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

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

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Thus, an equation linking Q and Gain can be <strong>de</strong>duced:<br />

R1<br />

1 ⎛ 1 1 ⎞<br />

Q = . Gain<br />

K R ⎜ +<br />

3 R1<br />

R ⎟ . (IV.5)<br />

⎝<br />

2 ⎠<br />

From the transfer function formula, a stability issue comes out. In<strong>de</strong>ed, the<br />

<strong>de</strong>nominator should only have roots with negative real parts, so the jCω term of this second<br />

or<strong>de</strong>r polynomial has to be negative as well. This leads to the condition:<br />

⎛ 1 2 ⎞<br />

K < ⎜ + ⎟<br />

⎟R3<br />

+ 1.<br />

(IV.6)<br />

⎝ R1<br />

R2<br />

⎠<br />

If the voltage gain K of the amplifier does not fulfill this condition, the filter is<br />

unstable.<br />

IV.1.c Rauch Filters<br />

IV.1.c.i Negative feedback Rauch filter<br />

In the literature, the “Rauch” filter is always <strong>de</strong>scribed in a negative feedback<br />

configuration [IV.2], as <strong>de</strong>picted in Figure 140, where K is the voltage gain of the voltage<br />

amplifier.<br />

H R−<br />

Figure 140. Negative feedback Rauch bandpass filter schematic<br />

The transfer function of the filter can be computed, which gives:<br />

jCωKR3<br />

−<br />

⎛ 1 1 ⎞<br />

R1<br />

( 1+<br />

K ) ⎜ + ⎟<br />

⎝ R1<br />

R2<br />

⎠<br />

( jω<br />

) =<br />

2 (IV.7)<br />

1 ⎛ 1+<br />

K 1 1 ⎞ ( jCω)<br />

1+<br />

jCω<br />

⎜<br />

⎜2<br />

+ + ⎟ +<br />

1+<br />

K ⎛ 1 1 ⎞ ⎝ R3<br />

R1<br />

R2<br />

⎠ 1 ⎛ 1 1 ⎞<br />

⎜ + ⎟<br />

⎜ + ⎟<br />

R3<br />

⎝ R1<br />

R2<br />

⎠<br />

R3<br />

⎝ R1<br />

R2<br />

⎠<br />

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