Download Procedure and Questions

Experiment 10 – Bipolar Junction Transistor: Common-Emitter Amplifier
Physics 242 - Electronics
Introduction
Transistors are particularly useful in applications involving the amplification of signal
power. In this lab, you will investigate the common emitter amplifier, one of the most
commonly used transistor circuits.
Procedure and Questions
1. Measure the component values first, then build the common emitter amplifier shown
above. With the input voltage disconnected, measure the DC voltages VCC, VC, VE, and VB
characterizing the quiescent operating point. VC, VE, and VB refer to the voltages at the
collector, emitter, and base terminals of the transistor, measured with respect to ground.
Q1: Assuming  = 150 and 𝑣𝐵𝐸 = 0.6 V, calculate the predicted values of VCE and 𝐼𝐶 .
Show your calculations and use measured component values. Find the percent difference
between the measured and predicted values of collector current IC and collector-emitter
voltage 𝑉𝐶𝐸 . Note that you can find your measured VCE from your experimentally
measured values as 𝑉𝐶𝐸 = 𝑉𝐶 − 𝑉𝐸 , and you can find your measured 𝐼𝐶 from your
𝑉 −𝑉
measured values as 𝐼𝐶 = 𝐶𝐶𝑅 𝐶.
𝐶
Q2: Solve for the predicted values IC and 𝑉𝐶𝐸 , assuming  = 50. How much (by what
percent) are these predicted values moved from the predicted values you found in Q1
where you assumed that  = 150.
2. Use the sine-wave generator to provide a 1 kHz sinusoidal input signal, and monitor
output waveform on the ‘scope. Reduce the amplitude of the input signal so that the
output waveform is not clipped. Measure the amplitudes of the input and output
waveforms (from which the voltage gain can be found). Then increase the amplitude of
the input signal until the wave begins to clip, and sketch the waveform, indicating
maximum and minimum voltage levels with respect to ground.
Q3: Calculate the expected value of the voltage gain
𝑣𝑜
𝑣𝑖
for your circuit (using measured
component values). Find the percent difference between measured and predicted gains.
Q4: Show your sketch. Indicate where clipping occurs. Explain why the clipping occurs
at the voltage you observed and show a supporting calculation. Don't expect exact
agreement, since it is difficult to detect the exact onset of clipping.
3. The combination of the capacitor and the parallel combination of 𝑅1 , 𝑅2 , and the
effective resistance looking into the emitter 𝑅𝐸,𝑒𝑓𝑓 form a high-pass filter for input
signals. Vary the frequency of the input signal to find the cut-off frequency 𝑓𝑐 , the
1
frequency at which the gain has dropped by a factor of or 0.707.
√2
Q5: Show your calculation of the predicted cut-off frequency, and calculate the percent
difference between the predicted and measured values.
4. In this part, you will measure the input impedance 𝑅𝑖 of your common-emitter
amplifier. Set the signal generator to provide a 10 kHz sine wave that is not clipped.
First, measure the peak-to-peak voltage of the input signal, with the signal generator
connected directly to the 'scope, and disconnected from the circuit. Then, insert a resistor
𝑅𝑥 as shown in the figure above and measure the voltage 𝑉𝑖𝑛 with the 'scope. Make this
measurement for the following resistance values of 𝑅𝑥 : 2 k, 5 k, 10 k, and 20 k.
Q6: a. Make a plot from which you can find the input resistance of your common-emitter
amplifier circuit. In finding the input resistance, the circuit is modeled as a resistor 𝑅𝑖 as
shown in the picture above. To decide what to plot, write the voltage divider equation
describing the simplified circuit shown in the picture. Find the input resistance using the
slope or y-intercept values from your plot.
b. Calculate the predicted input resistance of the circuit. Use measured component
values. You may assume the capacitor is effectively a short circuit at 10 kHz. Find the
percent difference between the predicted and measured input resistance values.
5. In this part, you will measure the output impedance of the amplifier. Set the signal
generator to provide a 10 kHz sine wave that is not clipped. Then connect a 0.5 F
capacitor 𝐶𝑜 and a load resistor 𝑅𝐿 as shown in the picture above. The function of the
coupling capacitor is to pass the AC part of the output signal while blocking the DC
level. Measure the output voltage 𝑉𝐿 across the load resistor using the 'scope. Make this
measurement for the following resistance values for 𝑅𝐿 : 2 k, 5 k, 10 k, 20 k, and
open circuit (omit 𝑅𝐿 and connect the 'scope between the circuit output and ground,
where 𝑅𝐿 would be).
Q7: a. Make a plot from which you can find the output resistance of your commonemitter amplifier circuit. In finding the output resistance, you should model the circuit as
a Thevenin-equivalent source 𝑉𝑇 in series with a Thevenin-equivalent resistor 𝑅𝑇 as
shown in the picture above. Find the output resistance 𝑅𝑇 from your plot.
b. Calculate the predicted output resistance of the circuit. The resistance looking into the
collector is very large (it is a reverse-biased diode); you may assume it is infinite. Use
measured component values. You may assume the capacitor is effectively a short circuit
at 10 kHz. Find the percent difference between the predicted and measured input
resistance values.