Lecture 15-1

Common Collector (Emitter Follower) Amplifier

• Gain is never better than unity, however, has some desirable input and output impedance characteristics --- acts as a buffer

VCC

RB

RC

RE

-VEE

C1

C2

C3

RL

vs

RS

Lecture 15-2

Common Collector (Emitter Follower) Amplifier

• Without RC there is no need for C3

VCC

RB RE

-VEE

C1

C2

RL

vs

RS

Lecture 15-3

Emitter FollowerExample

• Calculate the voltage gain, current gain, input resistance and output resistance

• Assume capacitors are infinite

• 1) Calculate dc operating point

+10V

100kΩ 10kΩ

-10V

10kΩ vs

10kΩ

β 100=

VA 100=

Lecture 15-4

Emitter Follower Example

• 2) Establish small signal model

100kΩ

10kΩ 10kΩ

vs

10kΩ

rπ βibro

Lecture 15-5

Amplifier Input Resistance, Rib

100kΩ

10kΩ 10kΩ

vs

10kΩ

rπ βibro

• 3) Calculate input resistance

Lecture 15-6

Transistor’s Input Resistance, Rib

100kΩ

10kΩ 10kΩ

3kΩ βib

119kΩ

• “Reflect” impedances into base from emitter for simplified input resistance calcuation

Rib

Lecture 15-7

Amplifier Input Resistance, Ri

100kΩ

10kΩ 10kΩ

3kΩ βib

119kΩ

• RB is part of the amplifier circuit, and adds to the input resistance to the transistor

Ri

• Amplifier input resistance is limited by RB in this circuit. Why not make it bigger?

Lecture 15-8

Emitter Follower --- Buffer

100kΩ

10kΩ RL

vs

RS

rπ βibro

• Even though the load resistance is 10kΩ, the input resistance is much higher

• This is a desirable feature of a buffer amplifier, especially if RS is large, or RL is small

• Is Rin big enough for our example? What is vi?

vi

Lecture 15-9

Emitter Follower --- Buffer

10kΩ RL

re

αie

• Further voltage division for the amplifier stage

• Most easily seen using other small signal model

vi

ro

Lecture 15-10

Analysis by Inspection

• Experienced analog designers just analyze the circuit directly by inspection

+10V

100kΩ 10kΩ

-10V

10kΩ

vs

10kΩ

Lecture 15-11

Analysis by Inspection

• What if RL is much less than RE?

+10V

10kΩ

-10V

RL

vs

10kΩ

Lecture 15-12

Emitter Follower Current Gain

100kΩ

10kΩ 10kΩ

vs

10kΩ

rπ βib

119kΩ

• No voltage gain, but acts as a buffer to drive small impedance loads

• Provides good current amplification

Lecture 15-13

Current Gain

100kΩ

10kΩ 10kΩ

vs

10kΩ

rπ βib

119kΩ

Lecture 15-14

Output Resistance, Ro

100kΩ

10kΩ

0

10kΩ

rπ βibro

• 4) Calculate output resistance

Ro

Lecture 15-15

Output Resistance, Ro

Lecture 15-16

SPICE Results

Q1CustomIIN

0.000 pA

C21E-6F+ -

RB100E3Ω

+ -

10VVCC

IO

0.000 pA

C11E-6F+ -

+ -10VVEE

RS10E3Ω

IC

839.882 µA

+ SINVSSIN

+

-

VIN

-758.300 mV

RE10E3Ω

RL10E3Ω

+

-

VOUT

0.000 pV

+

-

VINAC

0.000 pV

dc solution

Lecture 15-17

SPICE Results

voltage gain

time0.0 0.1 0.2 0.3 0.4 0.5 0.6 ms

-10

0

10

mV

VINAC VOUT

Lecture 15-18

SPICE Results

current gain

time0.0 0.1 0.2 0.3 0.4 0.5 0.6 ms

-1

0

1

uA

IO IIN

Lecture 15-19

Common Base Amplifier

• Gain is close to unity

• Very low input impedance --- great for impedance matching (e.g. 50 ohms)

• Large output impedance -- approximately RC

• Great high frequency behavior -- more on this later...

VCC

RC

I

-VEE

C1

vs

RS

vo

Lecture 15-20

Current Source Biasing

• Instead of resistors, a current source is used to bias the transistor in this example

• Current sources can be used to bias other amplifier types too

• Building a current source is less expensive than building a resistor on an IC --- we’ll be addressing IC issues such as this much more extensively beginning with the next lecture

VCC

RC

I

-VEE

C1

vs

RS

vo

Lecture 15-21

Current Buffer

• Can be used as a current buffer

• Macromodel:

io

αiivi voRoRi

ii

Lecture 15-22

Small Signal Equivalent Circuit

• Since the base is grounded, we would probably select the T-model from Lecture 13

ic

E

B

C

αie

ie

ib

re

• What parameters are being ignored in this simplified T-model? And what is the potential impact?

Lecture 15-23

Common Base Small Signal Analysis

Lecture 15-24

Common Base Small Signal Analysis