Amplifier Frequency Response

Chapter 10

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

12

1

CRf

inc

Circuit Analysis at Low Frequencies

22

1

RCf c

)(emitterRinRR E

ac

THe

RremitterRin

')(

sTH RRRR 21Bypass Capacitor:

Circuit Analysis at Low Frequencies

3)(2

1

CRRf

LCc

Low Frequency Amplifier Response

Putting it all together:

Individual responses

Overall frequency response

What is the GAIN?

)(

)(

')(totalinS

totalin

e

LCmidv RR

R

r

RRA

So What about the Internal Capacitance of the Transistor?

Note: Cobo=Cob=CbcCibo=Cib=Cbe

Ignore stray capacitance

)1()( VbcMillerin ACC

V

VbcMillerout A

ACC

)1()(