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

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

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B.1 General Consi<strong>de</strong>rations<br />

APPENDIX B<br />

B.1.a “Single-tone” input signal<br />

Though classical electronic components such as amplifiers are consi<strong>de</strong>red as linear<br />

elements for small signals, they have non-linear transfer characteristics. On Figure 197 are<br />

shown the theoretical transfer characteristic of an amplifier as well as its simulated one. It<br />

consists in a linear zone between two saturated portions. Let’s consi<strong>de</strong>r<br />

x ( t)<br />

= X cos( ωt) = X cosφ<br />

(B.1)<br />

For X < X max , the response to a sinusoidal input remains sinusoidal since the transfer<br />

characteristic is linear. In the frequency domain, a single frequency tone in both input and<br />

output is then obtained.<br />

Figure 197. Theoretical and simulated saturated gain of an amplifier<br />

Now, if max<br />

X<br />

X > , the input signal will be subject to saturation in the amplifier. The<br />

output signal will no longer be sinusoidal since the sine edges are cut. Thus discontinuities<br />

will lead to harmonics in the frequency domain. This phenomenon is called distortion.<br />

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