BJT CE

ELECTRONICS LABORATORY I

1

by Fatemeh Khorramshahi

Outline

BJTCommon Emitter

○ Determining quiescent conditions○ Calculating small signal performance

Voltage GainInput ImpedanceOutput ImpedanceCut-off frequency

Common CollectorCommon Base

2

npn transistor

simplified structure

Note: Normally Emitter layer is heavily doped, Base layer is lightly doped and Collector layer has Moderate doping.

B

C

E

Schematic Symbol

3

• Curve TracerProvides a graph of the characteristic curves.

• Digital multimeters (DMM)Some DMMs measure βDC or hfe.

• Ohmmeter

Transistor Testing

4

Small signal models of bjt

b

e

hoe

hie

hrevce hfeib vbe

ib ic

vce

c

e

+ _

+ +

_ _

h-parameter model

Hybrid-π model

5

Common Emitter (CE)

DC analysis AC analysis

6

Collector Characteristics Base Characteristics

Common-Emitter Characteristics

7

Features of Common Emitter

High voltage gain High current gain Medium input impedance due to high

current gain High output impedance. For HF capacitive

loading will need to be resonated reducing bandwidth.

Bad HF & bandwidth as falling beta with frequency reduces gain.

8

Input Impedance

IN

ININ i

vr

iIN iB

iRB

Input impedance, rIN, is the ratio of the small signal input voltage and the small signal input current

BRBIN iii

B

INRB R

vi

mINC

B

gvii

9

Input Impedance

iIN iB

iRB

mIN

B

INBRBIN

gv

R

viii

mB

mBIN

ININ g

RgRi

vr

||//1

1

10

Output Impedance

One way to measure rOUT is: Short the input to 0 V Output now looks like just rOUT

11

Output Impedance (cont)

00 CIN iv

Applying Kirchoff’s current law:

RCOUTOUTRCC iiiii 0

RC

OUTC

RC

C

i

vR

i

v

By Ohm’s law:

CCRC

OUT

OUT

OUTOUT RR

iv

iv

r

VCC

12

Capacitors

Capacitor COUT is needed to remove the d.c. component of the collector voltage

Capacitor CIN is needed to allow the base voltage to be offset from 0V

In both cases this is known as coupling

Both capacitors are chosen to look like short circuits at operating frequencies

Their reactance will, however, become significant at low frequencies

VCC

0 V

13

14

Frequency response Midband:

The frequency range of interest for amplifiers Large capacitors can be treated as short circuit and small capacitors can be

treated as open circuit Gain is constant and can be obtained by small-signal analysis

Low-frequency band: Gain drops at frequencies lower than fL

Large capacitors can no longer be treated as short circuit The gain roll-off is mainly due to coupling and by-pass capacitors For calculation we use dominant pole approximation

○ If there is a dominant pole, the cutoff frequency is determined mainly by this pole.

High-frequency band: Gain drops at frequencies higher than fH

Small capacitors can no longer treated as open circuit The gain roll-off is mainly due to parasitic capacitances of the MOSFETs and

BJTs

Considering the effect of each capacitor separately

Considering only Cc1:

15

Considering the effect of each capacitor separately

Considering only CE:

16

By using Norton equivalent circuit

Considering the effect of each capacitor separately

Considering only Cc2:

17

Determining the lower 3-dB frequency

Coupling and by-pass capacitors result in a high-pass frequency response with three poles

The lower 3-dB frequency is simply the highest-frequency pole if the poles are sufficiently separated

The highest-frequency pole is typically ωp2 due to the small resistance of RE

An approximation of the lower 3-dB frequency is given by

Selecting values for the coupling and by-pass capacitors These capacitors are typically required for discrete amplifier designs CE is first determined to satisfy needed fL

CC1 and CC2 are chosen such that poles are 5 to 10 times lower than fL 18r

fC

CCC

102

12,1

Common Base (CB)

Current gain of approximately 1 (alpha) Low input impedance

(due to low current gain) High output impedance High voltage gain

(if input impedance matched) Good HF & bandwidth as falling beta

with frequency matters less.

19

Common Collector (CC)

Voltage gain of almost exactly 1 High current gain High input impedance

(due to high current gain) Low output impedance (Good for

unknown loads) Good HF & bandwidth as falling beta

with frequency matters less.

20