Experiment No. -1
Aim:
To Plot the Frequency Response of a single stage RC Coupled Amplifier with feed back
and without feed back and find the following:
1. Voltage Gain
2. Lower cut off Frequency
3. Upper cut off Frequency
4. Bandwidth
5. Gain Bandwidth Product
Apparatus:
1. Digital Storage Oscilloscope (DSO)
2. Power Supply
3. Bread Board
4. Signal Generator
Component:
Resistances: 100k Ω = 01, 1k Ω = 01, 22k Ω = 01, 220k Ω = 01, 220 Ω = 01
Capacitors: 10µf = 02, 47 µf = 01
Transistor
BC107 = 01
Connecting wire and DSO Probe
Introduction
A practical amplifier circuit is meant to raise the voltage level of the input signal. This signal
may be obtained from anywhere e.g. radio or TV receiver circuit. Such a signal is not of a single
frequency. But it consists of a band of frequencies, e.g. from 20 Hz to 20 KHz. If the
loudspeakers are to reproduce the sound faithfully, the amplifier used must amplify all the
frequency components of signal by same amount. If it does not do so, the output of the
loudspeaker will not be the exact replica of the original sound. When this happen then it means
distortion has been introduced by the amplifier. Consider an RC coupled amplifier circuit shown
fig 1 shows frequency response curve of a RC coupled amplifier. The curve is usually plotted on
a semilog graph paper with frequency range on logarithmic scale so that large frequency range
can be accommodated. The gain is constant for a limited band of frequencies. This range is
called mid-frequency band and gain is called mid band gain. AVM. On both sides of the mid
frequency range, the gain decreases. For very low and very high frequencies the gain is almost
zero.
In mid band frequency range, the coupling capacitors and bypass capacitors are as good as short
circuits. But when the frequency is low. These capacitors can no longer be replaced by the short
circuit approximation.
At low frequency, output capacitor reactance increases. The voltage across RL reduces because
some voltage drop takes place across XC. Thus output voltage reduces.
The XC reactance not only reduces the gain but also change the phase between input and output. It
would not be exactly 180o but decided by the reactance. At zero frequency, the capacitors are open
circuited therefore output voltage reduces to zero.
The gain is constant over a frequency range. The frequencies at which the gain reduces to 70.7%
of the maximum gain are known as cut off frequencies, upper cut off and lower cut off frequency
fig. 2, shows these two frequencies. The difference of these two frequencies is called Band width
(BW) of an amplifier.
BW = f2 – f1.
fig. 2
At f1 and f2, the voltage gain becomes 0.707 Am(1 / 2). The output voltage reduces to 1 / 2
of maximum output voltage. Since the power is proportional to voltage square, the output power
at these frequencies becomes half of maximum power. The gain on dB scale is given by
20 log10(V2 / V1) = 10 log 10 (V2 / V1)2 = 3 dB.
20 log10(V2 / V1) = 20 log10(0.707) =10 log10 (1 /
2)2 = 10 log10(1 / 2) = -3 dB.
If the difference in gain is more than 3 dB, then it can be detected by human. If it is less than 3 dB
it cannot be detected.
Circuit:
Procedure:
1. Connect the circuit as shown in the diagram.
2. Apply a sinusoidal input signal of = from a signal generator.
3. Connect the output to the DSO
4. Measure output voltage and calculate gain
5. Keeping the input voltage constant vary the input frequency and note the output
voltage of the Amplifier till the output decrease upto a -3dB point.
6. Draw a graph between input frequency Vs output voltage
7. For without feedback short terminal A and B by a connecting wire and repeat
procedure 1 to 6.
Observation table
Vin = 20mv
S. No
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
Frequency
Output voltage
Voltage Gain
10 Hz
50 Hz
100 Hz
200 Hz
500 Hz
800Hz
1k Hz
5K Hz
10KHz
20KHz
50KHz
100KHz
200KHz
300KHz
500KHz
800KHz
1MHz
2MHz
Result:
1. Voltage Gain
………………………….
2. Lower cut off Frequency ……………………………
3. Upper cut off Frequency …………………………….
4. Bandwidth
……………………………
5. Gain Bandwidth Product …………………………..
Voltage gain in
dB