EXPERIMENT #1: COMMON EMITTER AMPLIFIER
THEORY:
In the amplifier circuit shown in the figure, the resistor R1, R2 and RE fix the operating
point. The resistor RE stabilizes it against temperature variations. The capacitor CE bypasses
the resistor RE for the ac signal. As it offers very low impedance path for ac, the emitter
terminal is almost at ground potential. When the ac signal is applied to the base, The baseemitter voltage changes, because of which the base current changes. Since the collector
current depends on the base current, the collector current also changes. When this collector
current flows through the load resistor, ac voltage is produced at the output. As the output
voltage is much greater than the input voltage, the circuit works as an amplifier.
Figure 1. Circuit Diagram of a Common Emitter Amplifier
1. AIM
i.
Determine the operating point parameters.
ii.
To study and plot the frequency response curve of CE amplifier.
iii.
Determine the bandwidth and voltage gain of the amplifier in mid frequency range.
2. APPARATUS REQUIRED
i.
Regulated DC power supply (12V)
ii.
Function generator
iii.
Dual trace oscilloscope
iv.
Digital multimeter (DMM)
3. COMPONENTS FOR DESIGN
i.
Transistor BC 107
ii.
Resistor R1= 56k Ω, R2= 10K Ω, RC= 2.2K Ω, RE= 500 Ω, RL= 820 Ω - 1 each.
iii.
Capacitors C1 and C2 = 10F each
iv.
Capacitor, CE = 100F
v.
Bread Board & Connecting Wires
4. SPECIFICATIONS:
BC 107 (N-P-N)
Hfe = 500 at IC= 2mA, VCE = 5V
ICBO=15 uA
ICM = 200 mA
VCEO =45V
5. PROCEDURE TO DETERMINE THE OPERATING POINT PARAMETERS
i.
Disconnect the signal generator and apply Vcc = 12V.
ii.
Connect the positive(red) probe of one of the DDM’s to the collector of the
transistor and the COM probe to the emitter of the transistor to measure VCE.
iii.
Connect another DDM in series with resister RC in the circuit to measure the
collector current (IC).
iv.
Ensure that the transistor is operating in the active region by noting that VCE is
about half of Vcc.
6. PROCEDURE TO DETERMINE THE FREQUENCY RESPONSE.
i.
Make the circuit connections as show in figure 1.
ii.
Set input ac signal at 1KHz using the signal generator and observe the amplified
waveform on oscilloscope. Increase the input signal voltage till the output wave
shape starts to distort. Measure this input signal voltage. (It’s the maximum signal
that the amplifier can amplify without distortion).
iii.
Set Vs = 50 mVp-p using the signal generator (sinusoidal signal is applied).
iv.
Keeping the input voltage constant, vary the frequency from 20Hz in steps as
provided in the table and note down the corresponding output voltage.
7. RESULTS
i. Operating point parameters
ii. Frequency response
1. Determine the maximum signal that can be handled by the amplifier without
introducing distortion at the input frequency of 1KHz.
2. Plot the graph of gain vrs frequency as shown in figure 2.
3.
4.
5.
6.
Lower cutoff frequency, fL =
Upper cutoff frequency, fH =
Voltage gain at mid-frequency Vo/Vs =
Find the band width = fH - fL
Figure 2. Frequency Response of Common Emitter Amplifier
Vs = ……………….(p-p)(mV)
GAIN (dB)
FREQUENCY(Hz) Vo(p-p) (mV)
20
30
40
50
100
200
500
700
1k
20k
30k
40k
50k
70k
100k
Vo/Vs
20log (Vo/Vs)
200k
500k
1M
10M
50M
200M
500M
1G