Download emt 112 / 4 analogue electronics i

EMT 112 / 4 Analogue Electronics I
EMT 112 / 4 ANALOGUE ELECTRONICS I
BIPOLAR JUNCTION TRANSISTOR AMPLIFIER
A. Current Relationships
1. Explain, with the help of an example, the meaning of the term ‘biasing’.
2. What are the bias voltages need to be applied to an npn bipolar transistor such that
the transistor is biased in the forward-active, cut-off and saturation modes of
operations?
3. With the aid of a diagram, state the relationships between collector, emitter and
base currents in a bipolar transistor biased in the forward-active mode.
4. In a bipolar transistor biased in the forward-active mode, the base current is
iB  6.0A and the collector current is iC  510A . Determine  ,  and i E .
1
january 14, 2008
EMT 112 / 4 Analogue Electronics I
B. BJT Common-Emitter Amplifiers
5. The transistor parameters for the circuit in Figure 1 are   120
and VBE ( on)  0.7V .
The circuit elements are VCC  5V , VBB  2V RB  200k and RC  4k .
Figure 1
Determine:
(a) base current, IB
(b) collector current, IC
(c) collector-emitter voltage, VCE
2
january 14, 2008
EMT 112 / 4 Analogue Electronics I
6. The transistor parameters
and VEB( on)  0.7V .
for
the
circuit
in
Figure
2
are
  80
The circuit elements are V   5V , VBB  2.8V and RB  325k .
Figure 2
Given that VEC  2V , determine:
(a) base current, IB
(b) collector current, IC
(c) emitter current, IE
(d) collector resistor, RC
3
january 14, 2008
EMT 112 / 4 Analogue Electronics I
7. The circuit elements in Figure 4 are VCC  12V , VBB  8V ,
RC  0.4k , RE  1.2k and RB  30k .
Figure 3
Let   75 and VBE ( on)  0.7V .
(i)
Determine:
(a)
(b)
(c)
(d)
(ii)
base current, IB
collector current, IC
emitter current, IE
collector-emitter voltage, VCE
Sketch the DC load line
4
january 14, 2008
EMT 112 / 4 Analogue Electronics I
8. Assuming   100 , design the circuit in Figure 5 such that IC = 1.5 mA and VC =
+ 4 V.
Figure 4
5
january 14, 2008
EMT 112 / 4 Analogue Electronics I
9. Given that   120 , VCC = 5 V and VBE(ON) = 0.7 V for the circuit shown in
Figure 6, design the circuit such that IC Q = 0.25 mA and VCEQ = 2.5 V.
Figure 6
6
january 14, 2008
EMT 112 / 4 Analogue Electronics I
10. Refer to Figure 6, given RE=0.6kΩ, RC=5.6kΩ, β=120, VBE(on)=0.7V, R1=250kΩ,
R2=75kΩ.
a)
For VA=∞, determine small-signal voltage gain, Av
b)
Determine the input resistance looking into the base of the transistor.
(Ans: Av= -8.27 and Rib=80.1kΩ)
Figure 6
Draw Thevenin equivalent circuit. Refer to Amplifier dc equation slide in BJT Commonemitter lecture note. Label all the current and voltages for the circuit.
We know that,
I CQ   I BQ
I EQ  (1   ) I BQ
The given circuit has RE connected at emitter terminal. Apply KVL to base-emitter loop,
we will get:
VTH  I BQ RTH  V BE ( on)  I EQ R E  0
VTH  I BQ RTH  V BE ( on)  (1   ) I BQ R E  0
I BQ RTH  (1   ) R E   VTH  V BE ( on)
I BQ 
VTH  V BE ( on)
RTH  (1   ) R E
If we need to find VCEQ,
Apply KVL around collector-emitter loop for common-emitter circuit yield to,
7
january 14, 2008
EMT 112 / 4 Analogue Electronics I
VCC  I CQ RC  VCEQ  I EQ RE  0
VCEQ  VCC  I CQ RC  I EQ R E
 VCC   I BQ RC  (1   ) I BQ R E
 VCC  I BQ  RC  (1   ) R E 
Thevenin resistance, RTH and thevenin voltage, VTH are given as below:
8
january 14, 2008
EMT 112 / 4 Analogue Electronics I
***Assignment 1***
11. Let β=100, VBE(on)=0.7V and VA=100V.
Determine:
a) Small-signal voltage gain
b) Input resistance seen by the signal source, Rin
c) Output resistance looking back into the output terminal, Ro
(Ans: ICQ = 0.439mA, Av= -148, Rin=6.09kΩ, Ro = 9.58kΩ)
9
january 14, 2008
EMT 112 / 4 Analogue Electronics I
Figure 7
10
january 14, 2008
EMT 112 / 4 Analogue Electronics I
***Assignment 1***
12. Assume that β=100, VA=∞, R1=10kΩ and R2=50kΩ for the circuit in Figure 8.
a. Plot the Q-point on dc load line
b. Determine the small-signal voltage gain
(Ans: ICQ=1.19mA, VECQ = 8.42V and Av= -1.94)
Figure 8
11
january 14, 2008
EMT 112 / 4 Analogue Electronics I
For small-signal equivalent circuit shown above, make sure label all the current, voltages
and Ri and Rib.
100(0.026)
 2.18k
1.19
VO  g mV RC
Write KVL from input around B-E loop,
r 
12
january 14, 2008
EMT 112 / 4 Analogue Electronics I
V
V S  V     g mV
 r
given g m r   ,
V 

