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Effect of Capacitors
Capacitor have capacitive reactance (Xc)
– Coupling capacitors (C1 and C3)– Bypass capacitors (C2)– Internal
Xc = 1/2fc – Very high frequency
Xc=0; AC ground is is effectively zero
– Very low frequency Xc is significant AC ground is not short anymore
Capacitors
Bypass capacitors– RE is parallel with Xc
Coupling capacitors– Low frequency High reactance High
voltage drop: Xc.Ic = Vc– Low frequency more shift in phase
Internal capacitance– High frequency lower capacitive
reactance (Xc)
Remember: Phase shift is due Xc = |Xc| |_ Angle
Decibel
Logarithmic measurement of the ratio of one power/voltage to another.
– Ap (dB)= 10log Ap– Av (dB) = 20log Av
Example– Ap = 0.5 W Ap (dB) = 10 log (0.5) = -3dB– Av = 0.707 V Av (dB) = 20 log (0.707) = -3dB
dBm is the unit to measure power with reference to 1mW – 32 mW 10log (32mW/1mW) = 15 dBm
Center Frequency
Center frequency (fc) also known as critical /cut off frequency or roll-off frequency – @ fc -3dB – @ 0.1 x fc -20dB – @ 0.01 x fc -40dB
Mid-frequency refers to higher frequencies
Center frequency(-3 dB)
Center frequency: frequency at which the gain is down 3dB (called the -3dB point)
Center Frequency and Bode Plots
A plot of dB voltage gain verses frequency on a semilog graph paper
Estimation
Actual fequency response
At fc: R = Xc
Circuit Analysis at Low Frequencies
22
1
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sTH RRRR 21Bypass Capacitor:
Low Frequency Amplifier Response
Putting it all together:
Individual responses
Overall frequency response
What is the GAIN?
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totalin
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LCmidv RR
R
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RRA
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