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UNIVERSITY OF CALIFORNIA AT BERKELEY
College of Engineering
Department of Electrical Engineering and Computer Sciences
EE140: Lab 1
Due: Saturday March 15th, 2008
8am in the HW drop box
Instruction
For this lab, you may consult the professor, the TAs, the textbook, and any other inanimate
objects, with the exception of your peers' lab reports, for reference. You may obtain data in
pairs, but must submit your own written report. Be concise.
Objective
This lab focuses on the analysis of a simple two stage bipolar op-amp, shown in Figure 1 below.
Your goal will be to characterize the circuit’s low frequency performance by measuring the DC
bias current, common mode and differential voltage gains, and the offset voltage. This lab also
introduces feedback and the effects of resistive load on an amplifier.
Figure 1: Lab 1 Op-Amp
Preliminaries
For this lab, the only things that you need to know about bipolar transistors are:
1) Ic = IS(eVbe/Vt-1)
2) IB = IC/
3) gm = IC/VT
4) ro = VA/IC
5) r = VT/IB =  /gm
You can get the values for IS, , VA and from the datasheets (or the spice models below, which
come from the datasheets)
http://www.fairchildsemi.com/ds/2N/2N3904.pdf
http://www.fairchildsemi.com/ds/2N/2N3906.pdf
NPN model Q2N3904
.model ee140_npn npn (Is=6.734f Xti=3 Eg=1.11 Vaf=74.03 Bf=416.4
Ne=1.259 Ise=6.734 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 model Q2N3906
.model ee140_pnp pnp (Is=1.41f Xti=3 Eg=1.11 Vaf=18.7 Bf=180.7 Ne=1.5
Ise=0 Ikf=80m Xtb=1.5 Br=4.977 Nc=2 Isc=0 Ikr=0 Rc=2.5 Cjc=9.728p
Mjc=.5776 Vjc=.75 Fc=.5 Cje=8.063p Mje=.3677 Vje=.75 Tr=33.42n
Tf=179.3p Itf=.4 Vtf=4 Xtf=6 Rb=10)
The only difference between our simple low frequency small signal model for the BJT and the
MOSFET is the addition of r between the base and emitter. When we drive the base with a
voltage source, this is irrelevant.
Note that for large signal analysis when calculating bias points, the base and emitter will never
be more than ~0.7 volts apart if the base/emitter diode is forward biased.
Prelab and Homework 5
1. Read all the lab tutorials in the course handouts section on the course website.
2. Differential amplifier analysis. Assuming R1 = R2 = 1kΩ and Rtail = 5.1kΩ.
a. Plot Vtail, Itail, IC2, IB2, gm2, and Vout1 versus the input common mode voltage
for Vi,CM= -9 to 9V. (if you do it right, this is just 2 curves with many different
axis labels)
b. Calculate and label the values at Vi,CM={-7, 0, 7} volts.
c. Calculate the value for ro2 at Vi,CM={-7, 0, 7} volts. Do we need to consider ro2
when calculating the first stage gain?
d. Calculate the value for the first stage differential voltage gain, AVDM1, at Vi,CM={7, 0, 7} volts
e. Use HSPICE and .OP to verify all of your hand calculations at Vi,CM={-7, 0, 7}
volts. Comment on any values that are different by more than 10%.
3. Second stage analysis.
a. Calculate the collector current IC3 required for the PNP BJT drives the second
stage output to Vout2 = {-8, 0, 8} volts.
b. Calculate the base/emitter bias voltage VBE3 required to achieve those 3 output
bias voltages, and the corresponding output resistance ro3 and input resistance r
of Q3 at those bias voltages. Do we need to consider the output resistance of Q3
in the voltage gain calculation of the second stage? Do we need to consider the
input resistance of Q3 in the voltage gain calculation of the first stage?
c. Calculate the second stage gain AV2 at Vout2 = {-8, 0, 8} volts.
d. Use HSPICE and .OP to verify your hand calculations at Vout2 = {-8, 0, 8} volts.
Comment on any values that are different by more than 10%.
