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ECEN 3314 Electronic Devices and Applications
Fall 2007
Lab. 6 BJT Amplifier
1. Objective
To investigate the principles of BJT amplifier circuits, learn the BJT bias-stability design
to compensate for process and temperature variation, and implement common-emitter (CE) and
common-base (CB) BJT amplifiers circuit.
2. Components Required
BJTs, resistors, capacitors and circuit board.
3. Equipment Required
Curve tracer
DC power supply
Oscilloscope
Function generator
Digital multimeter.
4. Topics
I-V curve of npn and pnp BJTs.
DC bias point of an npn BJT amplifier.
CE BJT amplifier.
CE BJT amplifier with an emitter degeneration.
5. Pre-lab
1) Simulate circuit 1 with DC sweep, the primary is V1 (0 to 15V, increment 0.05V), the
secondary sweep is I1 (10 mA to 200 mA, increment 10 mA). The BJT can be found in
the library ‘ Bipolar ’. Q2N2222 is a widely used general purpose npn BJT, its forward
current gain in the PSpice model is b=255.9. Note this is maximum beta in SPICE and
the actual Beta will vary from 50 to 250 across various operating points.
2) Simulate circuit 2 to verify a stable operating point after finding after find a stable Qpt at
ICQ = 4 mA, VCEQ 8 to 10 V and VRE = 2.4V. Assume 12 V for VCC. Confirm Qpt
stability by simulating you Qpt design at -25 C, 50 C and 125 C.
3) Simulate circuit 2 to find the relationship between the output and input voltages verse
frequency from 25 Hz to 10 MHz. Use VCC equal 15 V. Select a load CL to achieve a 1
MHz bandwidth and a collector resistor, RC to establish a midband gain of -160. Show
the input and output currents (ac ib and ic and ac vbe, vin and vo) and voltages in
different windows ( they are different in orders of magnitude, so it is hard to show them in the same
window ). This example shows that a small base current can control a large emitter current.
From a power perspective, ib x vbe and ic x vo, it is a power amplifier. Note for a 4 Vpp
output the input must be less than 20 mV. How will you develop a 20 mV signal in lab?
Circuit 2 is a simple BJT amplifier, the output can be quite difficult, clipping, distortion,
high gain variation across temperature if you are not careful about the DC bias point (Q-
point). The critical parameters are ICQ and VCEQ. Ideally it should be approximately ½ of
VCC. Using small signal analysis find the midband input and output impedance, verify by
simulation.
4) ( Emitter Degeneration ) From the above step you have realized that the amplifier circuit
is unreliable, as the current gain of the BJT will change with temperature. Now you can
design a BJT amplifier circuit with gain stability via emitter degeneration (circuit 3).
You have specified the current in the path of transistor (through RC and RE) as 4 mA and
the current in the bias path (through RB 1 and RB 2 ) is around 1 mA or >> IBQ. Select an
emitter resistor Re (an un by passed portion of RE) such that the emitter degeneration
circuit if circuit 3 has a gain of -20.
Simulate circuit 3, to find the relationship between the output and input voltages verse
frequency from 25 Hz to 10 MHz. Using small signal analysis find the midband input and
output impedance, verify by simulation. Gain A = gm x RC/(1 + gm Re).
5) Circuit 4 is a common base amplifier, simulate it and find the small signal or ac voltage
gain from 50 to 10 MHz. Note, to convert Circuit 2 to Circuit 4 remove the generator and
generator resistance, 200 ohms and tie the input side of CC1 to ground and apply the
signal generator to ground side of CE.
Simulate circuit 4, to find the relationship between the output and input voltages verse
frequency from 25 Hz to 10 MHz. Using small signal analysis find the midband input and
output impedance, verify by simulation.
6. Laboratory Work
( Demo ) Observe the I-V characteristics of a BJT with the curve tracer.
Construct circuit 2 from the guide lines obtained in the pre lab simulation and verify a
stable operating point at ICQ = 4 mA, VCEQ 8 to 10 V and VRE = 2.4V. Assume 12 V for
VCC. Record your DC output voltage and current through the transistor.
- Construct circuit 2 to find the relationship between the output and input voltages verse
frequency from 25 Hz to 10 MHz. Use VCC equal 15 V. Select a load CL to achieve a 1
MHz bandwidth and a collector resistor, R C to establish a mid band gain of -160. Record
the input and output voltage (DC as well as AC waveforms) for input frequency of 10Hz,
1 KHz, 1 MHz and 20MHz. Explain what is happening in the circuit, by observing the
waveforms recorded.
- ( Emitter Degeneration ) Construct a BJT amplifier circuit with gain stability via emitter
degeneration as shown in circuit 3. You have specified the current in the path of
transistor (through RC and RE) as 4 mA and the current in the bias path (through RB 1 and
RB 2 ) is around 1 mA or >> IBQ. Select an emitter resistor Re (an un by passed portion of
RE) such that the emitter degeneration circuit of circuit 3 has a gain of -20.
Measure the input and output voltages for an input frequency of 10Hz, 1 KHz, 1 MHz
and 20MHz.Verify by measurement that Gain A = gm x RC/(1 + gm Re).
8. Circuits
Circuit 1 I-V characteristics
Circuit 2. Stable BJT amplifier
AC
R
C
R
E C 2
C
3
RL
R
B 1
RB 2
C
1
R 4
10 V
47 u
47 u
47 u
10 K
V
off
V
ampl = 30 mV
Freq = 10 K
Circuit 3. Amplifier with Emitter Degeneration Resistor
AC
RC
RE
C 2
C 3
RL
RB 1
RB 2
C 1
R 4
10 V
500
47 u
47 u
47 u
10 K Voff
V ampl = 30 mV
Freq = 10 K
RDG
Circuit 4. CB BJT amplifier
RC
RE
C
3
R
L
R
B 1
R
B 2
C 1
10 V
47 u
47 u
10 K
Vampl = 30 mV
Freq = 10 K
AC Voff
