Neamen Microelectronics Chapter 6-1 14 February 2012 McGraw-Hill

Microelectronics Circuit Analysis and Design

Donald A. Neamen

Chapter 6

Basic BJT Amplifiers

Neamen Microelectronics Chapter 6-2 14 February 2012 McGraw-Hill

In this chapter, we will:

Understand the concept of an analog signal and the principle of a linear amplifier. Investigate how a transistor circuit can amplify a

small, time-varying input signal. Discuss and compare the three basic transistor

amplifier configurations. Analyze the common-emitter amplifier.

Understand the ac load line & determine the maximum symmetrical swing of the output.

Analyze the emitter-follower amplifier. Analyze the common-base amplifier.

Analyze multitransistor or multistage amplifiers. Understand the concept of signal power gain in an

amplifier circuit.

Neamen Microelectronics Chapter 6-3 14 February 2012 McGraw-Hill

Common Emitter with Time-Varying Input

Neamen Microelectronics Chapter 6-4 14 February 2012 McGraw-Hill

IB Versus VBE Characteristic

bB

T

beBQB iI

V

vIi )1(

Neamen Microelectronics Chapter 6-5 14 February 2012 McGraw-Hill

ac Equivalent Circuit for Common Emitter

Neamen Microelectronics Chapter 6-6 14 February 2012 McGraw-Hill

Small-Signal Hybrid p Model for npn BJT

p

p

rg

I

Vr

V

Ig

m

CQ

T

T

CQ

m

Phasor signals are shown in parentheses.

