ECE 248
Lab 4
Spring 2016
Lab 4: Emitter Follower (May 03/04)
GOAL
To better understand how to make voltage buffers with a bipolar junction transistor (BJT).
OBJECTIVES
To build, test, simulate, and understand emitter
followers based on the following circuits:
1) Regulated DC power supply based on a zener
follower
2) Speaker driver based on a Darlington pair.
Fig. 0: (Left) Bipolar junction transistor (Center) DC motor (Right) speaker. GENERAL GUIDELINES
1) Each student must build, test, and demo all circuits.
2) During the lab session, students may need to share test stations.
3) Students are allowed (even encouraged) to help each other. Buma and/or the lab teaching assistant will be around to
help as well.
4) Ask questions! The more questions you ask, the more you learn (assuming Buma can provide adequate answers ).
5) Build your circuits with neat wiring! Messy circuits will result in a 10 pt deduction from your lab demo grade.
6) Please keep your lab kit and work area organized.
PARTS AND MATERIALS
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Lab kit (breadboard, wire stripper, wire)
Oscilloscope, scope probes, multimeter
Function generator, coaxial cable (with alligator clips), benchtop power supply
Transistors:
2N2222A (1), 2N3904 (1), 2N4401 (1)
Resistors:
68 ohm
(blue/gray/black)
(1)
100 ohm
(brown/black/brown) (3)
1 k ohm
(brown/black/red)
(1)
33 kohm
(orange/orange/orange) (1)
100 kohm
(brown/black/yellow) (1)
Capacitors:
10 uF (1), 470 uF (1)
Zener diode:
1N4742A
(1)
DC motor
16 ohm speaker
1 ECE 248
Lab 4
Spring 2016
PART 1: ZENER FOLLOWER

Step 1a: Build and test the basic zener circuit shown in Fig. 1a.
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Do NOT cram your circuit into a tiny area! You will need to make many current measurements.
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The 1N4742A zener is the “big” orange diode.

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VCC comes from the benchtop supply.
o
Use neat and color-coded wiring on your breadboard!
o

As always, the black band is the cathode.
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RED wire = (+) voltage supply (14V)
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BLACK wire = ground
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YELLOW wire = everything else
Use the multimeter to make DC measurements of VZ and IZ.

Complete the first row of Table 1 (next page).
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You should get values very close to VZ = 12V and IZ = 20 mA.
Step 1b: Attach the 12V DC motor to your previous circuit (see Fig. 1b).
o
While the motor is running, use the multimeter to make DC measurements of VZ, IZ, and ILOAD.
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Complete the second row of Table 1 (next page).
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VZ will probably be between 6 to 8 V, which means the 12V zener has dropped out!
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IZ should be really small – if it shows up as 0.00 mA, that is fine.
Rs
100Ω
14V
Vcc
14V
Vcc
Rs
100Ω
14V
Vcc
Rs
100Ω
Q1
2N2222A
VZ
D1
1N4742A
IZ
(a)
D1
1N4742A
(b)
MOTOR
D1
1N4742A
(c)
MOTOR
Fig. 1: (a) Zener diode circuit without load (b) The motor should cause zener drop out (c) The emitter follower should drive the motor at close to full speed! Do NOT build three separate circuits – keep adding on to the previous version until you end up with the zener follower.
2 ECE 248
Lab 4
Spring 2016
Table I: Zener circuit measurements Circuit
Vz
ILOAD
IB
VE
---------------
--------------
--------------
--------------
--------------
IZ
#1a
#1b
#1c
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Step 1c: Modify your circuit by adding the emitter follower (see Fig. 1c).
o
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Keep the transistor SEPARATE from the zener.
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Use a wire to connect the zener to the transistor base.
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This makes it MUCH easier to measure the base current with the multimeter.
The pin diagram for the 2N2222A is on the course website.
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o
The motor should be running much faster than before!
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o
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The “tab” on the transistor case is the emitter (see data sheet).
This is because the motor is being driven by the emitter follower rather than the zener!
Use the multimeter to make DC measurements of VZ, IZ, ILOAD, IB, and VE.
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Complete the third row of Table 1.
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You should find that VZ is back to 12V again. Nice! 
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IZ should be around 19 mA, which is still plenty to maintain regulation. Nice! 
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ILOAD should be around 80 mA. If the motor current is 100 mA or more, it may need some oil!
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IB should be roughly 0.5 mA.
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VE should be roughly VZ – 0.7V.
Compute the transistor power dissipation.
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Use P = IBVBE + IEVCE.
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It will probably be around 200 mW – this will make the transistor pretty warm!
Disconnect the motor when you are done, but DO NOT DISASSEMBLE THIS CIRCUIT!
(see next page for Part 2)
3 ECE 248
Lab 4
Spring 2016
PART 2: AUDIO SPEAKER

RS
Step 2a: Build the circuit shown in Fig. 2.
VS
(Function
Generator)
This mimics the undesirable situation where a high impedance source (e.g.
common emitter) is directly attached to a low impedance load.
o
Set the Agilent function generator to “High Z” and a 0.2 VPP sine wave
at 1 kHz.
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
Press “Utility”, then select “Output Setup”, then select “High
Z”, and finally press “Done”.
The 1 kohm resistor is the source impedance RS.
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The speaker is the load.
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If you put your ear right next to the speaker, you should hear a very faint sound …
Step 2b: Configure the scope.
Attach TWO scope probes to the circuit:
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Attach the CH1 probe to VS and the CH2 probe to VL.
Press the “Default” button on the scope and press “Force Trigger” (enables front panel control).

