This document discusses the frequency response of audio frequency amplifiers. It explains that an ideal amplifier would provide consistent amplification across all audio frequencies from 20Hz to 20KHz. However, real amplifiers have cut-off frequencies where the gain deviates from ideal. The frequency response curve plots gain against input frequency to determine the upper and lower cut-off frequencies, which define the bandwidth. Below the lower cut-off frequency and above the upper cut-off frequency, the gain drops to 70.7% of the maximum. The document also covers decibel measurements of gain and the significance of octaves and decades in measuring frequency changes.

1. FrequencyResponse
Prepared by
Mrs.V.SrirengaNachiyar, AP(SG)/ECE
RamcoInstitute ofTechnology
Rajapalayam

2. General shape of frequency response of an
amplifier:
• Let us consider an audio frequency amplifier
which operates over audio frequency range
extending from 20Hz to 20KHz.
• The audio frequency amplifiers are used in
everyday life.

3. • Eg:
1. Used in radio receivers
2. To address large public meeting
3. Used for various announcements
• Over the range of frequencies at which it is to
be used, an amplifier should ideally provide
the same amplification for all frequencies.
• The degree to which this is done by a curve
known as frequency response curve of an
amplifier.

4. • The curve is the plot of the voltage gain of an
amplifier against the frequency of an input
signal.
• To plot the frequency response curve, input
voltage to an amplifier is kept constant and
frequency of input signal is continuously
varied.
• The output voltage at each frequency of an
input signal is noted and the gain of an
amplifier is calculated.

5. • The frequency response is nearly ideal over a
wide range of mid-frequency. Only at low and
high frequency ends, the gain deviates from
ideal characteristics.
• Definition of cut-off frequencies &
Bandwidth.
• The two frequencies are indicated by fL & fH
called lower cut-off frequency & upper cut-off
frequency.

6. • Bandwidth:
Bandwidth of an amplifier is defined as
the difference between fH & fL.
Bandwidth = fH - fL.
• These 2 frequencies are also called as half
power frequencies. Since gain or output
voltage drops to 70.7% of maximum value &
this represents a power level of one-half power
at the reference frequency in mid- frequency
region.

7. Low frequency analysis of amplifier to
obtain lower cut-off frequency
• The decibel unit:
The basic for the decibel unit origins from
the logarithmic response of the human ear to the
intensity of sound.
The decibel is a logarithmic measurement
of the ratio of one power to another power or one
voltage to the another voltage.

8. • Voltage gain in dB is given by
Voltage gain = 20 log (AV)
• Power gain in dB is given by,
Power gain = 10 log (AP)
• When AV is greater than one, the dB is
positive.
• When AV is less than one, the dB is negative.
• The positive & negative sign of dB indicates
the amplification and attenuation respectively

9. Significance of Octaves and Decades
• The Octaves and Decades are the measures of
change in frequency.
• A ten times change in frequency is called a
decade.
• On the other hand, an octave correspond to a
doubling or halving of the frequency.

10. References
1. David A. Bell, Electronic Devices & Circuits,
5th Edition, Oxford University Press, 2008.
2. Donald. A. Neamen, Electronic Circuits
Analysis and Design, 3rd Edition, Mc Graw Hill
Education (India) Private Ltd., 2010.
3. Salivahanan and N. Suresh Kumar, Electronic
Devices and Circuits, 4th Edition, , Mc Graw
Hill Education (India) Private Ltd., 2017.