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

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IV.1.b Sallen-Key Filters<br />

The Sallen-Key second or<strong>de</strong>r bandpass filter is <strong>de</strong>scribed in the literature [IV.2]. The<br />

schematic is <strong>de</strong>picted in Figure 139, where K is the voltage gain of the voltage amplifier.<br />

Figure 139. Sallen-Key bandpass filter schematic<br />

The transfer function of this structure is given by:<br />

jCωKR<br />

⎛ 1 1 ⎞<br />

R1<br />

⎜ +<br />

⎟<br />

⎝ R1<br />

R3<br />

⎠<br />

( jω<br />

) =<br />

. 2<br />

(IV.1)<br />

1 ⎛ 2 1−<br />

K 1 ⎞ ( jCω)<br />

1+<br />

jCω<br />

⎜ + + ⎟ +<br />

1 ⎛ 1 1 ⎞ ⎝ R2<br />

R3<br />

R1<br />

⎠ 1 ⎛ 1 1 ⎞<br />

⎜ + ⎟<br />

⎜ + ⎟<br />

R2<br />

⎝ R1<br />

R3<br />

⎠<br />

R3<br />

⎝ R1<br />

R2<br />

⎠<br />

Thus, the resonant frequency is <strong>de</strong>fined by<br />

H SK<br />

f<br />

0<br />

=<br />

1<br />

2πC<br />

and the quality factor of this bandpass filter is:<br />

Q<br />

3<br />

1 ⎛ 1 1 ⎞<br />

⎜ + ⎟<br />

R3<br />

⎝ R1<br />

R2<br />

⎠<br />

1<br />

R<br />

⎛ 1<br />

⎜<br />

⎝ R<br />

1 ⎞<br />

+<br />

R ⎟<br />

⎠<br />

1 − K 1<br />

+<br />

R R<br />

2<br />

+<br />

R<br />

Filter gain at the resonant frequency is given by:<br />

1<br />

- 120 -<br />

2<br />

2<br />

(IV.2)<br />

3 1 2<br />

= . (IV.3)<br />

K<br />

Gain =<br />

.<br />

⎛1<br />

− K 1 2 ⎞<br />

R1<br />

⎜ + + ⎟<br />

⎝ R3<br />

R1<br />

R2<br />

⎠<br />

(IV.4)

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