Make sure both CH1 and CH2 are set to “1X”.
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Adjust the trigger level to be 50 mV.
Step 2c: Measure the output voltage using the CURSORS on the scope.
o
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Adjust the vertical and horizontal settings of the scope to show a few cycles of VS and VL.
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Make sure both CH1 and CH2 are set to “1X”
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CH2 will probably need the smallest scale (e.g. 2 mV/div).
Use the CURSORS (not “Measure”) to measure the peak-to-peak values of VS and VL and complete Table 2.
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VL will be really tiny and noisy (probably ≈ 3 mVPP).
Table II: Scope measurements for the “directly connected” speaker. VS
VL
YOU CAN DISASSEMBLE THIS CIRCUIT (SURPRISE!)
(see next page for Part 3)
4 Speaker
Fig. 2: The high impedance source is the function generator and the RS = 1 kohm resistor. o
o
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VL
1kΩ
VL/VS
ECE 248
Lab 4
Spring 2016
PART 3: DARLINGTON FOLLOWER
The extra high  of a Darlington pair means the low speaker impedance (16 ohm) looks like a much larger impedance to
the source (over 10 kohm!).
For this lab, we are using a “firm” divider, rather than a “stiff” divider, to bias the Darlington follower. A firm divider is
defined by R1//R2 < 0.1 (+1)RE. A firm divider is nice because it raises the input impedance ZIN by a factor of 10. This is
highly useful for a variety of applications.
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Step 3a: Build the Darlington follower shown in Fig. 3.
9V
Vcc
Q1
2N3904
Q2
2N4401
Cin
RS
From
(Function
Generator)
R1
33kΩ
1kΩ
Vin
VBB
10µF
VEQ
R2
100kΩ
IEQ
Vout
Cout
RE
68Ω
470µF
SPEAKER
Fig. 3: Darlington follower using a 2N3904 with a 2N4401 transistor.

o
Do NOT cram your circuit into a tiny area! You will need to make many scope measurements.
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The +9V power comes from the benchtop supply.
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The pin diagram for the 2N3904 and 2N4401 transistors are on the course website.
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Pay attention to the orientation of the electrolytic capacitors.
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The function generator settings are the same as in the previous task.
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Your circuit works when you the speaker emits a well-defined pitch.
Step 3b: Make some DC voltage measurements.
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Turn off the function generator output (just press the “Output” button) to silence the speaker.
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Use the multimeter to measure VBB, VEQ, and IEQ, and enter these values into Table 3 (next page).
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VBB should be around 6.7V.
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VEQ should be around 5.3V.
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IEQ should be around 80 mA.
5 ECE 248
Lab 4
Spring 2016
Table III: Darlington circuit measurements VEQ
VBB
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VOUT
VOUT/VS
ZIN
Turn on the output of the function generator (press the “OUTPUT” button).
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Adjust the scope to display a few cycles of VIN and VOUT.
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Use “swave” to record a snapshot of both VIN and VOUT for your lab report.
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Use the CURSORS to measure the peak-to-peak values of VIN and VOUT and enter these values in Table 3.
Your measured VIN should be 190 mVPP or slightly lower.
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Compute the voltage gain VOUT/VIN, the signal gain VOUT/VS, and update Table 3.
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Based on your measured gains, compute the input impedance ZIN, and complete Table 3.
ZIN should be less than 25 kohm.
Step 3d: Listen to some chirping.
o
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Configure the Agilent function generator to produce a frequency sweep from 100 Hz to 10 kHz:
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Keep the sine wave amplitude at 200 mVPP.
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Press the “Sweep” button
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Enter the proper “Start” and “Stop” frequencies.
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Enter a “Sweep Time” of 5 seconds.
Monitor the speaker voltage with CH2 of the scope (Horizontal setting = 500 us/div).
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What is the relationship between what you see (on the scope) and hear (speaker)?
Step 3e: Make some theoretical calculations.
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Turn off the output of the function generator to silence the speaker.
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Based on your measured IEQ, compute re’.
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Recall that re’ = 2*0.026/IEQ (the factor of 2 is only for Darlington).
Next, compute the voltage gain A = re / (re + re’).
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Recall that re = RE // RL.
Finally, compute the input impedance ZIN
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Recall that ZIN = (R1 // R2 ) // (12 + 1)(re’ + re).
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Assume 1 = 2 = 100.
Is there reasonable agreement between your theoretical and experimental voltage gain and input impedance?
6 A = VOUT/VIN
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VIN
Step 3c: Make some AC voltage measurements using the scope’s CURSORS.
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IEQ
ECE 248
Lab 4
Spring 2016
PART 4: CIRCUIT DEMOS
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Motor Driver
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Show your completed Table 1.
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Re-connect the motor to your circuit.
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The motor should be running.
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Use the multimeter to show the motor voltage (should be roughly 11.4V).
Speaker Driver
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Show your completed Table 2 and Table 3.
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Show your theoretical calculations for voltage gain and input impedance.
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Buma will turn off the function generator and ask you to set up:
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High Z output
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0.2 VPP sine wave
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Frequency sweep:
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o
Start = 100 Hz, Stop = 10 kHz, Sweep Time = 5 sec.
Buma will reset the scope and ask you to configure the following settings:
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CH1: Probe = 1X, Scale = 50 mV/div, Offset = -100 mV
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CH2: Probe = 1X, Scale = 50 mV/div, Offset = +100 mV
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HORIZONTAL: 500 us/div
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TRIGGER: Level = 50 mV
NOTE: BUMA FORBIDS ANY STUDENT FROM USING THE AUTOSCALE BUTTON.
Buma will deduct 3000 pts from your lab grade if he catches you using Autoscale!
o
We should be able to both hear and see the speaker happily chirping!
(End of Lab 4)
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