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Common Emitter Amplifier
Objectives:
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The purpose of the Common Emitter Amplifier (CE amplifier) lab is to offer hands-on experience with the title itself, the CE amplifier.
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This lab is designed to familiarize students with gain/bandwidth, the CE amplifier circuit, and to give experience with LTspice, a simulation program that is useful for analyzing circuits.
Equipment:
Pomona 20 dB attenuator
BK Precision 4040 function generator
Tektronix TDS2022 oscilloscope
Tenma DC power supply
Omega HHM90 digital multimeter(DMM)
The pre-lab:
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The pre-lab for this lab included the LTspice simulations which involved building a 4-resister bias circuit for the CE amplifier. There were both DC and AC versions of this circuit built using the simulation program. There were calculations involved, all of which were designed to reveal circuit functionality. This pre-lab was sort of a review from Digital Electronics, especially in terms of the 4-resistor bias circuit. The pre-lab introduces gain and various scenarios in which the output of the AC CE amplifier changes. Finally, The pre-lab closes with using LTspice to construct a Bode Plot, which is used to measure the half-power bandwidth.
Build and test a CE Amplifier
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The CE Amplifier in-lab activities truly begin with breadboarding the CE Amplifier that was analyzed using the LT Spice simulator.
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DC followed by AC circuit
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Measurements were recorded.
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Plots were graphed
The CE Amplifier lab kicks off with breadboarding the DC version of the CE amplifier as shown below and recording measurements of the base, collector and emitter of the transistor:
DC CE Amplifier
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Base Voltage = 1.83 V
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Collector Voltage = 5.61 V
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Emitter Voltage = 1.11 V
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Base-Emitter Voltage = V_B - V_E = 0.72
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Collector-Emitter Voltage = V_C - V_E = 4.5 V
The next step in this lab was to add on the AC components to the circuit and conduct an experiment of how gain(V/V) changes with various resistor values. The BK Precision 4040 Function Generator was used to provide the AC Signal. Also, if the audio amplifier was to drive an 8 ohm speaker, I believe the CE amplifier would perform differently depending on the chosen resistor values, in this case, the 100k ohm resistor gives the most gain:
Voltage GAIN TABLE
aC CE Amplifier
Voltage Gain
Load Resistor
10 ohm
36 mV / 19.2 mV = 1.88
100 ohm
1 kohm
10 kohm
216 mV / 20.8 mV = 10.38
860 mV / 20 mV = 43
1.18 V / 20.8 mV = 56.73
100 kohm
1.22 V / 20.8 mV = 58.65
GAIN VS. RESISTANCE
PLOT
bREADBOARDED ac CE AMPLIFIER
The last portion of this lab is similar to the first part. The difference here however is that Gain is measured at various frequencies with 1k ohm load resistance. Once that data was recorded, it was used to form a Bode Plot from which determined the 3 dB bandwidth (mid-band gain)
Useful Equations:
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Gain in dB = 20*log(Vout / Vin)
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BW = freqHi - freqLo
Bode plot data
Voltage Gain
Frequency
148 mV / 17.6 mV = 8.41
100 Hz
300 Hz
1 kHz
3 kHz
400 mV / 18.4 mV = 21.74
820 mV / 18.4 mV = 44.56
Gain (dB)
10 kHz
30 kHz
100 kHz
300 kHz
1 MHz
3 MHz
1.04 V / 16.8 mV = 61.9
1.04 V / 16.8 mV = 61.9
1.04 V / 16.8 mV = 61.9
1.04 V / 16.8 mV = 61.9
1.04 V / 16.8 mV = 61.9
1.04 V / 16.8 mV = 61.9
660mV / 14 mV = 47.14
18.50
26.74
32.98
35.83
35.83
35.83
35.83
35.83
35.83
33.47
10 MHz
256 mV / 17.6 mV = 14.55
23.25
Bode Plot
In measuring the gain on the oscilloscope, most of the input / output signals were without distortion; however, the 100 MHz being such a high frequency caused distortion in viewing on the ocilloscope.
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Distorted 100 MHz signal
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1 kHz voltage gain signal
This lab was very similar to the things learned in Digial Electronics, Circuit Analysis, and Linear Signals and Systems. The lab involved aspects from each of these classes in terms of breadboarding, interpreting signals, and being comfortable with various components. This lab was filled with great information on CE amplifiers and Bode plots. I look forward to the following labs which build onto this one.
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