Download Experiment 9 – Bipolar Junction Transistor Amplifier

ECE 3550 - Practicum
12/1/09
Fall 2009
Experiment 9 – Bipolar Junction Transistor Amplifier
Objectives
1. To become familiar with amplifier bias, i.e., the Q-point.
2. To examine the midband voltage gain of the four-resistor BJT amplifier.
Exercise 1: Transistor Beta
1. Look up the data sheet for the PN2222A npn transistor. This can be found on the ECE
3550 course web site at
http://www.homepage.villanova.edu/charles.mckeough/.
Go to the link Lab (Practicum), and under Lab Information select Technical Information,
and then 2N222A bipolar junction transistor.
Find the minimum and maximum value of hFE (we call it forward beta βF or simply β).
2. Obtain a 2N2222A npn transistor. Figure 0 shows a 3-D outline drawing for the
2N2222A transistor. Hold the transistor in your hand with the flat part up and the leads
pointing towards your eye! The pins are listed, from left to right, E – B – C.
2N2222A
E
B
C
E - emitter
B - base
C - collector
E
B
C
Figure 0. Pinout for the transistor
3. Go to the curve tracer (located on the left hand side of the lab) and insert the transistor in
the socket. The TA will show you how to use the instrument. I used the following
settings:
Limiting resistor 500 
Step range of 0.01 mA (which is 10 µA)
Horizontal sensitivity 1 V/div
Vertical sensitivity 2 mA/div
4. Display the iC versus vCE characteristics. On the curve tracer, limit the voltage vCE to 15
V and the collector current iC to 12 mA. Try step sizes of iB of 5 μA. Record sufficient
data to produce reasonably accurate graphs.
5. Plot these curves in your notebook.
6. Mark the Q-point, VCE = 5 V at IC = 0.5 mA, on your curves. Determine the
corresponding value of base current, IB.
7. Calculate the value of β from the values of IC and IB.
Exercise 2: Biasing
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1. Figure 1 shows the basic amplifier. Before you construct it you have to determine values
for bias resistors R1 and R2.
10 v olts
Vcc
RC
R1
4.7k
R
0
C1
Q2
Vo1
C
FG
Q2N2222
Vi
R2
RE
R
4.7k
Vo2
0
0
0
0
0
Note: use two 0.47 uF, in
parallel, for C1
Figure 1. Four resistor amplifier
2.
3.
4.
5.
6.
7.
8.
9.
We will assume that the desired Q-point is IC = 0.5 mA. Write the equation for the
Thevenin equivalent voltage source, VEQ, and the equation for the Thevenin equivalent
resistance, REQ, for the left hand side of the circuit. (These equations will be in terms of
VCC, R1, and R2).
Assume these values: REQ = 52 kΩ and VEQ = 3.2 V. Now calculate values for R1 and R2.
Select the closest nominal resistor values for R1, and R2.
Do a check: Using your nominal values for R1 and R2 calculate the value of the base
current IB by writing the VAL law around the left-hand loop in terms of VEQ, REQ, VBE, RE
and the value of β from Exercise 1.
Next, calculate the collector current IC. This should be quite close to the desired Q-point
value. Proceed if everything checks.
Before you do any wiring, set the current limit on the + 25 V Agilent dc supplies to 25
mA. This will provide protection in the event a short occurs on your circuit board.
Construct the circuit shown in Figure 1, but don’t wire the FG and capacitor C1 yet.
Measure all node voltages with the DVM.
To check your Q-point, compute these currents I1, I2, IC, IE, and IB. Compare your
measured values to the values in step 4. Make appropriate resistor value adjustments if
necessary.
Exercise 3: Amplifier
At this point the amplifier should be properly biased in the active region and you can connect
an input signal and display the output signal and then determine voltage gain. Capacitor C1 is
used to block bias current from entering the signal source.
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1. What is the nominal value of C1? Calculate its impedance at 10 kHz. Then wire in C1.
2. Connect a 10 kHz sine wave from the FG to the amplifier input. Initially set the input
signal, vi, for a peak-to-peak (p-p) value of about 300 mV. Display the output signal vO1
and compare it to the input signal for gain and phase shift. Make sure there is no
discernible distortion in these signals. If there is distortion it will be necessary to reduce
the amplitude of vi before proceeding.
