1. EKT104 ANALOG ELECTRONIC CIRCUITS [LITAR ELEKTRONIK ANALOG] BASIC BJT AMPLIFIER (PART I)
DR NIK ADILAH HANIN BINTI ZAHRI

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 operating mode
Time-varying output voltage is directly proportional to & larger than time-varying input voltage  linear amplifier
Fig (a) shows the circuit where the input signal vi, contains both ac and dc signal.
Vcc is dc voltage to bias the transistor at particular Q point.
Q point : Steady state voltage or current at a specific terminal of an active device with no input signal applied.
Forward active mode : BE junction is forward bias, where positive terminal connected to P region, and BC junction is reversed bias, where positive terminal is connected to n region.
If time varying input signal is superimposed on dc input voltage, the output voltage will changed.
If the output voltage is directly proportional to or larger than time varying input voltage, then the circuit is linear amplifier.

4. The Bipolar Linear Amplifier
Variable
Meaning
iB, vBE
Total instantaneous values
IB, VBE
DC values
ib, vbe
Instantaneous ac values
Ib, Vbe
Phasor values
Summary of notation

5. Graphical Analysis & AC Equivalent CircuitRB vs vO
VBB
VCC iC
vCE
vBE iB
In figure d, the sinusoidal signal source, vs will produce ac base current superimposed on q point base current.
Then, the base current will induced ac collector current superimposed on q point collector current.
Then, the collector current will produce voltage across Rc and induces ac collector emitter voltage.
The ac collector emitter output voltage will be larger than input signal.
Hence, the circuit is amplifier circuit and amplification occurred.
Figure (c)
(c) Common-emitter circuit with time varying signal source in series with base dc source
Figure (d)
(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 Figure (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:

8. Graphical Analysis & AC Equivalent Circuit
For C-E loop, considering time varying signals:
Base on (6), left side of (11) is 0. So:

9. Graphical Analysis & AC Equivalent Circuit
Definition of small signal
Small signal : ac input signal voltages and currents which 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.

10. 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

11. Graphical Analysis & AC Equivalent Circuit
Equations
Base-emitter loop (Input loop)
Collector emitter loop (Output loop) RC RB vs vO
vce
vbe ic ib + -
Thermal voltage, 0.026
AC equivalent circuit of C-E with npn transistor
(ac equivalent circuit of Figure (c))

12. 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
To do analysis against small signal, small signal equivalent circuit must be developed for the transistor.
One way is to use hybrid-pi model which is similar to the physics of the transistor.
The sinusoidal signal is small and the slope at Q-point can be treated as constant which has unit of conductance.
The inverse of this conductance is the small signal resistance defined as r-pi.
Therefore, the small signal input voltage is the product of small signal current and the small signal resistance.
Transconductance is the ratio of the current variation at the output to the voltage variation at the input.
Small signal hybrid-π equivalent circuit
for npn transistor using
transconductance (gm) parameter

13. Alternative Form of Small-signal Hybrid- Equivalent Circuit
Small signal collector current can be related with small-signal base current and is called as ac common emitter current gain, beta.
The figure shows the same circuit with the previous circuit. The difference is , instead of using transconductance parameter, this circuit use common emitter current gain parameter.
Where beta= ic/ib
Using common-emitter current gain (β) parameter

14. Constructing 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
When incorporating small signal hybrid-pi model of transistor into ac equivalent circuit, it is easier to start with 3 terminals of the transistor.
Then sketch the hybrid-pi equivalent circuit to this terminals.
Connect the remaining circuit elements such as Rb & Rc.
This technique will minimize errors in developing the small signal equivalent circuit. B E C rπ

15. Small-signal Voltage Gainr Vs RB RC Vo Ic Ib
gmVbe
Vbe
Vce + -
Output signal voltage
Input signal voltage

16. Voltage Gain MeasurementRC RB vs vO
VBB
VCC
Example
Given :  = 100, VCC = 12V
VBE = 0.7V, RC = 6k,
RB = 50k, and VBB = 1.2V
Calculate the collector-emitter voltage at q-point and small-signal voltage gain.

17. SOLUTIONS 1. 2. 3. 4. 5. 6.

18. Hybrid- Model and Early Effect
Collector voltage has some effect on collector current
Collector current increases slightly with increases in voltage  Early Effect
Modeled as a linear increase in total current with increases in VCE
The Early effect is the variation in the width of the base in a bipolar junction transistor (BJT) due to a variation in the applied base-to-collector voltage.
In small signal equivalent circuit, collector current is vary with the collector emitter voltage.
The figure shows the small signal model of BJT, which include output resistance due to early effect for the case of tranconductance in upper figure.
And current gain parameter in lower figure.

19. Hybrid- Model and Early Effect
Early Voltage (pg 296)
Early Voltage
(VA)

20. Hybrid- Model and Early Effect
transconductance
parameter
ro=VA/ICQ
The Early effect is the variation in the width of the base in a bipolar junction transistor (BJT) due to a variation in the applied base-to-collector voltage.
In small signal equivalent circuit, collector current is vary with the collector emitter voltage.
The figure shows the small signal model of BJT, which include output resistance due to early effect for the case of transconductance in upper figure.
And current gain parameter in lower figure.
current gain
parameter
ro = small-signal transistor output resistance
VA = early voltage

21. Basic Common-Emitter Amplifier Circuitvs RS R1 R2 RC CC vO
VCC
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.

22. SOLUTION Small-signal equivalent circuit R1 \\ R2 Vs RS RC rO r gmVVo Ri Ro
Ans: ICQ = 0.95mA, VCEQ =6.3V, Av =-163)

23. 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)

24. 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: