GATE Electrical – Analog Electronics
Page 17 : Bode Plot of Amplifiers
A Bode Plot is a graphical method used to represent the frequency response of an amplifier using a logarithmic frequency scale.
It shows how amplifier gain varies with frequency.
What is a Bode Plot?
A Bode plot consists of two graphs:
- Magnitude plot (Gain vs Frequency)
- Phase plot (Phase vs Frequency)
For most GATE questions, the magnitude plot is mainly used.
Gain in Decibels
In Bode plots, amplifier gain is expressed in decibels (dB).
Gain (dB) = 20 log10 (Av)
Where:
- Av = Voltage gain
Frequency Scale
Frequency is plotted on a logarithmic scale.
Example frequency values:
- 10 Hz
- 100 Hz
- 1 kHz
- 10 kHz
- 100 kHz
Slope of Bode Plot
The gain decreases with frequency at a certain rate.
Typical slopes are:
- -20 dB / decade
- -40 dB / decade
- -60 dB / decade
Each pole introduces a slope of -20 dB/decade.
Corner Frequency
The frequency at which gain starts decreasing is called the corner frequency.
At this point gain drops by:
3 dB
Important GATE Points
- Bode plot uses logarithmic frequency scale.
- Gain is expressed in decibels.
- Each pole contributes -20 dB/decade slope.
- Cut-off frequency corresponds to -3 dB point.
Next Page → Miller Effect in Amplifiers
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