Lect. 17: BJT Emitter Follower (Razavi 5.3.3)

Emitter Follower or Common Collector

Output follows input

Prof. Woo-Young ChoiElectronic Circuits 1 (09/2)

Lect. 17: BJT Emitter Follower

S ll i l M d lSmall-signal Model

- R :- Rin:

- Rout:

- Voltage Gain:

- Function

Prof. Woo-Young ChoiElectronic Circuits 1 (09/2)

Lect. 17: BJT Emitter Follower

Input Resistance

Ein RrR )1( βπ ++=

Prof. Woo-Young ChoiElectronic Circuits 1 (09/2)

Lect. 17: BJT Emitter Follower

Voltage Gain

EEout Rr

Rr

v1~1

1==

mEE

E

in

gRrR

Rrv 1

11

11 +

++⋅

++

ββππ

Thevenin equivalent circuit from output port ?

Prof. Woo-Young ChoiElectronic Circuits 1 (09/2)

Lect. 17: BJT Emitter Follower

Output Resistance

Eout RrR ||= πEout ||

1+β

Prof. Woo-Young ChoiElectronic Circuits 1 (09/2)

Lect. 17: BJT Emitter Follower

Common-Collector or Emitter Follower

-Rin: Large

-Rout: Smallout

- Voltage Gain: Close to unity

Voltage Bufferg

Prof. Woo-Young ChoiElectronic Circuits 1 (09/2)

Lect. 17: BJT Emitter Follower

Voltage AmplifierVoltage Amplifier

With RL, Ro should be as small as possible

What if a given amplifier configuration does not have small RWhat if a given amplifier configuration does not have small Ro

(for example CE)?

Use voltage buffer or emitter follower!

Prof. Woo-Young ChoiElectronic Circuits 1 (09/2)

Use voltage buffer, or emitter follower!