Department
of
Electronics & Communication Engineering
Laboratory Manual
Analog Electronic Circuits
EXPERIMENT NO.1
Design and measure the frequency response of an RC coupled amplifier using discrete components.
Aim: To study the frequency response of a single stage RC coupled amplifier in a CE configuration, at low, middle, high frequency and demonstrate that gain bandwidth product is constant.
Apparatus: Bread board, NPN-transistor-1 (BC 547), resistances (3.3kohm-1, 33kohm, 1kohm-1, 330 ohm, variable resistance 20kohm) capacitors (22μf, 0.1 μf, 100 μf), regulated power supply (+12), function generator, C.R.O., connecting wires.
Theory:
A circuit diagram of RC coupled amplifier in CE configuration is shown in fig-1.
The gain of an amplifier depends on frequency .As frequency decreases the gain starts to fall .The main cause of this is coupling capacitor Cc. Because it offers very high impedance at low frequency this causes decrease in output voltage. At high frequency the gain again falls because of the shunt capacitances made up of junction capacitances and wiring capacitances. The size of the coupling capacitor is so chosen that it offers negligible reactance to ac at operating frequencies. Ce acts as a by pass capacitor for ac signals.Cb is that it must pass the input ac signal unattenuated. Hence the lower cut off frequency of the amplifier is determined by the coupling capacitors. The Coupling capacitor Cb along with resistance combination forms a high pass filter whose cut off frequency,
fl = 1 / 2πRC
Shunt capacitance at the output and RL + hi acts as low pass filter (high frequency response).
The gain of the amplifier at different frequency is:
At middle frequency
The current gain is hfe R L
Ai =
hie + R L
The Voltage gain is
hfe R L
Av =
hie + R L
At low frequency
hfe R L
the current gain is Ai =
hie + R L - J w C
The Voltage gain is
hfe R L
Av =
hie + R L - J w C
At high frequency
hfe R L
the current gain is Ai =
hie + R L + Jw Cd R L hie
The Voltage gain is
hfe R L
Av =
hie + R L + Jw Cd R L hie
where hie is the input impedance
R L is the load impedance
Cd is the shunt capacitance(junction + wiring capacitance)
The lower cut off frequency at which gain the gain falls to 70.7% of its maximum value and the higher cut off frequency for which the gain falls to 70.7% of its maximum value.
Bandwidth of an amplifier is given by BW = fh-fl
This type of amplifier generally used in audio frequency range.
The frequency response curve of this amplifier is shown in fig-2.
Fig.-1 Circuit diagram of single stage RC Coupled Amplifier
Where Vcc = +12v
R1 33kΩ , Cb 0.1μf
R2 3.3kΩ , Ce 22μf
R3 1kΩ, Cc 100μf
R4 Variable 20kΩ
Gain BW = gain x (fh-fl)
Fig-2 Frequency response of RC-coupled Amplifier
The gain of RC coupled amplifier will fall at the rate of 20 db per decade from the unity gain bandwidth.
Procedure:
1. Make connection as shown in fig.1.
2. After making sure that transistor is biased in active region feed the input signal from function generator (sine wave amplitude of 1mv) such that the output signal is undistorted.
3. Vary the input signal frequency from the function generator and observe for the voltage output (Vo) at the CRO, adjust the pot meter R4 to get the overall gain of 10.
4. Find the maximum voltage output by varying the frequency of input signal. Make sure that the amplitude of the input signal should remain same.
5. Record the voltage output in observation table by decreasing the frequency in small steps, below the frequency at which maximum voltage is obtained. Take the reading till voltage output is readable on the CRO.
6. In the similar manner, find the higher cut off frequency (fh) by increasing the frequency from the frequency where maximum gain is obtained.
7. Record the voltage output in observation table by increasing the frequency in small steps, above the frequency at which maximum voltage is obtained. Take the reading till voltage output is readable on the CRO.
9. Plot the frequency response curve as per the reading noted in the observation table.
10. Find out the lower cut-off frequency and higher cut-off frequency, where the gain falls up to 70.7% of maximum gain.
10. Calculate the bandwidth (f’h – f’l).
11. Repeat the steps from step no.3 to 10, for the gain of 20 by adjusting the R4 and make sure that amplitude of input signal should not change.
