Chapter 5: BJT AC Analysis - الصفحات...

Chapter 5: BJT AC Analysis

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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky

Two-Port Systems ApproachThis approach:

• Reduces a circuit to a two-port system• Provides a “Thévenin look” at the output terminals• Makes it easier to determine the effects of a changing load

ooTh RZZ

ivNLTh VAE

With Vi set to 0 V:

The voltage across the open terminals is:

where AvNL is the no-load voltage gain.




Effect of Load Impedance on Gain

L

ivi R

ZAA

This model can be applied to any current- or voltage-controlled amplifier.

Adding a load reduces the gain of the amplifier:

vNLoL

L

i

ov A

RR

R

V

VA




Effect of Source Impedance on Gain

si

sii RR

VRV

The fraction of applied signal that reaches the input of the amplifier is:

vNLsi

i

s

ovs A

RR

R

V

VA

The internal resistance of the signal source reduces the overall gain:




Combined Effects of RS and RL on Voltage Gain

Effects of RL:

Effects of RL and RS:

L

ivi

oL

vNLL

i

ov

R

RAA

RR

AR

V

VA

L

isvsis

vNLoL

L

si

i

s

ovs

R

RRAA

ARR

R

RR

R

V

VA




Cascaded Systems

• The output of one amplifier is the input to the next amplifier• The overall voltage gain is determined by the product of gains of the

individual stages• The DC bias circuits are isolated from each other by the coupling

capacitors• The DC calculations are independent of the cascading• The AC calculations for gain and impedance are interdependent




R-C Coupled BJT Amplifiers

This image cannot currently be displayed.

Co RZ

Input impedance, first stage:

Output impedance, second stage:

Voltage gain:

e21i r||R||RZ

2v1vv

e

C2V

e

e21C1v

AAA

r

RA

r

r||R||R||RA




R-C Coupled BJT AmplifiersDraw the ac equivalent circuit and calculate the voltage gain, input resistance andoutput resistance for the cascade BJT amplifier in above Figure. Let the parametersare:

Solution

Dc Analysis

kRRkRRkRRkRR EECC 1,2.2,7.4,15 21214231

0)(21 ,7.0,200,20 rVVVV ONBEQQCC

mAI

AI

CQ

BQ

979.3

89.19

1

1

mAI

AI

CQ

BQ

979.3

89.19

2

2

k307.12211 ee rr




R-C Coupled BJT AmplifiersAc Analysis:The voltage gain at Q1 is:

The voltage gain at Q2 is

The overall gain is then,

The input resistance is:

The o/p resistance is:

06.102)36.957//2.2(153.01 kAV

6.336)2.2(153.02 kAV

353,34)6.336)(06.102(21 VVV AAA

36.957//// 21 ei BrRRR

kRR Co 2.22




Cascode Connection

This example is a CE–CB combination. This arrangement provides high input impedance but a low voltage gain.

The low voltage gain of the input stage reduces the Miller input capacitance, making this combination suitable for high-frequency applications.




Cascode Connection




Darlington Connection

The Darlington circuit provides a very high current gain—the product of the individual current gains:

bD = b1b2

The practical significance is that the circuit provides a very high input impedance.




DC Bias of Darlington Circuits

This image cannot currently be displayed.

BDBDE II)1(I

EEE RIV

EDB

BECCB RR

VVI

Base current:

Emitter current:

Emitter voltage:

Base voltage:

BEEB VVV




AC small signal of Darlington Circuits




Feedback Pair

This is a two-transistor circuit that operates like a Darlington pair, but it is not a Darlington pair.

It has similar characteristics: • High current gain• Voltage gain near unity• Low output impedance• High input impedance

The difference is that a Darlington uses a pair of like transistors, whereas the feedback-pair configuration uses complementary transistors.




DC Analysis of Feedback Pair




AC Analysis of Feedback Pair




Current Mirror Circuits

Current mirror circuits provide constant current in integrated circuits.




Current Mirror Example

Find I?




Current Mirror More Configurations




Current Source CircuitsConstant-current sources can be built using FETs, BJTs, and combinations of these devices.

IE IC

E

BEZE R

VVII

more…




Current Source Circuits

VGS = 0VID = IDSS = 10 mA


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