Electronic Devices and Circuits – EET2222
Lab 11 – Upper and Lower Cutoff Frequency
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Reading: |
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Floyd Text,
Chapter 10, Sections 1 through 4. |
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Objectives: |
After completing this experiment, you will be able to: 1. Compute
the three lower break frequencies for a common-emitter (CE) amplifier and
determine the overall lower critical frequency. 2. Measure
the lower cutoff frequency of a CE amplifier. 3. Measure
the upper cutoff frequency of a CE amplifier. |
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Discussion: |
The purpose of this experiment is to determine the factors
that contribute to the low-frequency and high-frequency response of a common-emitter
transistor amplifier. The low-frequency response is determined by the input and
output coupling capacitors and the emitter bypass capacitor. The overall low-frequency response is
determined by the combination of three high-pass filter networks due to the
three capacitors. The lower cutoff
frequency is the highest of these three. The high-frequency response is determined by a combination
of internal transistor capacitances and stray capacitances from the circuit
wiring. |
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Procedure: Procedure: |
Lower
Cutoff Frequency 1. Measure and record the resistance of the
resistors for the circuit in Figure 1.
Use the curve tracer to measure and record the value of beta of the
2N2219A transistor you will be using in this lab. 2. Compute the following parameters for the
circuit in Figure 1: 3. Calculate the lower break frequencies using
the formulas below. The highest of
these three break frequencies will be the calculated lower cutoff frequency. 4. Construct the circuit of Figure 1. Measure and record VB, VE,
VC, and VCE. Recheck your work if the calculated and
measured values differ significantly. 5. Measure the voltage gain of your amplifier
at a frequency of 10 kHz using the scope.
Make sure that the output signal is not distorted (reduce the
amplitude of the input signal to eliminate distortion). Again, recheck your work if the calculated
and measured voltage gain values differ significantly. Measure and record the peak-to-peak output
voltage. 6. Determine the magnitude of the output
voltage at the lower cutoff frequency by multiplying the peak-to-peak output
voltage from step 5 by .707. 7. Decrease the input frequency until the
output drops to the value calculated in step 6. This is the measured lower cutoff frequency
of the common-emitter amplifier circuit.
Record this value. 8. Change the input coupling capacitor to 0.1 μF and repeat step 7. 9. Restore the input coupling capacitor to its
original value. Upper
Cutoff Frequency 10. Add a .01 μF
capacitor in parallel to the load resistor.
This will simulate the effects of stray wiring capacitances and other
capacitances in the amplifier circuit. 11. Starting at 10 kHz, increase the input
frequency until the output drops to the value calculated in step 6. This is the upper cutoff frequency of the
common-emitter amplifier circuit.
Record this value. 12. Complete the questions below. Your lab report is due at the end of the
period. |
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Questions: |
1. Based on your calculations and the results
of step 7, which capacitor is responsible for the lower cutoff frequency? 2. Based on your calculations, which capacitor
is responsible for the lower cutoff frequency in step 8? |
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Formulas |
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