Department of Physics, Stanford University
Physics 105, Analog Electronics
Lab 5.12
Page 1
Lab 5:
BJT Followers, Amplifiers
Read: Meyer, Sections 5.2.5, 5.3.1, 5.3.2 (also read 5.2.3, 4.2.4 if you haven’t already done so)
PRELAB
Part 1
Part 2:
For the emitter follower in Figure 1 of this lab, estimate the input and output impedance.
Assume β ~ 100.
The common emitter amplifier in Figure 3 of this lab will be built with the following
component values:
R1 = 56k
R2 = 5.6k
RC = 6.8k
RE = 680Ω
C = 0.33 uF
Using these values, calculate/estimate the following parameters:
AC Gain
IC, Quiescent
VOUT, Quiescent
VB, Quiescent
VCE, Quiescent
f3dB of input
RTh, Divider at base
Rin, transistor
Zin, total
Zout
i.e., | vout / vin |
(Is operation biased to the linear/active region?)
this is same as for the follower. For proper biasing, you want
RTh<< Rtransistor. Is this the case here?
The total impedance, accounting for the divider, transistor and
capacitor. See pp. 163-165 in the text.
this takes no calculation, just a glance at the circuit--remember the output
impedance of a current source, and that the collector is the output of the
transistor as current source.
If you connect a 45-ohm speaker to the output, what do you expect to happen to the
output waveform ( compare Zout to 45 ohms)?.
Part 3:
The circuit of Figure 4 has the common emitter amp driving the emitter follower. What
DC offset do you expect at the output of the emitter follower, Point B in Figure 4? (Hint:
what is the DC quiescent point of at the collector of the amplifier?)
Homework Problems
1. A scope has an input impedance of R||C. A 10x probe is connected, consisting of another parallel
combination of (9R)||(C/9). Treat this combination as a divider, and a) find the transfer function ,
and b) show that it has the desired behavior, ie, is a 10:1 divider, with no frequency dependence.
There’s not much complex algebra here—simplify it only to the point where you can easily get
the limits.
2. Meyer Section 5.7.1, Problems 1 and 2
Department of Physics, Stanford University
Physics 105, Analog Electronics
Lab 5.12
Page 2
LAB
Day 1: Part 1
Day 2: Parts 2,3,4
1.
2.
3.
4.
5.
Lab tips for this week:
This week you will start using a powered breadboard--with DC power terminals. Do not apply external
power to these terminals--you will blow the internal power supply, and will be rewarded with the
opportunity to learn how to repair it—it’s not that hard.
Rotate your breadboard 90° counterclockwise so the power terminals are on the left. Use the horizontal
rows of holes as your power “rails”.
The circuits this week can be noisy, so there is a premium on learning to wire things neatly, with resistors and
wires close to the board, rails on separated bus strips, etc. Remember, make the layout look like the schematic.
Use your needle nose pliers (see your lab drawer) to help get wires in and out of the board neatly.
ScopesmanshipI: Use a 10x probe to look at all output waveforms on the scope. The only time not to use one is
for the function generator output.
ScopesmanshipII: you will need to use either/or both of the following to see all of the effects in this lab:
.-- AC coupling to measure AC gains (amplification)
.-- DC coupling to see offset effects from followers.
Part 1. Emitter Follower: (repeated from Lab 4)
(25 pts)
Build this circuit in the upper right hand corner of your
breadboard. Save the circuit – you will use it again in Part 3 of
the lab.
Vcc (+15V)
270
In
2N3904
a. Drive the circuit in Figure 1with a 1kHz, 1V peak (2Vpp)
sine wave. Use a 10x scope probe to look at the output
on the scope. Submit the input and output waveforms on
one plot. Show the 0.6V drop on the positive half of the
cycle.
Out
3.3k
Vee
b. Explain why the output is not a replica of the input.
Figure 1
c. Increase the amplitude until you see bumps below ground.
Explain; be quantitative. (Hint: the VBE reverse breakdown
spec for this transistor is 6V).
d. Return to 1V input. Connect the lower part of the emitter
resistor (labeled Vee) to-15V. Submit the input and output
waveforms again.
e. Explain the improvement.
f.
Replace the 270 ohm resistor with a 10k; this will give you a
high impedance source. See Figure 2. Use a 1V peak input
and measure the output impedance of this follower circuit. Do
this by measuring the output voltage with and without the 1k
“load” connected, then use the voltage divider concept. (this
won’t be precise, but you should conclude that Zout is low).
Show your circuit and explain how you used your measurements
to arrive at Zout
+15V
In
10k
Out
2N3904
3.3k
1k Load
-15V
Figure 2
Department of Physics, Stanford University
Physics 105, Analog Electronics
Lab 5.12
Page 3
g. Remove the 1k load, then use a 10V peak signal to measure the input impedance of the follower
by measuring the signal on both sides of the 10k resistor (there is nothing connected to the output
for this measurement, not even the scope). Show your circuit and explain how you used your
measurements to arrive at Zin .
Remember to save your circuit from Part 1 for Part 3 of the lab.
Vcc (+15V)
Part 2: Common Emitter Amplifier:
(25 pts)
Build this circuit on your breadboard to the left of the Emitter
Follower from Part 1—you will eventually connect the two.
Where not specified, use a 100mv peak input (200mVpp, pp≡peak-topeak), and 1kHz.
R1
Out
In
Build the amplifier in Figure 3 to the left of the emitter follower on
your breadboard (you will connect them later). Use these component
values:
R1 = 56k
R2 = 5.6k
RC = 6.8k
RE = 680Ω
C = 0.33 uF
[For questions 1-4 below, provide your prelab calculations for comparison to
measurement]
C
2N3904
R2
Re
Vee
Figure 3
1.
With no AC input (function generator disconnected), measure all DC
quiescent voltages. Compare with prelab prediction.
2.
Connect the function generator. show a plot of input and output waveforms at 1 kHz, and
measure amplifier AC gain and phase shift at this point. Compare with predicted.
3.
measure 3dB frequency of the amplifier – measured from input to output with NO LOAD.
Compare with predicted.
4.
measure Zout @ 1kHz. Specify an appropriate resistive load for this measurement.
Compare with predicted.
5.
Attach a 45 ohm speaker to the output. Show waveform (or, at least what’s left of it) . Pay
attention to the speaker volume — this will improve in Part 3 below.
Keep your circuit built on your breadboard to use in Part 3.
Rc
Department of Physics, Stanford University
Physics 105, Analog Electronics
Part 3: Emitter Follower Buffer
(15 pts)
In this part of the lab you will
demonstrate the benefit of a follower.
Connect the output of your amplifier
(remove the speaker) to the input of the
emitter follower from Part 1. See point
A in Figure 4. Keep your input to 100
mV peak as before.
1.
Apply an AC input to the
amplifier—no speaker—and
look at the output of the follower
(Point B in Figure 4). Submit
the input and output waveforms.
Lab 5.12
Page 4
Vcc (+15V)
R1
In
Rc
270
A
C
2N3904
R2
2N3904
3.3k
Re
Vee
Vee
Figure 4
2.
Now connect the speaker to the output, Point B in Figure 4. Compare with Part 2 the
difference both in speaker volume (for the same input) and overall AC gain (measure
signal voltage delivered to the speaker).
3.
Explain this improvement quantitatively in terms of impedances: Zout of amp, Zin and Zout
of follower. Specifically, show the Thevenin equivalent circuits, and voltage divider
calculations, of
a. the amplifier driving the follower,
b. the follower driving the speaker.
Out
B