ECE137A class notes, UCSB, Mark Rodwell, copyright 2019

ECE137A, notes set 5:

Source Follower, Emitter Follower

Mark Rodwell, Doluca Family Chair, ECE DepartmentUniversity of California, Santa Barbararodwell@ece.ucsb.edu

ECE137A class notes, UCSB, Mark Rodwell, copyright 2019

Emitter Follower, Source Follower

ECE137A class notes, UCSB, Mark Rodwell, copyright 2019

Analysis

EER LR

eqLLEECE RRRR ,||||

CER

ECE137A class notes, UCSB, Mark Rodwell, copyright 2019

Analysis

eqLR ,

ECE137A class notes, UCSB, Mark Rodwell, copyright 2019

Emitter Follower Voltage Gain

ECE137A class notes, UCSB, Mark Rodwell, copyright 2019

Impedances in the Emitter Follower

on.degenerati-emitterfor derivedearlier formulas from Follows

CER

ECE137A class notes, UCSB, Mark Rodwell, copyright 2019

Emitter Follower: Example (1)

ECE137A class notes, UCSB, Mark Rodwell, copyright 2019

Emitter Follower: Example (2)

ECE137A class notes, UCSB, Mark Rodwell, copyright 2019

Emitter Follower: Example (3)

ECE137A class notes, UCSB, Mark Rodwell, copyright 2019

Emitter Follower: Example (4)

ECE137A class notes, UCSB, Mark Rodwell, copyright 2019

Emitter Follower: Example (5)

ECE137A class notes, UCSB, Mark Rodwell, copyright 2019

Emitter Follower: Example (6)

ECE137A class notes, UCSB, Mark Rodwell, copyright 2019

Source Follower Example (1)

regioncurrent -constantin iticscharacters FET Recall gsD VI

( / 2)(1 )

for , where

D v gs th DS

gs th v inj gs g

I K V V V V

V V V K v c W

2( ) (1 )

for , where / 2

D gs th DS

gs th gs g g

I K V V V

V V V K c W L

ECE137A class notes, UCSB, Mark Rodwell, copyright 2019

Device model: NMOS @ 15nm gate length

plot for 1 micron gate width

ignores VDS term

2

15nm

0.3 V

16mA/V ( /1 m)

2.40mA/V ( /1 m)

75mV

1/ =5V

g

th

g

v g

L

V

K W

K W

V

0.3V

/ 2 0.337VthV V

0.375VthV V

2.4 mA/V/ m

216.0 mA/V / m

ECE137A class notes, UCSB, Mark Rodwell, copyright 2019

Source Follower Example (3)

1

We've chosen to bias the FET @ 0.4V 0.3V=0.1V and 0.6V.

Velocity-limited: 2.4mA/V ( /1 m)( / 2)(1 ) 1mA.

1mA=2.4mA/V ( /1 m)(0.1 75mV)(1+0.6V/5V) 14.9 m.

Setting 200k

gs th DS

D g gs th DS

g g

g

V V V

I W V V V V

W W

R

2 and 800k provides 0.8V.

0.4V/1.0mA=400 .

g g

ss

R V

R

ECE137A class notes, UCSB, Mark Rodwell, copyright 2019

Source Follower Example (4)

(1 )

14.9 m 0.6V 2.4mA/V 1

1 m 5V

40mS.

1/ 5V/1mA 5k

m v DS

DS D

g K V

R I

AC small-signal analysis

200k

800k 40010k

10k5.6 mgW

|| || 357

/ / ( 1/ ) 0.93

Leq DS SS L

out in Leq Leq m

R R R R

V V R R g

,

, ,

200k || 800k 160k

/ / ( ) 0.94

in A

in gen in A in A gen

R

V V R R R

ECE137A class notes, UCSB, Mark Rodwell, copyright 2019

Emitter Followers vs Source Followers

ECE137A class notes, UCSB, Mark Rodwell, copyright 2019

Source Follower Output Impedance

,More carefully working from the s.s. model shows that (1/ ) ||out T m DSR g R