. Singh, Transistor ;ECC, ALD.
Construction of Bipolar junction transistors
Base region(very narrow)
Emitter region
Collector region
Collector
Base
Emitter
Emitter-base junction
Collector-base junction
Saturday, April 18, 2020 21Anil K. Singh, Transistor ;ECC, ALD.
Standard bipolar junction transistor symbols
Depending on the biasing across each of the junctions, different modes of operation are obtained – cutoff, active and saturation
Saturday, April 18, 2020 22Anil K. Singh, Transistor ;ECC, ALD.
Saturday, April 18, 2020 23Anil K. Singh, Transistor ;ECC, ALD.
There are three basic configurations for single-stage BJT amplifiers:– Common-Emitter – Common-Base– Common-Collector
VBB VCC
RC
NN Pc
e
b
(a)
VBB VCC
RcN
N
P
c
e
b
(b)
VBB
VCC
Re
N
N
P
c
e
b
(c)
E B CV V V E B CV V V E B CV V V
Saturday, April 18, 2020 24Anil K. Singh, Transistor ;ECC, ALD.
BJT in Active Mode
Two external voltage sources set the bias conditions for active mode
– EBJ is forward biased and-- CBJ is reverse biased
Saturday, April 18, 2020 25Anil K. Singh, Transistor ;ECC, ALD.
Hybrid Parameters
Condition
hi Input resistance Output shorted
hr Voltage feedback ratio Input open
hf Forward current gain Output shorted
ho Output conductance Input open
Second subscript indicates common base (b), common emitter
(e), or common collector (c)
Saturday, April 18, 2020 26Anil K. Singh, Transistor ;ECC, ALD.
Hybrid Parameters
= b
= Slope of curve
Saturday, April 18, 2020 27Anil K. Singh, Transistor ;ECC, ALD.
Hybrid Parameters
hie = VB/IB Ohm’s Law
hie =input impedance
hre = VB/VC
Saturday, April 18, 2020 28Anil K. Singh, Transistor ;ECC, ALD.
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
Saturday, April 18, 2020 29Anil K. Singh, Transistor ;ECC, ALD.
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
Saturday, April 18, 2020 30Anil K. Singh, Transistor ;ECC, ALD.
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
Saturday, April 18, 2020 31Anil K. Singh, Transistor ;ECC, ALD.
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
Saturday, April 18, 2020 32Anil K. Singh, Transistor ;ECC, ALD.
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
b
1Let
EC
BCEOBC
BBCE
II
IIII
IIII
bb
b )1(
CBOBC III )1( bb
B
C
I
Ib ---common-emitter current gain Beta:
Saturday, April 18, 2020 33Anil K. Singh, Transistor ;ECC, ALD.
+ +
- -
vBE vCE
b iB
iB iC
iE
BJT Equivalent Circuits
BJT DC model
+ +
- -
VBE=Von VCE
bIB
IB IC
IE
•Use a simple constant-VBE
model– Assume VBE = 0.7V
Saturday, April 18, 2020 34Anil K. Singh, Transistor ;ECC, ALD.
BJT DC Analysis
• Make sure the BJT current equations andregion of operation match
VBE > 0, VBC < 0, VE < VB <VC
• Utilize the relationships (β and α) betweencollector, base and emitter currents to solvefor all currents
EC
BC
BBCE
II
II
IIII
b
b )1(
Saturday, April 18, 2020 35Anil K. Singh, Transistor ;ECC, ALD.
C-E CircuitsI-V Characteristics
Base-emitter Characteristic(Input characteristic)
CCEvBEB vfi
)(
Saturday, April 18, 2020 36Anil K. Singh, Transistor ;ECC, ALD.
C-E Circuits I-V Characteristics
Collector characteristic (output characteristic)
CiVC BCEfi )(
AμiB 40=
Saturday, April 18, 2020 37Anil K. Singh, Transistor ;ECC, ALD.
C-E Circuits I-V Characteristics
Collector characteristic (output characteristic)CiVC BCE
fi )(
Saturday, April 18, 2020 38Anil K. Singh, Transistor ;ECC, ALD.
Saturation
Vsat
C-E Circuits I-V Characteristics
Collector characteristic
Saturation occurs whenthe supply voltage, VCC, isacross the total resistanceof the collector circuit, RC.
