BJT Frequency Response -...

07/10/58

1

BJT Frequency Response

LF Response, Miller Theorem & HF Response

M2-3 Electrinics 1/30

FR; Outline

- Introduction

- Decibels

- Bode Plot (Frequency Response)

- Low Frequency Analysis

- Miller Theorem

Hi h F A l i- High Frequency Analysis

M2-3 Electrinics 2/30

07/10/58

2

Introduction

- Previously, effects of capacitive elements y, pare ignored

- Capacitor affect the LF & HF of amplifiers

- Swept Frequencies are used to test- Logarithmic scales are both used in gain & frequency axes

M2-3 Electrinics 3/30

Logarithm (I)

- Logarithm

M2-3 Electrinics 4/30

07/10/58

3

Logarithm (II)

- Some useful logarithm

M2-3 Electrinics 5/30

Logarithm (III)

- Semilog scale

M2-3 Electrinics 6/30

07/10/58

4

Important points in log scale

M2-3 Electrinics 7/30

Decibels- Bel is the surname of Alexander Graham Bell

M2-3 Electrinics 8/30

07/10/58

5

Advantage of Logarithm

Gain calculation made easier

M2-3 Electrinics 9/30

Example of gain calculation

M2-3 Electrinics 10/30

07/10/58

6

General Frequency Considerations

RC coupling

M2-3 Electrinics 11/30

Cut-Off or Half-Power Frequency

M2-3 Electrinics 12/30

07/10/58

7

Bandwidth, BW

Linear

Log

M2-3 Electrinics 13/30

Phase plot

M2-3 Electrinics 14/30

07/10/58

8

Low Frequency Analysis

@ Hi h F@ High Frequency

@ DCM2-3 Electrinics 15/30

Voltage Divider

M2-3 Electrinics 16/30

07/10/58

9

Gain Equation

M2-3 Electrinics 17/30

Bode Plot of Low Frequency Region

M2-3 Electrinics 18/30

07/10/58

10

Example of LF Bode Plot

M2-3 Electrinics 19/30

Phase response of LF region

M2-3 Electrinics 20/30

07/10/58

11

Miller Effect Capacitance (I)

M2-3 Electrinics 21/30

Miller Effect Capacitance (II)

CMo ~ Cf ; AV >>1M2-3 Electrinics 22/30

07/10/58

12

High Frequency Analysis

M2-3 Electrinics 23/30

Parasitic Capacitances

M2-3 Electrinics 24/30

07/10/58

13

HF Model of BJT Amplifier

M2-3 Electrinics 25/30

Parameter Variation

M2-3 Electrinics 26/30

07/10/58

14

Full Frequency Response

M2-3 Electrinics 27/30

Square-wave Testing

M2-3 Electrinics 28/30

07/10/58

15

Results of Square-wave Tesing

M2-3 Electrinics 29/32

Rise Time & Tilt

orWh f i hi h t ff & f or

M2-3 Electrinics 30/32

Where fHi is high cut off & fLo

is low cut off frequencies.The fs is the frequency of square wave signal

07/10/58

16

Rise Time & Tilt; example

M2-3 Electrinics 31/32

Memo

M2-3 Electrinics 32/32