Electronics- I
Lab 9: Common Base Amplifier
Names Abdul Wahab
Muhammad Rehan Tabassum
Registration
FA20-BEE-010
Number
FA19-BEE-139
Class BEE-3A
Instructor’s Name Ma’am Riffat Shaista
Lab Assessment
Post Lab Total
Pre-Lab In-Lab
Data Presentation Data Analysis Writing Style
Objectives:
1: Verify through circuits measurements, the voltage gain and phase shift of common base
amplifier.
2: Experimentally determine the input impedance of common base amplifier.
3: Explore three common failures of common base amplifiers
Equipment:
Breadboard
Function Generator
Oscilloscope
Digital Multimeter
DC Voltage Source: VDC: -15V - +15V
Resistor: 1x1KΩ, 1x20kΩ, 1x10KΩ, 1x2.7KΩ, 1x100Ω
Capacitor: 2 x 1µF
Transistor: 2N2222
In-Lab I: Common Base Amplifier Functional Experiment
Parameters Measured Simulated
Vin 500mV 500mV
Vout 1.30V 1.356V
Av 2.6 2.712
Delta T 120µs 130µs
Φ 43.2° 46.8°
Taking readings from the Voltage Divider:
Parameters Measured Simulated
Vin 38.6mV 35.2mV
Vout 1.30V 1.356V
Av 33.67 38.522
Delta T 180 µs 193 µs
Φ 64.8° 69.48°
Amplifier Impedance:
Parameter Measured Simulated
Ii 0.480mA 0.468mA
Vi 500mV 500mV
re (26mV/Ii) 54.1Ω 55.55 Ω
Zi (Re||re) 54 Ω 54.43 Ω
Common Base Amplifier with C2 Open
Parameter Measured Simulated
Vout 1.34V 0
VC 3.142V 2VPP Shifted DC +5VDC
VE -0.614V 10VPP Shifted -0.635VDC
VC
VE
VIN vs VOUT
Common Base Amplifier with RE Open
Parameter Measured Simulated
Vout 0
VC 10 VDC
VE 45mVPP Shifted +8mVDC
Vout Vs VE
Common Base Amplifier with C2 Shorted
Parameter Measured Simulated
Vout 12mVPP Shifted -636mVDC
VC 12mVPP Shifted -636mVDC
VE 45mVPP Shifted +8mV
Vout Vs VE
Post Lab
Q1: You should have found that the gain of the common base amplifier is high and the
input impedance is very low. What effect do you think the low in-put impedance would
have on most input signals to the amplifier?
In the lab we observed that the gain of common base amplifier is 38.5 max which is very
low and not a desirable, which questions the circuit whether it is correct or not.
However, we did observe the configuration has low input impedance which is desirable
for low input current sources. As the low input impedance would allow most of the input
current to flow through.
Q2: Even though the common base amplifier has a high gain, why do you think it is not
very popular for use as an audio amplifier?
Although low input impedance is preferred for audio amplifiers, the common base
amplifier doesn’t have any current gain which limits the output power. Moreover,
common base configuration has high output impedance which is not recommended as
audio speakers usually have low impedances in range of 4Ω, 8Ω and 16Ω.
Q3: Considering that the impedance of most coaxial cables used to carry RF signals is in
the range of 50 to 75 OHM, do you feel that there would be an application in RF use for
the common base amplifier? Give an example to explain your answer.
The common Base amplifier has a low input impedance which is desirable in the RF signal
transmission as it amplifies the Voltage while maintaining the same current level. Thus,
resulting in no current loss and high voltage gain which increases the power of the signal.
Moreover, the configuration is also preferred while working with High Frequencies.
Critical Analysis:
1: In this lab we build a common base amplifier circuit to study the configuration with an
attached load of 20KΩ and observed a gain of 38.5 which is very less however, the fault
lies in the circuit design.
2: Moreover, we observed the behavior of the circuit by Shorting the Collector Capacitor
and observed that the both RE and RC resulted the output having a DC offset. As we
know the job of the capacitor is to remove the DC offset such behavior is expected.
3: We also observed the configuration by removing the Output DC coupling
Capacitor and observed voltage shift at Collector and Emitter, while the Voltage
across the Load was effectively 0 as it was removed from the circuit.
4: We conducted the experiment with RE open and observed constant 10VDC at
the Collector terminal while the load observed 0VDC due to the coupling capacitor.
5: Overall, in this lab we couldn’t get desirable outcome, as the Voltage gain was
significantly less and the methodology mentioned in the manual is hard to follow
and as such the lab was not completed in the lab.
