1. ANALOG ELECTRONIC CIRCUITS 1
EKT 204
Basic BJT Amplifiers (Part 1)

2. Analog Signals & Linear Amplifiers
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
Low signal power
High signal power
DC power
Block diagram of a compact disc player system
(a)
(b)
Signal
source
Amplifier
Load

3. The Bipolar Linear Amplifier
(a) Bipolar transistor inverter circuit; (b) inverter transfer characteristics
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

4. 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

5. Graphical Analysis & AC Equivalent Circuit
Fig. D RC RB vs vO
VBB
VCC
Fig. C iC
vCE
vBE iB
(C) Common-emitter circuit with time varying signal source in series with base dc source
(D) Common-emitter transistor characteristics, dc load line, and sinusoidal variation in base current, collector current, and collector-emitter voltage

6. Graphical Analysis & AC Equivalent Circuit
Base on Fig. C & D
(time-varying signals linearly related & superimposed on dc values)
If signal source, vs = 0:

7. 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:

8. 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.

9. 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

10. Graphical Analysis & AC Equivalent CircuitRB vs vO
vce
vbe ic ib + -
Equations
Input loop
Output loop
0.026 V
AC equivalent circuit of C-E with npn transistor

11. Small-signal hybrid- equivalent circuit
vbe = ibrπ rπ
= diffusion resistance /
base-emitter input resistance
1/rπ
= slope of iB – VBE curve
gm=ICQ/VT
r=VT/ICQ
Using transconductance (gm) parameter

12. Small-signal hybrid- equivalent circuit
Using common-emitter current gain (β) parameter

13. How to construct Small-signal hybrid-RC RB vs vO
VBB
VCC
We know that
i across B  ib
i across C βib
i across E  (β+1)ib
rπ between B -E
Place a terminal for the transistor
Common Terminal as ground B E C
βib B E C rπ

14. Small-signal equivalent circuit
Small-signal hybrid- equivalent circuit
Small-signal equivalent circuit r Vs RB RC Vo Ic Ib
gmVbe
Vbe
Vce + -
Output signal voltage
Input signal voltage

15. Small-signal hybrid- equivalent circuit
Example
Given :  = 100, VCC = 12V
VBE = 0.7V, RC = 6k,
RB = 50k, and VBB = 1.2V
Calculate the small-signal voltage gain. RC RB vs vO
VBB
VCC

16. Solutions 1. 2. 3. 4. 5. 6.

17. Hybrid- Model and Early Effect
transconductance
parameter
ro=VA/ICQ
current gain
parameter
ro = small-signal transistor output resistance
VA = early voltage

18. Hybrid- Model and Early Effect
Early Voltage (pg 299)
Early Voltage
(VA)

19. Basic Common-Emitter Amplifier Circuit
Example
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. vs RS R1 R2 RC CC vO
VCC

20. Solution Small-signal equivalent circuit R1 \\ R2 Vs RS RC rO r gmVVo Ri Ro

21. Self-Reading
Textbook: Donald A. Neamen, ‘MICROELECTRONICS Circuit Analysis & Design’,3rd Edition’, McGraw Hill International Edition, 2007
Chapter 5:The Bipolar Junction Transistor
Page:
Chapter 6: Basic BJT Amplifiers
Page:

22. Exercise
The 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)