Quiz 4 performance
1
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Chapter 8: Frequency Response
for
8.1 BODE PLOTS
Logarithmic frequency scales:
For f << fb, |Av(f)|dB =0
For f >> fb
For f = fb
Phase of Av(f):
Fig. 8.3: Bode plot for the low-pass RC filter
Fig. 8.3: Bode plot for the low-pass RC filter
The Magnitude Bode Plot: The Phase Bode Plot:
Note: The actual responses are usually approximated by the asymptotes
The Magnitude and Phase Bode Plots
The results is a ratio of polynomials in s, we have a pole at s = -1/(R1+R2)C, and a zero at s = -1/R2C
Magnitude of Av(f) =
In decibels:
Let
We have,
Bode Plot for an RC Circuit with one pole and one zero
Example 8.1:
Steps: 1.Plot each of the functions 2.Note the corner frequencies3.Determine the superimposed plot based on the corner frequencies
The Magnitude Bode Plot
The Phase Bode Plot
At a very high frequencies, the capacitor behaves as a short circuit, then the circuit reduces to a resistive voltage divider with a gain of
Simple checks of the Bode Plots:
Thus the gain is -20 dB at high frequencies, similar to what we have plotted.
Magnitude of Av(f):
Example
In decibels,
Magnitude Plots
9
Phase Plots
FET Common Source Amplifier model at High Frequencies
where
Thus to obtain higher value of upper freq:
1.Reduce Cgs and Cds
2.Reduce Rsig and RL
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