19EEE114 Electronic Circuits Lab #4
Exp. No #4
FREQUENCY RESPONSE OF COMMON EMITTER AMPLIFIER
OBJECTIVE
The purpose of the experiment is to design a common emitter amplifier. To analyze and plot the frequency
response of the amplifier with and without bypass capacitor. Also to compare the bandwidth of the amplifier
with and without feedback.
EQUIPMENT AND COMPONENTS USED
30 MHz Dual Channel Cathode Ray Oscilloscope
3 MHz Function Generator
0-30 V dc regulated power supply
4 ½ digit Digital Multimeter
Transistor BC107
Resistors ¼W
Electrolytic Capacitors
Breadboard and Connecting wires
BNC Cables and Probes
CIRCUIT DIAGRAM
Figure1. Common Emitter Amplifier
DESIGN
Output requirements: Mid-band voltage gain of the amplifier, AV = 50 V/V
Select the general purpose transistor BC107.
Specifications of BC107
Type: NPN
Nominal ratings: VCE = 5 V, IC=2 mA, hFE =110 to 450
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Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
19EEE114 Electronic Circuits Lab #4
DC biasing conditions Choose VCC=12 V, IC=5 mA
VRC=40% of VCC =
VRE=10% of VCC =
VCEQ=50% of VCC =
Design of Collector resistor RC
VRC = IC x RC
=> VCEQ = VCC - VRC
=> VRC = VCC - VCEQ
=> RC = (VCC - VCEQ ) / IC =
Design of Emitter resistor RE
VRE = IE x RE
=> RE =
Design of voltage divider R1 and R2
IB = IC/hFE Take hFE = 200
=
Assume current through R1=10 IB and that through R2=9 IB to avoid loading the potential divider network R1
and R2 by the base current.
VR2 = voltage across R2 = VBE + VRE =
Also VR2 = 9IBR2 =
Then R2 =
VR1 = voltage across R1 = VCC – VR2 =
Also VR1=10IBR1 =
Then R1 =
Design of Load RL
Take RL = 100 kΩ
Design of coupling capacitors CC1 and CC2
Choose XC1 ≤ Rin/10.
So CC1 = 0.1 µF
XC2 ≤ Rout/10, Then CC2 = 1 µF
Design of bypass capacitors CB
To bypass the lowest frequency (say 100 Hz), XCE should be equal to one-tenth or less than the resistance
RE.
XCB ≤ RE/10 => CB = 10 µF
PRACTICE PROCEDURE
DC Operating Point
1. Connect the voltage divider bias network from the circuit shown in Figure 1.
2. Apply DC bias voltage VCC and measure the operating point quantities: VB, VBE, VC, VCE, VRE, IC.
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Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
19EEE114 Electronic Circuits Lab #4
Table1: Operating Point measurements
Quantities Measured Value Designed Value
Base Voltage, VB
Base-emitter voltage, VBE
Collector Voltage, VC
Collector–emitter voltage, VCE
Collector Current, IC
Emitter Voltage, VRE
Amplifier Gain Measurements
1. Connect the circuit as shown in Figure 1.
2. Apply an input sine wave signal of 100mV, 1 kHz from the function generator.
3. Observe the output in CRO. Calculate the corresponding gain and compare with the designed values.
Table2: Amplifier gain measurements
Amplitude (V) Time Period (ms) Frequency (kHz)
Small signal Input voltage, Vs
Amplified output voltage, Vo
Voltage gain, AV =
Frequency Response
1. Keeping the magnitude of the input same, ie.,100mV, vary the frequency of the input signal and
tabulate the output voltage for different frequencies.
2. Compute the gain and plot the Frequency versus Gain (dB) using semi-log sheet.
3. From the plot, determine the values of (a) Mid band voltage gain, AV(mid), (b) Lower cut-off
frequency, (c) Upper cut-off frequency and (d) Bandwidth.
Repeat the above by removing the bypass capacitor.
Table3: Frequency response with bypass capacitor
Input voltage, Vs = mV
Signal frequency Output voltage, Vo Gain = Vo 20 log10(Gain)
(Hz) (Volts) VS dB
Table4: Frequency response without bypass capacitor
Input voltage, Vs = mV
Signal frequency Output voltage, Vo Gain = Vo 20 log10(Gain)
(Hz) (volts) Vs dB
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2019 – 2020 Page 11
Department of Electrical & Electronics Engineering, Amrita Vishwa Vidyapeetham, Coimbatore.