3. Measure the midband-frequency voltage gain Av1 = VO1/Vi, where VO1 and Vi are p-p
values. The term midband-frequency refers to signal frequencies where (i) the input
coupling capacitor is essentially a short circuit and (ii) the frequency effects of the
transistor can be neglected.
4. Measure the phase shift of vO1 with respect to vi.
5. Display the output signal vO2 and compare it to the input signal for gain and phase shift.
Again, make sure there is no discernible distortion in these signals.
6. Measure the midband-frequency voltage gain Av2 = VO2/Vi, where VO2 and Vi are p-p
values.
7. Measure the phase shift of vO2 with respect to vi.
8. Next we will place a bypass capacitor in parallel with the emitter resistor – we will use 20
μF. Obtain two 10 μF capacitors and connect them in parallel across RE, the emitter
resistor. You must connect the negative “─” terminals together and to ground.
Important Note: Observe the polarity shown on the side of the capacitor – the tiny
arrows point to the negative “─” lead. If the capacitor is wired incorrectly, the
capacitor may explode causing serious injury to your eyes or skin.
9. Examine output signal vO1 on the scope– it should be distorted since the voltage gain is
now of the order of 80 or so. In order to eliminate the distortion, you will have to reduce
the input signal to a p-p value of about 30 mV or less. Once the distortion is eliminated,
measure the updated value of the voltage gain Av1 = VO1/Vi. Check the phase shift from
input to output.
Parts and Equipment
Transistor: 2N2222A npn BJT
Resistors: ¼ W
Capacitors: 4.7 μF, 10 μF (2 of each)
Adjustable DC power supply
Digital multimeter
Protoboard and wire
Scope
Report
The first page must have this information: (a) title of the experiment, (b) date experiment was
performed, (c) name of the author of the report, and (d) the names of the lab partners. The
second page starts with the objectives of the experiment (write a few sentences). The main part
of the report follows.
I. Amplifier Bias Values
 Draw the amplifier circuit shown in Figure 1. Place on the circuit the component symbols
(e.g. RC, R1, etc) and their nominal values (e.g. 4.7 kΩ – don’t use the measured values).
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 Show the i-v output characteristic curves. Mark, on the curves, the desired Q point (IC and
VCE). Determine the value of beta from the curves.
II. Amplifier – No Emitter Bypass
 Draw the small-signal equivalent circuit for the complete amplifier in Figure 1 (no emitter
bypass). Using the value of β from part II, compute the parameters gm and rπ. Place all
numerical values on the small-signal equivalent circuit.
 Give the theoretical equation for voltage gain vo/vi. Compute the theoretical value and
compare it to your measured value. Put the results in a table similar to the following.
Input
Voltage
(V p-p)
Output
Voltage
(V p-p)
Measured
Gain
Theoretical
Gain
Percentage
Difference
III. Amplifier – Emitter Bypass
 Repeat the same steps that you did in part II.
 Compute the impedance of the bypass capacitor at a frequency of 10 kHz, and comment on
the significance of this impedance compared to the impedance of the emitter resistor, RE.
Appendix 1 – Scope Setup
Set the scope for external trigger: get a coax cable and connect one end to the SYNC output
of the FG and the other end to the Ext Sync terminal on the rear panel of the scope. In the
Trigger section of the scope, press Edge and select Ext.
Appendix 2: How to Change the FG Display for Peak-to-Peak Amplitude
The default display for the Agilent function generator (33120A) shows the peak-to-peak
amplitude of the output, if the FG output terminals are shunted (or terminated) by 50 ohms.
Typically, in our lab, we do not have that situation.
In order to have the FG display the actual peak-to-peak amplitude, enter the following
commands on the front panel of the FG [Ref.: page 40 of the manual].
Keys to Press
What is in the Display
Shift Menu
A: MOD MENU
On/Off
→ → →
↓
↓
→
Enter
D: SYS MENU
1: OUT TERM
50 OHM
HIGH Z
ENTERED
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