frequency response of an amplifier

1. Frequency Response of an Amplifier
Experiment No 3
Batch B1 Bock 1
Omkar Rane SETB118
Rohit Mane SETB102
Abhishek Sainkar SETB104
Tanmay Kale SETB112
JULY 31, 2017
MIT ACADEMY OF ENGINEERING, DEPARTMENT OF ENTC ENGG.
ANALOG ELECTRONICS LAB MANUAL

2. S.Y.B. Tech (ET): 2017-18 Analog Electronics
EXPERIMENT No: 3
Frequency Response of an Amplifier
Introduction: -
A practical amplifier circuit increases the voltage level of the input signal. Signal obtained
from radio or TV receiver circuit consists of a band of frequencies, e.g. from 20 Hz to 20
KHz. The amplifier must amplify all the frequency components of signal by same amount
so that output of the loudspeaker will not be the exact replica of the original sound. If the
loudspeakers are to reproduce the sound faithfully, then no distortion or attenuation has to
be introduced by the amplifier
Fig.1. a Fig. 1.b
Objective: -
 To observe the frequency response of an amplifier
 To calculate Bandwidth of the circuit
Pre-lab Questions: -
Write significance of Frequency Response
Ans) Frequency Response of an electric or electronics circuit allows
us to see exactly how the output gain (known as the magnitude

3. S.Y.B. Tech (ET): 2017-18 Analog Electronics
response) and the phase (known as the phase response) changes at
a particular single frequency, or over a whole range of different
frequencies from 0Hz, (d.c.) to many thousands of mega-hertz, (MHz)
depending upon the design characteristics of the circuit.
Generally, the frequency response analysis of a circuit or system is
shown by plotting its gain, that is the size of its output signal to its
input signal, Output/Input against a frequency scale over which the
circuit or system is expected to operate. Then by knowing the
circuits gain, (or loss) at each frequency point helps us to
understand how well (or badly) the circuit can distinguish between
signals of different frequencies.
Lab Exercise: -
1. Draw the circuit diagram of Single stage CE amplifier here
Post Lab. Question: -
2. Procedure: - (Write in your own words)
Practical method:

4. S.Y.B. Tech (ET): 2017-18 Analog Electronics
1. Setup voltage divider bias CE amplifier on breadboard or multisim
software.
2. Using signal generator set the input voltage for signal generator
Vin.
3. Put coupling capacitor C1 in series with source.
4. +Vcc =12 V is provided by DC dual regulated power supply and
ground the required connection as per circuit diagram.
5. C2 capacitor is bypass capacitor take reading of different voltages
from CRO without and with bypass capacitor. Also note frequency
values as per varying voltages.
6. Plot these values Voltage Vs frequencies on semi log paper. In this
way, we can draw a frequency response curve.
Software Method:
1. Using multisim software setup voltage divider single stage
amplifier on workspace of multisim.
2. Drag marker tool to input and output of amplifier. set in input
voltage of Vi=20V.
3. In simulate menu click on AC analysis option.
4. Set value to decibel and select voltage parameter.
5. Click on simulate option to get the frequency response graph.
3. List the Equipments / Component required for the experiment
Equipment / Components Specifications/Values

5. S.Y.B. Tech (ET): 2017-18 Analog Electronics
Function Generator CADDO,4061,3MHz Pulse
Generator with 40 MHz Frequency
Counter
CRO HAL-TEC plus, HT4030,30MHz,
Oscilloscope
Resistor R1=8k, R2=2.7k,RC=3.3k, RE=1k
Breadboard
Variable Power Supply Radial Industry SVP030002D
Transistor BC547{npn}
National instruments Multisim
software
Ver 8 or 12
4. Observation Table (Multisim readings)
Parameters
With Bypass
Capacitor
Without Bypass
Capacitor
Gain 818mV/40mV =
20.45
129mV/40mV=
3.225
Bandwidth F2 = 19.0353MHz
F1=12.4894KHz
B=F2-F1
= 19.0353MHz
F2 = 22.8292MHz
F1=77.0535Hz
B=F2-F1
= 22.8291MHz

6. S.Y.B. Tech (ET): 2017-18 Analog Electronics
5. Observation Table (Hardware readings)
Vin = 20mV
Frequency Voltage(Vout) Gain Gain in dB
Low Frequency Region
50 Hz 4.76 V 238 47.531
100 Hz 8.13 V 406.5 52.181
500 Hz 10.21 V 510.5 54.159
Mid Frequency Region
1 KHz 10.22 V 511 54.169
50 KHz 10.22 V 511 54.169
200 KHz 10.22 V 511 54.169
1 MHz 10.22 V 511 54.169
High Frequency Region
10 MHz 9.25 V 462.5 53.302
50 MHz 4.06 V 203 46.149
100 MHz 2.27 V 113.5 41.019

7. S.Y.B. Tech (ET): 2017-18 Analog Electronics
Frequency Response Curves: (With Capacitor and Without Curve)
Frequency Response curve (With Capacitor)
Frequency Response curve (Without Capacitor)

8. S.Y.B. Tech (ET): 2017-18 Analog Electronics
Circuit diagram on multisim.
Amplifier with Bypass Capacitor
Amplifier without Bypass Capacitor
Conclusion: -
We have verified frequency response graph on multisim. Gain is
inversely proportional to bandwidth. In the presence of bypass
capacitor gain is reduced so bandwidth will increase. In case without
bypass capacitor gain will decrease so bandwidth will increase in
this case. We have observed this in frequency response graph.

9. S.Y.B. Tech (ET): 2017-18 Analog Electronics
Post Lab. Questions: -
1. Why there is drop in gain at lower frequency and higher frequency
Ans 1) At higher frequencies, the internal junction capacitors of
transistor come into play, thus reducing the output and therefore the
gain of amplifier. The capacitor reactance decreases with increase in
frequency bypassing the majority of output. In some cases, the output
gets fed back to input as negative feedback.
2. As the frequency of the input signal increases, the gain of the
amplifier reduces. The factor responsible for the reduction in gain at
higher frequencies is the presence of various capacitors as shown in
fig. 2. They are not physically connected but inherently present with
the device. Justify
Fig. 2
Ans 2) These are parasitic capacitor present in transistor.
At low frequencies, parasitic capacitance can usually be ignored,
but in high frequency circuits it can be a major problem.
In amplifier circuits with extended frequency response, parasitic

10. S.Y.B. Tech (ET): 2017-18 Analog Electronics
capacitance between the output and the input can act as
a feedback path, causing the circuit to oscillate at high frequency.
3. Why we take 3 dB down from the maximum gain to measure BW?
Ans 3) The -3dB, come from 20 Log (0.707).
Gain dB= 20 log(Av).
Note :- Kindly attach screenshot of your performed experiments