IC(sat) = VCC/RC
Once the base current is high enough to produce saturation, further increases inbase current have no effect on the collector current and the relationship IC = bIB isno longer valid. When VCE reaches its saturation value, VCE(sat), the base-collectorjunction becomes forward-biased.
Saturday, April 18, 2020 39Anil K. Singh, Transistor ;ECC, ALD.
C-E Circuits I-V Characteristics
Collector characteristic
Cutoff
When IB = 0, the transistor is incutoff and there is essentially nocollector current except for avery tiny amount of collectorleakage current, ICEO, which canusually be neglected. IC 0.
In cutoff both the base-emitterand the base-collector junctionsare reverse-biased.
Saturday, April 18, 2020 40Anil K. Singh, Transistor ;ECC, ALD.
C-E Circuits I-V Characteristics
Collector characteristic
linearity
Δ
Saturday, April 18, 2020 41Anil K. Singh, Transistor ;ECC, ALD.
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:
Saturday, April 18, 2020 42Anil K. Singh, Transistor ;ECC, ALD.
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:Saturday, April 18, 2020 43Anil K. Singh, Transistor ;ECC, ALD.
Amplifiers CircuitsSingle-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.
o
C
CE
R
c
ii
BBE
C
i vviivv CBC 12 b
DC + small signal
OC vi Bi iv CB ii
coupling capacitor(only passes ac signals)
Saturday, April 18, 2020 44Anil K. Singh, Transistor ;ECC, ALD.
C-E Amplifiers
iV
iV
Vi
+
iV
Saturday, April 18, 2020 45Anil K. Singh, Transistor ;ECC, ALD.
C-E Amplifiers
vBE=vi+VBE
bBB iIi
Apply a small signal input voltage and see ib
Saturday, April 18, 2020 46Anil K. Singh, Transistor ;ECC, ALD.
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.
Saturday, April 18, 2020 47Anil K. Singh, Transistor ;ECC, ALD.
C-E Amplifiers Considering (all the capacitors are replaced by open circuits)
CV
Considering (all the capacitors are replaced by short circuits)
iV
Saturday, April 18, 2020 48Anil K. Singh, Transistor ;ECC, ALD.
C-E Amplifiers
Considering (all the capacitors are replaced by open circuits)
CV
Considering (all the capacitors are replaced by short circuits)
iV
Saturday, April 18, 2020 49Anil K. Singh, Transistor ;ECC, ALD.
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?Saturday, April 18, 2020 50Anil K. Singh, Transistor ;ECC, ALD.
Graphical Analysis
VCC
')//( LcLCcce RiRRiv
Saturday, April 18, 2020 51Anil K. Singh, Transistor ;ECC, ALD.
Graphical Analysis
Q-point is centered on the ac load line:
VCC
Saturday, April 18, 2020 52Anil K. Singh, Transistor ;ECC, ALD.
VCC
Graphical Analysis
Clipped at cutoff(cutoff distortion)
Q-point closer to cutoff:
Saturday, April 18, 2020 53Anil K. Singh, Transistor ;ECC, ALD.
VCC
Graphical Analysis
Clipped at cutoff(saturation distortion)
Q-point closer to saturation:
Saturday, April 18, 2020 54Anil K. Singh, Transistor ;ECC, ALD.
Graphical Analysis
Saturday, April 18, 2020 55Anil K. Singh, Transistor ;ECC, ALD.
Steps for using small-signal models1. Determine the DC operating point of the BJT
- in particular, the collector current2. Calculate small-signal model parameters: rbe3. Eliminate DC sources
– replace voltage sources with short circuits and current sources with open circuits4. Replace BJT with equivalent small-signal models5. Analysis
Saturday, April 18, 2020 56Anil K. Singh, Transistor ;ECC, ALD.
IC ≈ βIB, IE = IC + IB = (1+β)IB
eEBEbBCBC RIVRIR)II(V
))(1( eb
BEC
BRRR
VVI
b
)( eECCCCE RRIRIVV
Example 1
Saturday, April 18, 2020 57Anil K. Singh, Transistor ;ECC, ALD.
Example 2
vs
CC
bb
bB V
RR
RV
21
2
eB
e
BEBEC RV
R
VVII /
b C
B
II
)RR(IVV eCCCCCE
Saturday, April 18, 2020 58Anil K. Singh, Transistor ;ECC, ALD.
Common-Base Amplifier (C-B Configuration)
(a) 共基极电路
Rc
2b
2b1b
CCB R
RR
VV
eEBEB RIVV
e
B
e
BEBEC
R
V
R
VVII
bC
B
II )( eCCCCeECCCCCE RRIVRIRIVV
Ground the base and drive the input signal into the emitter
Saturday, April 18, 2020 59Anil K. Singh, Transistor ;ECC, ALD.
Common-Base Amplifier
(a) 共基极电路
Ri Ro
be
Lc
beb
Lccv
r
RR
ri
RRiA
)//()//( b
i
o
iI
IA
1
)1(
)(
//)1(
)(
C
ELC
C
ebe
be
LC
C
I
IRRR
Rr
r
RRR
b
b
b
b
For RL<<RC, CEi IIA
since1)1( b
be
be Rr
//)1( b
Ri=
Ro≈RCSaturday, April 18, 2020 60Anil K. Singh, Transistor ;ECC, ALD.
Common-Base Amplifier
(a) 共基极电路
be
Lcv
r
RRA
)//(b
i
o
iI
IA
LC
CLC
C
RR
RRRR
)1(
)(
b
b
For RL<<RC, 1)1(
b
biA
)1(//
)1( bb
bee
be rR
rRi=
Ro≈RCCB 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 responseSaturday, April 18, 2020 61Anil K. Singh, Transistor ;ECC, ALD.
C-C C-E C-B
Input
Output
Functions
Summary for three types of diodes:
BI BIBI
EI CI CIZout < Zin
Vout > Vin
Zout > Zin
Vout > VinVout ≈ Vin
Zout > Zin
Saturday, April 18, 2020 62Anil K. Singh, Transistor ;ECC, ALD.
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、Parasitic capacitive elements of the network (at high f)3、Dissappearance of changing current (for transformer coupled amp.)
ecs CCC ,,
Saturday, April 18, 2020 63Anil K. Singh, Transistor ;ECC, ALD.
vs
Frequency Response of the CE Amplifier
At low frequencies, C1, C2 are anopen circuit and the gain is zero.Thus C1 has a high pass effect on thegain, i.e. it affects the lower cutofffrequency of the amplifier.
)////( 2111 bebbs rRRRC
2 is the time constant for C2. 12 ---is neglected
1
12
1
Lf
Saturday, April 18, 2020 64Anil K. Singh, Transistor ;ECC, ALD.
vs
Frequency Response of the CE Amplifier
)////( 2111 bebbs rRRRC
12 ---is neglected
Capacitor Ce is an open circuit. Thepole time constant is given by theresistance multiplied by Ce.
eebesb
e CRrRR
//
1
)//(
b
22
2
2
11.1 LeLLL ffff e
Lef2
1
Saturday, April 18, 2020 65Anil K. Singh, Transistor ;ECC, ALD.
C'rbe C'
Frequency Response of the CE Amplifier
vs )1)(1(HL
Lvmv
f
fj
f
fj
f
fj
AA
frequency-mid0,, —vmv
HL
HL AAf
f
f
ffffFor
frequency-low
1
,0),( —
L
Lvmv
H
HL
f
fj
f
fj
AAf
fffffFor
frequencyHigh
f
fj
AAf
fffffFor
H
vmvL
LH
—1
1,0)(
Saturday, April 18, 2020 66Anil K. Singh, Transistor ;ECC, ALD.
decadedecade
0
Frequency Response of the CE Amplifier
Saturday, April 18, 2020 67Anil K. Singh, Transistor ;ECC, ALD.
Class-A Amplifiers
Class-B Amplifiers
Saturday, April 18, 2020 68Anil K. Singh, Transistor ;ECC, ALD.
Class-AB Amplifiers
Saturday, April 18, 2020 69Anil K. Singh, Transistor ;ECC, ALD.
• THANKS
Saturday, April 18, 2020 70Anil K. Singh, Transistor ;ECC, ALD.