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Lab 6–DC and AC Analysis of BJT-Amplifier Circuits
Circuits and Electronics 1 (Swinburne University of Technology)
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Swinburne University of Technology
Faculty of Science, Engineering and Technology
2021-HS1-EEE20006-Circuits & Electronics 1
Lab 6 – DC and AC Analysis of BJT-Amplifier Circuits (Multisim Based)
Student Name:
.
Assessment -This lab section has a weighting of 5% of the total subject marks. Preliminary work is
worth 20% of this lab. Assessment is based on demonstration in the NI Multisim and correct
completion of this lab sheet. The experiment is to be completed individually, and then the lab sheet
should be submitted in CANVAS.
1. Aims
To study DC and AC analysis of BJT-amplifier circuits.
2. Introduction
2.1 Transistor
Transistor is a kind of active and three-terminal device, which is a fundamental component for the
integrated circuits found in electronics and computers. There are two types of transistors: bipolar
junction transistors (BJTs) and field-effect transistors (FETs). In our lab session, we only consider BJTs
which are still commonly used today. There are two sorts of BJTs: NPN and PNP. Each type has three
terminals, designed as emitter (marked as E), base (B), and collector (C).
In DC analysis of the NPN transistor, the currents and voltages are illustrated as in Fig. 1. Applying KCL
and KVL gives:
I E =I B+ I C
V CE +V EB +V BC =0
Figure 1 Currents and voltages of NPN transistor
When transistors operate in the active mode, we have
I C =β DC I B
(1)
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Lab 6 – DC and AC Analysis of BJT-Amplifier Circuits
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where
β DC is the DC current gain.
In AC analysis of the NPN transistor, the transistor works around the the quiescent ( Q ) point -- (
I CQ ,V CEQ ), and we can use a simplified r -parameter model to equivalently reprent the
transistor in AC circuit. For the convenience of illustration, here the relation of the transistor symbol
to r -paramter model is given in Fig 2.
Figure 2 The relation of transistor symbol to
It should be noted here that
r 'e =
[
26 mV T +273 ℃
IE
298 ℃
( 2)
r -paramter model
r 'e , the internal emitter resistance, can be evaluated by
]
where I E is the (DC) emitter current at the
our later simulations, T =27 ℃ ).
Q -point,
T
is the ambient temperature. (In
2.2 Equipments
Oscilloscope
Resistors
Capacitors
Potentiometer
DC power supply
AC power supply
Transistor
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Lab 6 – DC and AC Analysis of BJT-Amplifier Circuits
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3. Preliminaries (20% of this lab.)
Workings for your preliminaries must be shown in the space provided.
Figure 3 AC voltage amplifier
3.1 Consider the AC voltage amplifier in Fig. 3, please give the DC equivalent circuit below: (5 marks)
3.2 The DC circuit obtained in 3.1 can be equivalently replaced by the following circuit. Please
calculate the RTh and V Th . (5 marks)
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Lab 6 – DC and AC Analysis of BJT-Amplifier Circuits
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3.3 Please give the AC equivalent circuit of the AC voltage amplifier in Fig. 3 below: (5 marks)
(Please keep the capacitors in the AC equivalent circuit.)
3.4 Capacitor C1 and C3 play critical roles in the above AC voltage amplifier, please explain
why we need to add these two capacitors in our circuit design. (5 marks)
In this circuit all the output goes through C3 while all of the input goes through C1. This can prevent
any Dc elements performance and power amplidyers and distortion is increased due to DC signals.
These two capacitors need to be added because a capacitor allows high frequency AC signals pass
while blocking low frequency DC signals. To low frequency components a for AC signals a capacitor
displays low impedence, while for DC signals it displays high impedence.
4. DC Analysis of BJT Circuit
Construct the circuit in Fig. 4.
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Lab 6 – DC and AC Analysis of BJT-Amplifier Circuits
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Cs
Bs
Es
Figure 4 AC voltage amplifier
(a) Measure the DC voltage V BE =¿ 647mV(ms), the DC current
DC current I B=¿ 10.2yA(ms).(9 marks)
I E =¿
2.16mA(ms) and the
(b) Please only use the measured voltage and and currents in (a) to calculate the DC current
and the DC current gain β DC below: (6 marks)
IC
5. AC Analysis of Single Stage Amplifier
Considering the amplifier in Fig. 4, we first adjust the amplitude of
multimeters to measure the AC currents at node B and C.
V s to be 0.01V and then use
(a) Based on the above measurements, please estimate the AC current gain
2021-HS1-EEE20006-Circuits & Electronics 1
β ac below: (5 marks)
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Lab 6 – DC and AC Analysis of BJT-Amplifier Circuits
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(b) Recover the initial settings of V s (with amplitude 0.1 volts
Replace the BJT in your AC equivalent circuit in 3.3 with a simplified
give the modified circuit below: (5 marks)
and frequency 1 kHz ).
r -parameter model. Please
(c) It can be seen from the property of the BJT in our simulation environment, namely Multisim, that
'
the BJT works at 27 ℃ . Please use equation (2) to estimate the internal emitter resistance r e
in (b). (5 marks)
(d) Assume that the AC voltage across R7 is denoted by V out , the AC voltage across R2 is
denoted by V ¿ and the sinusoidal input signal is denoted by V s . Please derive the following
transfer functions:
V out ( ω )
V ¿ ( ω)
(3)
H oi ( ω )=
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Lab 6 – DC and AC Analysis of BJT-Amplifier Circuits
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H is ( ω )=
V ¿ (ω )
V s (ω )
(4 )
V out ( ω )
V s ( ω)
(5 )
H os ( ω )=
[Please be noted that all the quantities in (3-5) are stated in phasor forms, and thus your analytical
expressions of these transfer functions should also be in phasor forms .] (15 marks)
(e) Tune the resistance of the potentiometer
R6
H os ( ω )
to such a value that the magnitude of the
is equal to 1. Please measure the required
(based on results in (a), (c) and (d) ) below: (10 marks)
(f) Tune the resistance of the potentiometer
R6
R6
and also calculate this resistance
to such a value that the magnitude of the
H os ( ω ) is equal to 2. Please measure the phase angle ∠ H os ( ω ) and also calculate this angle
(based on results in (a), (c) and (d) ) below: (10 marks)
(g) Continue the operations in (f).
(i) Please derive the following transfer function when the bypass capacitor
cancelled.
C2
is
(5 marks)
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Lab 6 – DC and AC Analysis of BJT-Amplifier Circuits
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V out ( ω )
V s ( ω)
(6 )
H 'os ( ω )=
(ii) Please measure the magnitude of H os ( ω ) and also calculate the this magnitude
(based on
results in (a), (c) and (6) ) below: (10 marks)
'
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Lab 6 – DC and AC Analysis of BJT-Amplifier Circuits
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