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ANALOGUE ELECTRONICS I
EMT 112/4
Basic BJT Amplifiers (Part 1)
Analog Signals & Linear Amplifiers Analog signals
Natural analog signals: physical sense (hearing, touch, vision) Electrical analog signals: e.g. output from microphone, output signal from
compact disc – form of time-varying currents & voltages Magnitude: any value which vary continuously with time
Analog circuits Electronic circuits which produce analog signals E.g. linear amplifier
Linear amplifier Magnifies input signal & produce output signal that is larger & directly
proportional to input signal
DC voltage source
Amplifier LoadSignal source
DC powera) Low signal powerb) High signal power
(a) (b)
Block diagram of a compact disc player system
The Bipolar Linear Amplifier
To use circuit as an amplifier, transistor needs to be biased with DC voltage at quiescent point (Q-point) transistor is biased in forward active region
Time-varying output voltage is directly proportional to & larger than time-varying input voltage linear amplifier
(a) Bipolar transistor inverter circuit; (b) inverter transfer characteristics
The Bipolar Linear Amplifier
Summary of notation
Variable Meaning
iB, vBE Total instantaneous values
IB, VBE DC values
ib, vbe Instantaneous ac values
Ib, Vbe Phasor values
Graphical Analysis & AC Equivalent Circuit
(D) Common-emitter transistor characteristics, dc load line, and sinusoidal variation in base current, collector current, and collector-emitter voltage
RC
RB
vs
vO
VBB
VC
C
(C) Common-emitter circuit with time varying signal source in series with base dc source
iB
iC
vBE
vCE
Fig. C
Fig. D
Graphical Analysis & AC Equivalent Circuit Base on Fig. C & D
(time-varying signals linearly related & superimposed on dc values)
If signal source, vs = 0:
(4)
(3)
(2)
(1)
beBEQBE
ceCEQCE
cCQC
bBQB
vVv
vVv
iIi
iIi
(6) loop) E-(C
(5) loop) E-(B
CEQCCQCC
BEQBBQBB
VRIV
VRIV
Graphical Analysis & AC Equivalent Circuit
For B-E loop, considering time varying signals:
Rearrange:
Base on (5), left side of (7) is 0. So:
For C-E loop, considering time varying signals:
Base on (6), left side of (11) is 0. So:
(7) )()( beBEQBbBQBEBBsBB vVRiIvRivV
(8) sbeBbBEQBBQBB vvRiVRIV
(9) beBbs vRiv
(11)
(10)
ceccCEQCCQCC
ceCEQCcCQCECCCC
vRiVRIV
vVRiIvRiV
)()(
(12) 0 cecc vRi
Graphical Analysis & AC Equivalent Circuit
Definition of small signal Small signal : ac input signal voltages and
currents are in the order of ±10 percent of Q-point voltages and currents.
e.g. If dc current is 10 mA, the ac current (peak-to-peak) < 0.1 mA.
Graphical Analysis & AC Equivalent Circuit
Rules for ac analysis Replacing all capacitors by short circuits Replacing all inductors by open circuits Replacing dc voltage sources by ground
connections Replacing dc current sources by open circuits
RC
RB
vs
vO
vce
vbe
ic
ib
+
+
--
AC equivalent circuit of C-E with npn transistor
Graphical Analysis & AC Equivalent Circuit
Equations Input loop
Output loop
beBbs vRiv
beT
BQb v
V
Ii
0 ceCc vRi
bc ii
0.026 V
Small-signal hybrid- equivalent circuit
Using transconductance (gm) parameter
gm=ICQ/VT
r=VT/ICQ
vbe = ibrπ
rπ
= diffusion resistance /base-emitter input resistance
1/rπ
= slope of iB – VBE curve
BQFCQ
CQ
TF
BQ
T
b
be
II
I
V
I
Vr
i
v
,
)( b
b
I
i
Using common-emitter current gain (β) parameter
Small-signal hybrid- equivalent circuit
How to construct Small-signal hybrid-
Place a terminal for the transistor
Common Terminal as ground
B
E
C
We know that
i across B ib
i across C βib i across E (β+1)ib
rπ between B -E
RC
RB
vs
vO
VBB
VCC
B
E
C
βibrπ
Small-signal equivalent circuit
Cbemceo RVgVV sB
be VRr
rV
B
Cms
ov Rr
rRg
V
VA
gain, voltage signal Small
B C
E
rVs
RB
RC
Vo
Ic
Ib gmVbeVbe Vce
+ +
--
Small-signal hybrid- equivalent circuit
Output signal voltage
Input signal voltage
RC
RB
vs
vO
VBB
VC
C
Example
Given : = 100, VCC = 12V
VBE = 0.7V, RC = 6k,
RB = 50k, and VBB = 1.2V
Calculate the small-signal voltage gain.
Small-signal hybrid- equivalent circuit
Solutions
AR
VVI
B
onBEBBBQ 10
50
7.02.1)(
mAAII BQCQ 1)10(100
VRIVV CCQCCCEQ 6)6)(1(12
kI
Vr
CQ
T 6.21
)026.0)(100(
VmAV
Ig
T
CQm /5.38
026.0
1
4.11
B
Cms
ov Rr
rRg
V
VA
1.
2.
3.
4.
5.
6.
Hybrid- Model and Early Effect
transconductance parameter
current gain parameter
ro=VA/ICQ
ro = small-signal transistor output resistanceVA = early voltage
Early Voltage (VA)
Hybrid- Model and Early Effect
Early Voltage (pg 299)
Basic Common-Emitter Amplifier Circuit
vs
RS
R1
R2
RC
CCvO
VCCExample
Given : = 100, VCC = 12V
VBE(on) = 0.7V, RS = 0.5k,
RC = 6k, R1 = 93.7k, R2 = 6.3k
and VA = 100V.
Calculate the small-signal voltage gain.
Como RrVgV
s
S
VRrRR
rRRV
21
21
Co
S
ms
ov Rr
RrRR
rRRg
V
VA
21
21
Coo RrR
rRRRi 21
B C
E
R1 \\ R2
Vs
RS
RCrOr gmV
Vo
Ri Ro
Solution
Small-signal equivalent circuit
Self-Reading
Textbook: Donald A. Neamen, ‘MICROELECTRONICS Circuit Analysis & Design’,3rd Edition’, McGraw Hill International Edition, 2007
Chapter 5:The Bipolar Junction Transistor
Page: 334-339
Chapter 6: Basic BJT Amplifiers
Page: 370-388.
ExerciseThe circuit parameters in Figure are changed to VCC = 5V, R1=35.2kΩ, R2=5.83kΩ, RC=10kΩ and RS =0, β =100, VBE(on) =0.7V and VA =100V. Determine the quiescent collector current and collector-emitter voltage and find the small-signal voltage gain.
Ans: ICQ = 0.21mA, VCEQ =2.9V, Av =-79.1)
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