13. Find the gain bandwidth product and verify that this product is constant.
Observations Table:
S.No. / Frequency of input signal / Output VoltageGain (10) Gain (20)
For single stage:
Higher cut off frequency (fh)=………….
Lower cut off frequency (fl)=…………
BW = fh - fl
Precautions:
1. Make connection very carefully.
2. Make connection firm & tight
3. Use C.R.O. carefully.
EXPERIMENT NO.2
Design a two stage RC coupled amplifier and determine the effect of cascading on gain and bandwidth.
Aim: To study the frequency response of two stages RC coupled amplifier at low, middle, high frequency and demonstrate that the gain bandwidth product is constant.
Apparatus: Breadboard, NPN-transistor (BC 547), resistances, capacitors, regulated power supply (+12), function generator, C.R.O., connecting wires.
Theory:
A circuit diagram of two stages RC coupled amplifier is shown in fig-1.
The gain of two stage amplifier depends on frequency .As frequency decreases the gain starts to fall .The main cause of this is coupling capacitor Cc. Because it offers very high impedance at low frequency this causes decrease in output voltage. At high frequency the gain again falls because of the shunt capacitances made up of junction capacitances and wiring capacitances.
The Higher cut off frequency for which the gain falls to 70.7% of its maximum value is given by:
f’h = fh (21/n -1 )1/ 2
The lower cut off frequency at which gain the gain falls to 70.7% of its maximum value is given by:
f’L =
Bandwidth of two stage amplifier is given by:
BW = f’h - f’L
Gain BW = gain x (fh-fl)
Where fh is single stage higher cut off frequency
fL is single stage lower cut off frequency
n is no. of single stage in cascading
f’h is multistage higher cut off frequency
f’L is multistage lower cut off frequency
Fig.-1 Circuit diagram of two stage RC coupled amplifier
Where Vcc = +12v
R1 33kΩ , Cb 0.1μf
R2 3.3kΩ , Ce 22μf
R3 1kΩ, Cc 100μf
R4 Variable 20kΩ
fig. 2 Frequency response of RC-coupled Amplifier
Procedure:
1. Make connection as shown in fig.1.
2. After making sure that transistors are biased in active region feed the input signal from function generator (sine wave amplitude of 1mv) such that the output signal is undistorted.
3. Vary the input signal frequency from the function generator and observe for the voltage output (Vo) adjust the pot meter R4 to get the overall gain of 10.
4. To observe the frequency response of the first stage disconnects the second stage by removing the right lead of Cc, which is connected, to the base of second transistor.
5. Find the maximum voltage output by varying the frequency of input signal.
6. Record the voltage output in observation table by decreasing the frequency in small steps, below the frequency at which maximum voltage is obtained. Take the reading till voltage output is readable on the CRO.
7. In the similar manner, find the higher cut off frequency (fh) by increasing the frequency from the frequency where maximum gain is obtained.
8. Record the voltage output in observation table by increasing the frequency in small steps, above the frequency at which maximum voltage is obtained. Take the reading till voltage output is readable on the CRO.
9. Plot the frequency response curve as per the reading noted in the observation table.
10. Find out the lower cut-off frequency and higher cut-off frequency from the graph, where the gain falls up to 70.7% of maximum gain.
11. Calculate the bandwidth (f’h – f’l).
12. Now to observe the effect of cascading, now before connecting the second stage with first stage, make sure that second stage of amplifier is identical and its gain as a single amplifier is same as stage first i.e.10 by adjusting the R4.
13 After making sure that transistors are biased in active region feed the input signal from function generator (sine wave amplitude of 1mv) such that the output signal is undistorted.
14 Find the maximum voltage output by varying the frequency of input signal.
15. Repeat the steps from step no.6 to 10.
16. Calculate the bandwidth of two stage amplifier as
BW = f’h - f’L
17. Find the gain bandwidth product for both stages.
Gain BW = gain x (fh-fl)
18. Verify the gain and bandwidth product is constant
Observations Table:
For single stage:
S.No. / Frequency of input signal / Output VoltageGain (10)
For double stage:
S.No. / Frequency of input signal / Output VoltageFor single stage:
Higher cut off frequency (fh)=………….
