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Department of Physics, Stanford University
Physics 105, Intermediate Laboratory Seminar: Analog Electronics
Lab 6.12
Page 1
Lab 6: Differential Amplifier, JFET’s
Read:
Day 1: Meyer Section 5.2.4, 5.4.4, 5.4.5
Day 2: Meyer Section 5.5 (all)
PRELAB
Part 2.1:
For the differential amplifier in Figure 1, calculate the following:
Differential Gain, Gdiff
(express as a ratio, and in dB)
Common Mode Gain, Gcm
“
CMRR (Common Mode Rejection Ratio), Gdiff/Gcm
“
Quiescent voltage at point “A”
VB, Quiescent for both transistors
IC, Quiescent for both transistors
VOUT, Quiescent
Part 2.6:
Calculate the output of the current source in Fig 3. Remember that the output is
sinking current.
Part 3:
For the variable resistor JFET circuit in Figure 2, check if the 100k potentiometer
can adjust the gate voltage (VGS) from 0 volts all the way to the pinch off, or
threshold, voltage VP (VGS (OFF) on the datasheet). Use the datasheet for the
2N5485 JFET in Coursework. Make sure you account for the entire range of
variability in VP. If the entire range is not available, resize the 330k potentiometer
(a potentiometer is the resistor with an arrow pointing at it, and is just a voltage
divider).
Note on terminology:
VP = pinch off voltage, the voltage at which a depletion mode JFET stops conducting.
VT = threshold voltage, the voltage at which an enhancement mode MOSFET starts conducting.
The two terms are sometimes used interchangeably
Department of Physics, Stanford University
Physics 105, Intermediate Laboratory Seminar: Analog Electronics
Lab 6.12
Page 2
LAB: 1st Day:
Part 1. Push Pull Follower:
(20 pts)
Vcc (+15V)
Make sure you understand why it’s a follower that requires no DC bias,
and why it’s called a “push-pull”. Note on transistor pinout; the
2N3904(npn) and 2N3906(pnp) have the same pinout: EBC from LR.
See 2N3904 spec sheet on Coursework.
If you melt things immediately, you probably got the pinout wrong.
Welcome to the club.
1.
2.
3.
4.
In
Build the circuit of Fig 1 and test it with a 1V peak sine
wave. Submit a waveform plot showing crossover
distortion (input and output waveforms). See Fig 5.39 in
Meyer for an example.
Explain the crossover distortion
Explain why DC offset is not needed at the base.
Change the function generator offset from zero to +1V.
Show the output waveform and explain what’s going on.
Out
470*
1k
*only if needed for
parasitic oscillations
Vee (-15V)
Figure 1
Part 2: Differential Amplifier
(40 pts)
(express gains as both a ratio and in dB; show
prelab calculations where
applicable)
Vcc (+15V)
7.5k
Vout
1.
2.
3.
4.
5.
Build the circuit of Figure 2.
Important: use 1% precision resistors
Vin (1)
for the matched emitter resistors RE .
Measure quiescent voltages and
compare w/ prelab
Measure differential gain—connect a
sine wave to Vin(1)--and compare w/
prelab. Specify the input amplitude you
use. (no waveform plots required—just
present input and output AC amplitude
data).
Switch the input – connect the sine
wave to Vin(2). Explain the difference
at the output compared to #2, above.
Connect the same sine wave simultaneously to
both inputs. Measure common mode gain and
compare w/ prelab
Calculate CMRR and compare w/ prelab.
1uF
1uF
100
Vin (2)
100
A
10k
10k
7.5k
Vee (-15V)
Figure 2
Department of Physics, Stanford University
Physics 105, Intermediate Laboratory Seminar: Analog Electronics
Lab 6.12
Page 3
Out
6.
7.
8.
Build the current source shown in Fig 3 and use it to replace the large 7.5k
resistor in the “tail”. Measure common-mode gain again and report the
CMRR.
Estimate output impedance of the current source based on your data from
#6. Should be a high number.
What is the function of the 10k resistor on each base?
LAB: 2nd Day
12k
2N3904
2.7k
Figure 3
1k
PART 3.1
(10 pts)
-15V
Note: use a 2N5485 JFET for all of Part 3 of the lab.
The spec sheet for this transistor is on Coursework
under Materials/Datasheets. The JFET is physically
symmetric, with the Drain and Source being
interchangeable.
+15V
A
Build the circuit of Fig 4a. Note where to place an ammeter
and a voltmeter.
a.
D
G
10k
Measure IDSS and VP. Compare with spec sheet ranges
for these parameters.
S
0.01uF
V
20k
-15V
Figure 4a
PART 3.2
(25 pts)
Vin
Rebuild your circuit per figure 4b. To operate the JFET
as a variable resistor, drive it with a small triangle wave,
0.2V peak @1kHz. Adjust the potentiometer on the gate
and observe the change in gain and distortion at the
output.
a.
Submit a printout of input/output waveforms
demonstrating the distortion. Use Eq 5.48 in
Meyer to explain the distorted shape:
ID ~ [(VGS – VP)VDS – VDS2/2]
10k
Vout
D
G
100k
1M
330k
-15V
Figure 4b
S
Department of Physics, Stanford University
Physics 105, Intermediate Laboratory Seminar: Analog Electronics
Lab 6.12
Page 4
To fix the distortion, add a 1M resistor and a 0.01uF
cap per Figure 4c.
Vin
10k
b.
c.
Drive the circuit as before. Submit a printout
of input/output waveforms demonstrating that
the distortion is eliminated.
Explain i) how the fix adds 1/2VDS to the gate,
and ii) why this eliminates the distortion (use
Eq 5.48 again).
0.01uF
1M
D
G
100k
Vout
1M
S
330k
-15V
Add a modulating signal to the gate circuit as in
Figure 4d. You will need a second function generator
for this. vin is now called the carrier signal. This
circuit effectively multiplies the two signals, where
vmod modulates the amplitude of vcarrier ; this is known
as “amplitude modulation” (AM). Use a 0.2Vpeak
sine wave for both inputs. Drive vin at 1 MHz, vmod at
~400Hz. Trigger the scope on the vmod signal.
d.
Submit a printout of waveform clearly
showing the modulated carrier. Indicate the
carrier and modulation frequencies on the plot.
Figure 4c
Vcarrier
1uF
10k
Vmod
0.01uF
1M
D
G
100k
1M
Vout
S
330k
-15V
Add a length of wire to the output, and wrap around
the antenna of an AM radio in the lab. Tune the radio to
1MHz (1000 kHz on the AM dial). Fine tune the radio
until you hear the 400 Hz tone through the radio.
e.
Figure 4d
Have your TA check off that you get the AM radio to work.
FOR FUN: Use the spectrum analyzer to view your signal (0-10MHz “vector signal analyzer”).
This device measures amplitude vs. frequency, instead of the amplitude vs. time you see on a
scope. Measure the frequency and relative amplitude of the carrier and the “sidebands”.