EE301-Fall 2015: Laboratory #5: BJT AMPLIFIER

EE301- Fall 2015
Laboratory #5: BJT AMPLIFIER
1. Objective:
In this experiment, you will construct the amplifier that you designed. You will measure the amplifier
gain, input and output impedances.
Equipment and circuit components: Signal Generator, Power supply with variable voltage, Multimeter,
Breadboard, BJT transistors (2N4401), Potentiometers, Resistors (of specified values in the manual).
2. Preliminary work:
2.1. In this lab, you will perform the experiment in a group of two students. Set-up a group with one of
your friends in the same lab section.
2.2. In the following circuit calculate R1, R2, RC and RE to achieve gain of 10. Note the followings:
 The supply voltage is 5V.

 Choose collector current around 1mA.
 Choose R1 and R2 such that the design is independent of Beta.
 Choose RE such that the design is not prone to VBE variations.
 Choose RC to get gain of 10.
Cir 2.2
2.3. Simulate the above circuit using SPICE to verify your calculations. Note the followings:
 Use NPN model that you used in the previous lab.

 The frequency of interest is 1 KHz.
1
 Choose C1 such that it does not affect the AC output but it is not too large, either. Start with
10 micro-farad capacitor and reduce it until it starts affecting AC amplitude then decide on the
value of C1.
 Gain: Apply a 10mV peak sine-wave to the input, check the output waveform using transient
analysis. By dividing the output amplitude to the input amplitude find the gain.
 Input impedance: Add a series resistor to the input source. Apply 10mV peak sine-wave and
check the output amplitude. Increase the value of the series resistor until the output amplitude
drops by half. At this point, the value of the resistor indicates the input impedance.
 Output impedance: Add a resistor at the output as load. (Remove the input resistor). Reduce
the value of the load resistor. The value of the load resistor when the output amplitude drops
by half is equal to the output resistance. (NOTE THAT YOU CANNOT ADD THE LOAD
RESISTOR directly to the output node without disturbing the DC bias point. You need to DC
decouple the load resistor from the collector node. Therefore you need to add a decoupling
capacitor between the load resistor and the output node. Use the same value of C1 that you
optimized in the previous part.)
3. Lab tasks:
In the lab you will be provided 2N4401 and a potentiometer. The pin diagrams are shown below:
A potentiometer is a three terminal device. When you turn the knob, you change the resistance
between the center pin and the side pins. In the experiment, you will use the center pin and one of the
side pins to create an adjustable resistor. The value of the potentiometer (i.e. 10K) indicates the
resistance between the two side pins (or the maximum resistance between the center pin and one of
the side pins).
Task 1.
 Construct the circuit shown in Cir 2.2 with a 2N4401 transistor. Use the values that you
calculated in the preliminary work for the resistors and capacitors.
 Use the fixed +5V output of the power supply.
 Set the function generator output to 10mV peak sine wave at 1KHz.
 Observe the output waveform on a scope screen. Measure the output amplitude and decide
on the gain.
 Sweep the frequency from 100Hz to 100 KHz at the following frequency points and note the
amplitude.
Frequency (Hz) Amplitude (mV)
100
200
300
500
600
2
700
800
1KHz
3KHz
5KHz
10KHz
30KHz
60KHz
70KHz
80KHz
90KHz
100KHz
 Set the frequency back to 1KHz. Make sure amplitude is 10 mV peak. Start increasing the
amplitude and observe the output waveform on the scope screen. Write down the maximum
peak-to-peak amplitude when the output is still a sine-wave.
 Go back to 1KHz, 10mVpeak sine-wave input. Change the transistor with another transistor
that you will borrow from another group and check the amplitude. See how much the output
amplitude changes when you change the transistor.
Task 2. (Input impedance)

 Set the signal generator to 1KHz, 10mVpeak.
 Add a series potentiometer between the signal generator and the amplifier. Set the value of
the potentiometer to zero initially. While observing the output wave-form start increasing the
value of the potentiometer. When the amplitude drops by half, remove the potentiometer from
the circuit and measure its value. This is the input impedance.
Task 3. (Output impedance)

 Reconnect the signal generator to the circuit without the potentiometer at the input side.
 Connect a capacitor between the collector of the transducer and the scope. The scope side of
the capacitor is now your new output node.
 Set the potentiometer to its maximum value (i.e. 100K). Connect it between the output node
and the ground.
 Reduce the value of the potentiometer until the output voltage amplitude drops by half.
Remove the potentiometer at this point and measure its value. This is the value of the output
impedance.

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EE301- Fall 2015

Laboratory #5: BJT AMPLIFIER

 The supply voltage is 5V.

 Use NPN model that you used in the previous lab.

Task 2. (Input impedance)

Task 3. (Output impedance)

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