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 = 1/2  fc – 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 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)

6. 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

7. Circuit Analysis at Low Frequencies

8. 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