4. 2 stage analysis.
a. What is the current through R1 that is necessary to produce Vout2 = 0 (you
calculated the necessary Vbe3 in the last problem)? How does that compare to the
current is flowing out of the base of Q3, Ib3?
b. What value of Vi,CM will give the right current through R1 to set Vout2=0?
c. In question 2, you calculated the values of Vout1 at Vi,CM = {-7, 0, 7}. How far are
these values from the value for Vout1 necessary to give Vout2 = 0? You know the
first stage voltage gain at these common mode input points. Calculate the input
offset voltage (i.e. the differential input voltage necessary to set Vout2=0) at VI,CM
= {-7,0,7}.
d. Use HSPICE and .DC to sweep the positive input (which one is it?) from -9 to 9
volts in 1mV steps, while keeping the negative input constant at {-7, 0, 7} volts.
i. Plot the tail voltage and first and second stage output voltages.
ii. On a separate plot, plot Vout2 and use the “measure/point” command in
awaves to label the gain at Vout2 = {-8, 0, 8} volts on each of the three
curves. How do these gains compare to the product of the corresponding
gains that you calculated above? Do all of them exist? Why not?
iii. When Vout2=0, what is the positive input voltage? The difference between
the positive and negative inputs is the input offset voltage. How does it
compare to the values that you calculated above?
5. Feedback
a. Use two 10k resistors to put the amplifier into feedback with a gain of (positive)
two. Use HSPICE to plot the input/output, sweeping the input from -9 to 9. Use
“measure” in awaves to find the gain at Vout={-8, 0, 8} volts.
b. Use two 10k resistors to put the amplifier into feedback with a gain of minus 1.
Use HSPICE to plot the input/output, sweeping the input from -9 to 9. Use
“measure” in awaves to find the gain at Vout={-8, 0, 8} volts.
Lab
1. Check your kits to make sure you have the following
1
2
2
1
PNP model Q2N3906
NPN model Q2N3904
1kΩ
5.1kΩ
Figure 2: BJT Pin Configuration
2. Build the circuit in Figure 1using R1 = R2 = 1kΩ and RS = 5.1kΩ.
3. Measurements
a. DC Biases
i. Ground the inputs and measure I1, I2, I3, and Vout1 and Vout2.
b. Common Mode Response
i. Use the HP4155 Parameter Analyzer to measure Vout vs. Vin,cm: connect Vin1 to
Vin2 and sweep the input from -9V to 9V. Use a VM (voltage measurement)
terminal to measure Vout. Do not forget to include ground while using the
parameter analyzer.
c. DC Offset Voltage
i. Use the Parameter Analyzer to measure Vout vs. Vin,cm. connect Vin1 to Vin2 and
sweep the input from -9V to 9V.
Use a VM (voltage measurement) terminal to measure Vout.
Do not forget to include ground while using the parameter analyzer.
ii. Graph Vos vs. Vin,cm.
d. Differential Mode Response
i. Tie Vin2 to 0V.
ii. Use the function generator to apply a 50mV amplitude 1kHz sine wave to Vin1.
Make sure to add an offset equal to your extracted the Vos for Vin,cm = 0V.
iii. Make sure the disable button is off.
iv. Use the oscilloscope to measure the peak-to-peak voltage of Vin1 and Vout.
v. Repeat with Vin2 = -1V and 1V.
vi. Estimate Adm for the circuit for Vin,cm = -1V, 0V, 1V
e. Resistive Feedback
i. Connect a 510Ω resistor between Vout and Vin2.
ii. Using the Parameter Analyzer, sweep Vin1 from -9V to 9V.
iii. Now disconnect the feedback resistor and connect it between Vout and Vin1.
iv. Sweep Vin2 from -9V to 9V.
f. Varying the Resistive Load.
i. Keep R1 a constant 1KΩ while varying R2.
Extract Acm and Vos for several different R2 values.
ii. Repeat the measurements for a constant R2 of 1KΩ while varying R1.
Postlab
1. Provide two schematics of the circuit, one with the hand calculated and the other with
simulated DC bias voltages and nominal currents. How do the measured values of currents
compare with your hand calculations and SPICE simulation?
2. Use your measurements to extract Acm vs. Vin,cm.
How do your results compare to the SPICE simulation?
3. How close are your measured values of Vos to your simulated ones? (Error percentage)
Why does our circuit have an offset?
4. Estimate the the differential mode gain from your data.
5. Find the gain for the two feedback circuits. Are they different? Explain.
6. Graph Acm vs. R1 (holding R2 constant) and Acm vs. R2 (holding R1 constant).
7. How well does this circuit work as an op-amp?
How can we improve the performance of this circuit (without scraping the entire circuit)?
Deliverables (by 8am on Saturday, March 15th in the homework drop box.)
Prelab: spice deck, hand calculations, plots of Acm vs. Vin,cm, Vos vs. Vin,cm, Adm vs. Vin,cm.
Lab: values/graphs for, Vos vs. Vin,cm, Adm vs. Vin,cm
Postlab: schematic x2, Acm vs. Vin,cm, Acm vs. R1, Acm vs. R2, postlab questions
Remember to explain, or at least try to explain, all discrepancies between your calculations and
measurements.
Note: No late reports accepted. Legibility is required. Succinctness is strongly encouraged.