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III. Gm-C Filtering - Epublications - Université de Limoges

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D.3 Linearity Enhancement of an Emitter-Follower<br />

D.3.a Initial Linearity<br />

Let’s consi<strong>de</strong>r the emitter follower illustrated in Figure 207.<br />

Figure 207. Emitter Follower Schematic<br />

Assuming that collector and emitter currents are equal, the current flowing through the<br />

load RL can be approximated to ie. Hence [D.2]:<br />

v<br />

out<br />

i e = (D.27)<br />

RL<br />

Then, consi<strong>de</strong>ring a biasing current I, the output can be computed by:<br />

⎛ I + ie<br />

⎞<br />

Vout + vout<br />

= Vin<br />

+ vin<br />

−U<br />

T ln ⎜<br />

⎟<br />

(D.28)<br />

⎝ I s ⎠<br />

Computing, this leads to:<br />

⎛ vout<br />

⎞<br />

⎜ I + ⎟<br />

⎜ RL<br />

V + = + −<br />

⎟<br />

out vout<br />

Vin<br />

vin<br />

U T ln<br />

⎜ I ⎟<br />

(D.29)<br />

s<br />

⎜ ⎟<br />

⎝ ⎠<br />

Thus<br />

V<br />

out<br />

+ v<br />

out<br />

= V<br />

in<br />

+ v<br />

Now, DC biasing leads to<br />

in<br />

−U<br />

T<br />

⎛ vout<br />

⎞ ⎛ I s ⎞<br />

ln ⎜<br />

⎜1+<br />

⎟ + U T ln⎜<br />

⎟<br />

(D.30)<br />

⎝ RL<br />

I ⎠ ⎝ I ⎠<br />

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