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Amplifier Frequency Response

Amplifier Frequency Response . Chapter 10. Effect of Capacitors . Capacitor have capacitive reactance (Xc) Coupling capacitors (C1 and C3) Bypass capacitors (C2) Internal Xc = 1/2 p fc Very high frequency Xc=0; AC ground is is effectively zero Very low frequency Xc is significant

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Amplifier Frequency Response

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  1. Amplifier Frequency Response Chapter 10

  2. Effect of Capacitors • Capacitor have capacitive reactance (Xc) • Coupling capacitors (C1 and C3) • Bypass capacitors (C2) • Internal • Xc = 1/2pfc • Very high frequency • Xc=0; AC ground is is effectively zero • Very low frequency • Xc is significant • AC ground is not short anymore

  3. 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

  4. 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

  5. Center frequency (-3 dB) 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: frequency at which the gain is down 3dB (called the -3dB point)

  6. Center Frequency and Bode Plots • A plot of dB voltage gain verses frequency on a semilog graph paper Estimation At fc: R = Xc Actual fequency response

  7. Circuit Analysis at Low Frequencies

  8. Circuit Analysis at Low Frequencies Bypass Capacitor:

  9. Circuit Analysis at Low Frequencies

  10. Low Frequency Amplifier Response Putting it all together: Individual responses Overall frequency response What is the GAIN?

  11. So What about the Internal Capacitance of the Transistor? Note: Cobo=Cob=Cbc Cibo=Cib=Cbe Ignore stray capacitance

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