 R E

 VS
1  
 R E
1  
r
  
Voltage gain, Av
 RC
Av 
 1.94
r  (1   ) RE
***Assignment 1***
13. Refer to Figure 9, given β=180, VA=∞.
a. Find ICQ and VCEQ
b. Plot dc and ac load lines
c. Calculate small-signal voltage gain.
(Ans: ICQ= 15.6mA, VCEQ= 10.1V, rπ=0.30kΩ, Rib=34.2kΩ and Av= 0.806)
Figure 9
13
january 14, 2008
EMT 112 / 4 Analogue Electronics I
RTH  R1 R2  5k
 R2  
 10 
(V  V  )  V   
VTH  
18  9  0
 10  10 
 R 1  R2 
VTH  V BE  V 
0  0.7  9
I BQ 

 0.0869mA
RTH  (1   ) R E 5  181(0.5)
I CQ  15.6mA , I EQ  (1   ) R E  15.7 mA
VCEQ  18  (15.7)( 0.5)  10.1V
14
january 14, 2008
EMT 112 / 4 Analogue Electronics I
C. BJT Common-Base Amplifiers
14. The transistor parameters for circuit in Figure 10 are   75 and VEB(ON )  0.7V .
Design the common-base circuit such that IEQ = 0.25 mA and VECQ = 2 V.
Figure 10
15
january 14, 2008
EMT 112 / 4 Analogue Electronics I
15.
For the circuit shown in Figure 11, the measured value of VC is +6.34 V.
Figure 11
Determine:
(a)
(b)
(c)
(d)
(e)
(f)
base current, IB
collector current, IC
emitter current, IE
collector-emitter voltage, VCE
common-emitter current gain, 
common-base current gain, 
16
january 14, 2008
EMT 112 / 4 Analogue Electronics I
16. Assuming   50 for the circuit shown in Figure 12, determine:
(a) base current, IB
(b) collector current, IC
(c) emitter current, IE
(d) emitter-collector voltage, VEC
Figure 12
17
january 14, 2008
EMT 112 / 4 Analogue Electronics I
***Assignment 1**
17. For the circuit shown in Figure 13, the transistor parameters are   100 ,
VBE (ON )  0.7V , V A   .
The circuit elements are VCC  VEE  10V , RB  100k , RE  10k ,
RC  10k , RL  1k and RS  1k .
Figure 13
(a) Determine the small-signal transistor parameters:
(i) g m
(ii) r .
(iii) ro
io
.
ii
v
(c) Determine the small-signal voltage gain, Av  o
vs
(d) What is the input resistance, Ri ?
(e) Find the output resistance, Ro .
(b) Determine the small-signal current gain, Ai 
18
january 14, 2008
EMT 112 / 4 Analogue Electronics I
19
january 14, 2008
EMT 112 / 4 Analogue Electronics I
20
january 14, 2008
EMT 112 / 4 Analogue Electronics I
18. For the circuit shown in Figure 14, let   100 , VBE (ON )  0.7V , V A   ,
VCC  VEE  5V , RC  RL  2k , CB  0F and RS  0 .
Design RE and RB for a dc quiescent collector current of 1mA and a small-signal
voltage gain of 20.
21
january 14, 2008
EMT 112 / 4 Analogue Electronics I
D. BJT Common-Collector Amplifiers
19. For the circuit in Figure 15, the transistors parameters are   100 ,
VBE (ON )  0.7V and VA  125V . Assume RS  0 and RL  1k .
RS
Figure 15
(a) Design a bias-stable circuit i.e. determine the values of RE , R1 and
R2 such that I CQ  125mA and VCEQ  4V .
io
.
ii
(c) What is the output resistance looking back into the output
terminals, Ro ?
(b) Determine the small-signal current gain, Ai 
22
january 14, 2008
EMT 112 / 4 Analogue Electronics I
23
january 14, 2008
EMT 112 / 4 Analogue Electronics I
24
january 14, 2008
EMT 112 / 4 Analogue Electronics I
***Assignment 1**
20. For the circuit shown in Figure 16, VCC  5V
VBE (ON )  0.7V , VA  100V , RE  1k , R1  25k and R2  50k .
,
  120
,
Figure 16
(a) Determine the small-signal voltage gain, Av 
vo
.
vs
(b) Determine the input resistance looking into the base of the
transistor, Rib
25
january 14, 2008
EMT 112 / 4 Analogue Electronics I
26
january 14, 2008
EMT 112 / 4 Analogue Electronics I
27
january 14, 2008
EMT 112 / 4 Analogue Electronics I
28
january 14, 2008