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Op Amps for Everyone - The Repeater Builder's Technical ...

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Oscillators<br />

A.5.4<br />

Quadrature Oscillator<br />

Quadrature oscillators produce sine waves 90° out of phase, so they output sine/cosine,<br />

or quadrature waves.<br />

When R 1 C 1 = R 2 C 2 = R 3 C 3 , the circuit oscillates at ω = 2πf = 1/RC. Both op amps act as<br />

integrators causing two poles at 1/RC, thus the circuit oscillates when the loop gain crosses<br />

the 0-dB axis. <strong>The</strong> integrators ensure that gain is always sufficient <strong>for</strong> oscillation. <strong>The</strong>re<br />

is a slight bit of distortion at the sine output, and it is very hard to eliminate this distortion.<br />

C 1<br />

R 1<br />

C 3<br />

+V CC<br />

_<br />

V OUT(SINE)<br />

+<br />

V CC<br />

2<br />

R 3<br />

C 2<br />

R 2<br />

+<br />

_<br />

+V CC<br />

V OUT(COSINE)<br />

Figure A–40. Quadrature Oscillator<br />

A-40

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