Chương 7: Frequency Response docx

Frequency of applied signal has effect on response of circuit Plots of effect of frequency on gain and phase... Frequency response analysis model Capacitor is short-circuit equivalent

Frequency response

 Introduction

 Low-frequency analysis

 High-frequency analysis

 Low-frequency response of BJT & FET amplifier

 Miller effect capacitance

 High-frequency response of BJT & FET amplifier

Frequency of applied signal has effect on response of circuit Plots of effect of

frequency on gain and phase

Frequency response

analysis model

Capacitor is

short-circuit equivalent at high frequency open-circuit equivalent at low frequency

Low-frequency analysis

Low frequency,

C is open-circuit equivalent

⇒ V o =0, A v =0

High frequency,

C is short-circuit equivalent

High-frequency analysis

Low frequency,

C is open-circuit equivalent

High frequency,

C is short-circuit equivalent

⇒ V o =0 , A v =0

Bode plot

Cutoff frequency: gain

is reduced by a factor of 0.707

f1 cutoff frequency at low frequency

f2 cutoff frequency at high frequency

Low-frequency response of

BJT amplifier

Cutoff low-frequency

by source capacitance

fLs=1/[2π(Rs+Ri)Cs] with Ri=R1//R2//βre

Cutoff low-frequency

by output capacitance

fLo=1/[2π(Ro+RL)Cc] with Ro=RC//ro

Cutoff low-frequency

by emitter capacitance

fLE=1/[2πReCE] with Re=RE//(Rs’/β+re)

Low frequency response of FET

Q1 RD

RS

Rsig

Vs

CG

4

5

Cs

Cc

VDD 5V

VDD

6

3

1

7

0

Miller effect capacitance

voltage gain Input, output capacitance

effect capacitance

High-frequency response of

BJT amplifier

High frequency limitation is determined by parasitic

capacitors Cbc, Cce,

Cbe, Cwi, Cwo

High-frequency response of

BJT amplifier

Ci=CWi+Cbe+Cmi Co=CWo+Cce+CMi

High-frequency response of

BJT amplifier

High frequency response of FET

Q1 RD

RS

Rsig

Vs

CG

4

5

Cs

Cc

VDD 5V

VDD

CWi

Cgs

Cds Cgd

CWo

6

7

0

3 1

Read 11.10 page 546, Electronic devices and circuit theory

 Chapter 11: 10, 11, 15, 17, 18, 19,

22, 26, 28, 29, 32