Lower cut off frequency (fl)=…………
BW = fh - fL
For double stage:
Higher cut off frequency (f’h)=………….
Lower cut off frequency (f’l)=…………
BW = f’h - f’L
Precautions:
1.Make connection very carefully.
2. Make connection right & tight
3. Use C.R.O. carefully.
4. Draw curve carefully and it should be like fig.2
EXPERIMENT NO. 3
Design of RC phase shift oscillator using discrete components.
Aim: To design and realize RC phase shift oscillator of having frequency of 200 hz
using discrete components.
Apparatus: BC-547, resistances (10k ohm-1, 47k ohm-1, 3.3kohm-3, capacitors
(0.1μf -3), regulated power supply (+12v), CRO, connecting wires, breadboard.
Theory:
The circuit arrangement of a Phase –shift oscillator using N-P-N transistor in CE configuration is shown in fig-1. As usual, the voltage divider R1- R2 provides dc ec emitter base bias, Re andCe combination provides temperature stability and prevent ac signal degeneration and collector resistor Rc controls the collector voltage. The oscillator output voltage is capacitevely coupled to the load by Cc.In case of a transistor phase- shift oscillator, the output of the feedback network is loaded appreciably by the relatively small input resistance (hie) of the transistor. Hence instead of employing voltage series feedback, voltage shunt feedback is used for a transistor phase shift oscillator.
The output of the amplifier is fed back to the input through RC network. This RC network provides an overall phase shift of 1800 .This phase shift is achieved in three stages of RC network. The additional 1800 phase shift is obtained from transistor itself. Thus, the phase of the output is in same phase with the input. Output waveform of RC phase shift oscillator is shown in fig-2.
It can be shown that frequency at which RC network provides exactly 1800 phase shift is given by
fo = 1/ 2π √6 +4RC /R
If R=Rc
Then fo = 1/ 2π RC√10
When the feedback is, the overall gain of the amplifier is written as
Af = A/ (1-Aβ),
Where Aβ is feedback factor or loop gain.
If Aβ =1 , Af =∞. Thus the gain becomes infinity i.e. there is output without any input. In another words, the amplifier works as oscillator. This condition is known as Barkhausen criterion of oscillation.
For the loop gain to be greater than unity, the requirement of the current gain of the transistor is found to be
hfe > 23+29 (R/RC) + (Rc / R)
If R=Rc
hfe > (23+29 + 4) i.e. >56
The phase shift oscillator is well suited to the range of frequencies from 20 Hz to 200kHz, and so includes the audio frequency range up to 20kHz.
Fig.1 Circuit diagram of Phase shift oscillator
R1=33kohm Rc=1kohm Ce= 22microfarad C=0.1microfarad
R2 =3.3kohm Re=variable resistance (5kohm) R=3.3 kohm
Procedure:
1. Connect the circuit as shown in fig-1.
2. Adjust the pot-meter for the gain of 56 or more to get the oscillations ,observe the output from collector on the C.R.O.
3. Note this output will be 180o out of phase with respect to input at base of the transistor.
4. Measure the frequency of voltage out.
5. Calculate the frequency of oscillation from formula and compared with the observed value.
Fig-2 Phase shift oscillator out waveform
Precautions:
1. All the connection must be tight.
2. Amplifier gain should be more than 56 to sustain the oscillations.
EXPERIMENT NO. 4
Design and realize inverting amplifier, non - inverting amplifier and buffer amplifier using 741 Op Amp.
Aim: To design inverting amplifier, non –inverting amplifier for the gain of 10 and establishing the saturation input voltage level and design the dc buffer.
Apparatus: Op-amp 741, two resistors (1kΩ), one resistor (10kΩ), bread board, connecting wires, CRO, function generator, multimeter, regulated power supply (+12v & -12v), variable power supply (0-30v).
a) Inverting amplifier:
Theory: