2. Outline
BJT
Common Emitter
○ Determining quiescent conditions
○ Calculating small signal performance
Voltage Gain
Input Impedance
Output Impedance
Cut-off frequency
Common Collector
Common Base
2
3. npn transistor
simplified structure
Note: Normally Emitter layer is heavily doped, Base
layer is lightly doped and Collector layer has
Moderate doping.
B
C
E
Schematic
Symbol
3
4. • Curve Tracer
Provides a graph of the characteristic curves.
• Digital multimeters (DMM)
Some DMMs measure βDC or hfe.
• Ohmmeter
Transistor Testing
4
5. Small signal models of bjt
b
e
hoe
hie
hrevce
hfeib
vbe
ib ic
vce
c
e
+
_
+ +
_ _
h-parameter model
Hybrid-π model
5
8. Features of Common Emitter
High voltage gain
High current gain
Medium input impedance due to high
current gain
High output impedance. For HF
capacitive loading will need to be
resonated reducing bandwidth.
Bad HF & bandwidth as falling beta with
frequency reduces gain.
8
9. Input Impedance
IN
IN
IN
i
v
r
iIN iB
iRB
Input impedance, rIN, is the ratio of the
small signal input voltage and the small
signal input current
BRBIN iii
B
IN
RB
R
v
i
mINC
B
gvi
i
9
11. Output Impedance
One way to measure rOUT is:
Short the input to 0 V
Output now looks like just rOUT
11
12. Output Impedance (cont)
00 CIN iv
Applying Kirchoff’s current law:
RCOUTOUTRCC iiiii 0
RC
OUT
C
RC
C
i
v
R
i
v
By Ohm’s law:
CC
RC
OUT
OUT
OUT
OUT RR
i
v
i
v
r
VCC
12
13. Capacitors
Capacitor COUT is needed to remove
the d.c. component of the collector
voltage
Capacitor CIN is needed to allow the
base voltage to be offset from 0V
In both cases this is known as
coupling
Both capacitors are chosen to look
like short circuits at operating
frequencies
Their reactance will, however,
become significant at low frequencies
VCC
0 V
13
14. 14
Frequency response
Midband:
The frequency range of interest for amplifiers
Large capacitors can be treated as short circuit and small capacitors can be
treated as open circuit
Gain is constant and can be obtained by small-signal analysis
Low-frequency band:
Gain drops at frequencies lower than fL
Large capacitors can no longer be treated as short circuit
The gain roll-off is mainly due to coupling and by-pass capacitors
For calculation we use dominant pole approximation
○ If there is a dominant pole, the cutoff frequency is determined mainly by this pole.
High-frequency band:
Gain drops at frequencies higher than fH
Small capacitors can no longer treated as open circuit
The gain roll-off is mainly due to parasitic capacitances of the MOSFETs and
BJTs
18. Determining the lower 3-dB frequency
Coupling and by-pass capacitors result in a high-pass frequency
response with three poles
The lower 3-dB frequency is simply the highest-frequency pole
if the poles are sufficiently separated
The highest-frequency pole is typically ωp2 due to the small
resistance of RE
An approximation of the lower 3-dB frequency is given by
Selecting values for the coupling and by-pass capacitors
These capacitors are typically required for discrete amplifier designs
CE is first determined to satisfy needed fL
CC1 and CC2 are chosen such that poles are 5 to 10 times lower than fL
18
r
f
C
C
CC
10
2
1
2,1
19. Common Base (CB)
Current gain of approximately 1 (alpha)
Low input impedance
(due to low current gain)
High output impedance
High voltage gain
(if input impedance matched)
Good HF & bandwidth as falling beta
with frequency matters less.
19
20. Common Collector (CC)
Voltage gain of almost exactly 1
High current gain
High input impedance
(due to high current gain)
Low output impedance (Good for
unknown loads)
Good HF & bandwidth as falling beta
with frequency matters less.
20