Neamen Microelectronics Chapter 6-7 14 February 2012 McGraw-Hill

Small-Signal Equivalent Circuit Using Common-Emitter Current Gain

Neamen Microelectronics Chapter 6-8 14 February 2012 McGraw-Hill

Small-Signal Equivalent Circuit for npn Common Emitter circuit

))((B

CmvRr

rRgA

p

p

Neamen Microelectronics Chapter 6-9 14 February 2012 McGraw-Hill

Problem-Solving Technique: BJT AC Analysis

1. Analyze circuit with only dc sources to find Q point.

2. Replace each element in circuit with small-signal model, including the hybrid p model for the transistor.

3. Analyze the small-signal equivalent circuit after setting dc source components to zero.

Neamen Microelectronics Chapter 6-10 14 February 2012 McGraw-Hill

Transformation of Elements

Element DC Model AC Model

Resistor R R

Capacitor Open C

Inductor Short L

Diode +Vg, rf –

rd = VT/ID

Independent Constant Voltage Source

+ VS -

Short

Independent Constant Current Source

IS

Open

Neamen Microelectronics Chapter 6-11 14 February 2012 McGraw-Hill

Hybrid p Model for npn with Early Effect

CQ

AoI

Vr

Neamen Microelectronics Chapter 6-12 14 February 2012 McGraw-Hill

Hybrid p Model for pnp with Early Effect

Neamen Microelectronics Chapter 6-13 14 February 2012 McGraw-Hill

Neamen Microelectronics Chapter 6-14 14 February 2012 McGraw-Hill

Neamen Microelectronics Chapter 6-15 14 February 2012 McGraw-Hill

Neamen Microelectronics Chapter 6-16 14 February 2012 McGraw-Hill

Neamen Microelectronics Chapter 6-17 14 February 2012 McGraw-Hill

Neamen Microelectronics Chapter 6-18 14 February 2012 McGraw-Hill

Neamen Microelectronics Chapter 6-19 14 February 2012 McGraw-Hill

Neamen Microelectronics Chapter 6-20 14 February 2012 McGraw-Hill

Neamen Microelectronics Chapter 6-21 14 February 2012 McGraw-Hill

Neamen Microelectronics Chapter 6-22 14 February 2012 McGraw-Hill

Neamen Microelectronics Chapter 6-23 14 February 2012 McGraw-Hill

Neamen Microelectronics Chapter 6-24 14 February 2012 McGraw-Hill

Neamen Microelectronics Chapter 6-25 14 February 2012 McGraw-Hill

Neamen Microelectronics Chapter 6-26 14 February 2012 McGraw-Hill

Neamen Microelectronics Chapter 6-27 14 February 2012 McGraw-Hill

Expanded Hybrid p Model for npn

Neamen Microelectronics Chapter 6-28 14 February 2012 McGraw-Hill

h-Parameter Model for npn

p

fe

bie

h

rrrh

o

oe

re

rrh

r

rh

11

p

Neamen Microelectronics Chapter 6-29 14 February 2012 McGraw-Hill

T-Model of an npn BJT

Neamen Microelectronics Chapter 6-30 14 February 2012 McGraw-Hill

4 Equivalent 2-port Networks

Voltage Amplifier

Current Amplifier

Neamen Microelectronics Chapter 6-31 14 February 2012 McGraw-Hill

4 Equivalent 2-port Networks

Transconductance Amplifier

Transresistance Amplifier

Neamen Microelectronics Chapter 6-32 14 February 2012 McGraw-Hill

Common Emitter with Voltage-Divider Bias and a Coupling Capacitor

Neamen Microelectronics Chapter 6-33 14 February 2012 McGraw-Hill

Small-Signal Equivalent Circuit – Coupling Capacitor Assumed a Short

Neamen Microelectronics Chapter 6-34 14 February 2012 McGraw-Hill

npn Common Emitter with Emitter Resistor

Neamen Microelectronics Chapter 6-35 14 February 2012 McGraw-Hill

Small-Signal Equivalent Circuit: Common Emitter with RE

)()1(

)1(

21

Si

i

E

Cv

ibi

Eib

RR

R

Rr

RA

RRRR

RrR

p

p

Neamen Microelectronics Chapter 6-36 14 February 2012 McGraw-Hill

RE and Emitter Bypass Capacitor

Neamen Microelectronics Chapter 6-37 14 February 2012 McGraw-Hill

Problem-Solving Technique: Maximum Symmetrical Swing

1. Write dc load line equation that relates ICQ

and VCEQ.

2. Write ac load line equations that relates ic and vce

3. In general, ic = ICQ – IC(min), where IC(min) is zero or other minimum collector current.

4. In general, vce = VCEQ – VCE(min), where VCE(min) is some specified minimum collector-emitter voltage.

5. Combine above 4 equations to find optimum ICQ and VCEQ.

Neamen Microelectronics Chapter 6-38 14 February 2012 McGraw-Hill

Common-Collector or Emitter-Follower Amplifier

Neamen Microelectronics Chapter 6-39 14 February 2012 McGraw-Hill

Small-Signal Equivalent Circuit: Emitter Follower

)())(1(

))(1(

))(1(

21

Si

i

Eo

Eo

v

ibi

Eoib

RR

R

Rrr

RrA

RRRR

RrrR

p

p

Neamen Microelectronics Chapter 6-40 14 February 2012 McGraw-Hill

Output Resistance: Emitter Follower

oEo rRr

R

p

1

Neamen Microelectronics Chapter 6-41 14 February 2012 McGraw-Hill

Common-Base Amplifier

Neamen Microelectronics Chapter 6-42 14 February 2012 McGraw-Hill

Small-Signal Equivalent Circuit: Common Base

]1

)[(

)(

E

LC

Cmi

LCmv

Rr

RR

RgA

RRgA

p

Neamen Microelectronics Chapter 6-43 14 February 2012 McGraw-Hill

Input Resistance: Common Base

Rie = rp/(1+)

Neamen Microelectronics Chapter 6-44 14 February 2012 McGraw-Hill

Output Resistance: Common Base

RO = RC

Neamen Microelectronics Chapter 6-45 14 February 2012 McGraw-Hill

Common Emitter Cascade Amplifier

Neamen Microelectronics Chapter 6-46 14 February 2012 McGraw-Hill

Small-Signal Equivalent Circuit: Cascade Amplifier

Neamen Microelectronics Chapter 6-47 14 February 2012 McGraw-Hill

Darlington Pair

21iA

Neamen Microelectronics Chapter 6-48 14 February 2012 McGraw-Hill

Cascode Amplifier

Neamen Microelectronics Chapter 6-49 14 February 2012 McGraw-Hill

Small-Signal Equivalent Circuit: Cascode Amplifier

)(1 LCmv RRgA