EE12: Laboratory Project-II
ECE Department, Tufts University
Spring 2008
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Objective
This laboratory exercise provides the student with the opportunity to experience the process of designing analog circuits based on given specification,
verifying the design with PSpice and final verification by building the circuit
in the laboratory.
Vdd +12V
R2
RB
Q2
C2
C1
vi
vo
Q1
RC
Zo
Zin
R1
RE
Zee
Zbb
Figure 1: Circuit Diagram of the voltage amplifier
Q1 = 2N3904 Q2 = 2N3702 C1 = 1µF
C2 = 1µF
R1 = 51kΩ
R2 = TBD
RB = 12kΩ RC = TBD
RE = 1.8kΩ
Table 1: Parts List
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Procedure
DC Bias
For the circuit shown in Figure-1, determine:
• R2 such that the amplifier Q-point is optimal for symmetric input.
• RC such that the amplifier has a voltage gain of +3.
• Estimate the bias voltages and currents for nominal β and nominal
value resistors. Nominal values of β can be assumed to be the value of
the PSpice model parameter Bf .
• Estimate the maximum and minimum bias voltages and currents. The
max. and min. β for the transistors are extracted from the data-sheet.
Assume resistors to vary by ±5%.
• Determine the maximum amplitude of sinusoidal input for which the
output is undistorted.
• Calculate the nominal power dissipated by transistor Q1 and Q2 and
ensure it’s well within the data-sheet limit. Power dissipated by the
transistor is given by the expression Pd = VCE · IC .
Small-Signal Analysis
• Estimate nominal β1 and β2 for transistors Q1 and Q2 respectively from
the PSpice model parameter Bf .
• Calculate rπ1 and rπ2 . Assume VT = 25mV .
• Calculate the output impedance (ro2 ) for Q2 . Extract Early-voltage
from the PSpice model (parameter Vaf).
• Using the small-signal parameters of Q1 and Q2 draw small-signal
equivalent circuit for the amplifier. Estimate small-signal voltage gain
Avo . Also estimate the impedances: Zin , Zbb , ZO and Zee .
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PSpice Analysis
• For simulating the amplifier in PSpice, you have two options:
– Draw the schematic using any schematic capture software.
– Write the netlist as a text file and compile using PSpice.
• Similarly for the PSpice models for the transistors:
– Use the provided models if you are using the text mode.
– Use the in-build model (if available) in the schematic capture program.
• Do a DC simulation to determine the quiescent voltage and currents.
• Use AC simulation to determine the small-signal voltage gain AV . Also
determine the impedances Zin , Zbb , ZO and Zee .
Laboratory Experiment
• Construct the amplifier using the closest RMA values for the resistors.
For Q1 , if 2N3904 is not available, it can be substituted with 2N3704
without noticeable change.
• Measure the quiescent voltages and currents in the circuit.
• Measure the voltage gain AV and impedances Zin , Zbb , ZO and Zee .
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The Report
• The report is to be a presentation document. It is to be typed and
have acceptable figures. If the figures are hand drawn, they should be
made using ruler.
• Restate, in your own words, the objective of the experiment.
• Provide a theory of operation of the circuit using clear language.
• Show all calculations used to determine AV , Zin , Zbb , ZO and Zee .
• Include the PSpice file used to determine AV , Zin , Zbb , ZO and Zee by
simulation.
• State how circuit measurements were done.
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• Show in a table the circuit quiescent values calculated from a paper
model, the quiescent values from PSpice simulation and the measured
values. Also show the percentage error of each calculated value from
the simulated one.
• Show in a different table the values of AV , Zin , Zbb , ZO and Zee determined by each method and their percentage error from the measured
value.
• Comment on your results. In particular, speculate as to the causes of
errors in the paper model and the PSpice simulation.
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PSpice Models
NPN: 2N3904
.model 2N3904 NPN(Is=6.734f Xti=3 Eg=1.11 Vaf=74.03 Bf=416.4 Ne=1.259
+ Ise=6.734f Ikf=66.78m Xtb=1.5 Br=.7371 Nc=2 Isc=0 Ikr=0 Rc=1 +
Cjc=3.638p Mjc=.3085 Vjc=.75 Fc=.5 Cje=4.493p Mje=.2593 Vje=.75 +
Tr=239.5n Tf=301.2p Itf=.4 Vtf=4 Xtf=2 Rb=10)
PNP: 2N3702
.model 2N3702 PNP(Is=650.6E-18 Xti=3 Eg=1.11 Vaf=115.7 Bf=133.8
Ne=1.832 + Ise=97.16f Ikf=1.081 Xtb=1.5 Br=3.73 Nc=2 Isc=0 Ikr=0
Rc=.715 + Cjc=14.76p Mjc=.5383 Vjc=.75 Fc=.5 Cje=19.82p Mje=.3357
Vje=.75 + Tr=114.1n Tf=761.3p Itf=.65 Vtf=5 Xtf=1.7 Rb=10)
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