Advanced Applications

One of the major hazards involved in the application of operational amplifiers is that the user often finds that they oscillate in connections he wishes were stable. An objective of this book is to provide guidance to help circumvent this common pitfall. There are, however, many applications that require a periodic waveform with a controlled frequency, waveshape, and amplitude, and operational amplifiers are frequently used to generate these signals. If a sinusoidal output is required, the conditions that must be satisfied to generate this waveform can be determined from the linear feedback theory presented in earlier chapters. The Wien-bridge corifiguration (Fig. 12.1) is one way to implement a sinusoidal oscillator. The transfer function of the network that connects the output of the amplifier to its noninverting input is (in the absence of loading) V.(s) _ RCs V 0 (s) ~ R 2 Cess + 3RCs + 1 The operational amplifier is connected for a noninverting gain of 3. Com­ bining this gain with Eqn. 12.1 yields for a loop transmission in this positive-feedback system 3RCs C 2 2 L(s) = s 3RCs (12.2) R2Cs + 3RCs + 1 The characteristic equation R 2 C 2 2 3RCs s + 1 I-L(s) = 1-2 3RsR222+1 2 (12.3) R 2 C 2 s + 3RCs + 1 R 2 C 2 s + 3RCs + 1 has imaginary zeros at s = ±(j/RC), and thus the system can sustain constant-amplitude sinusoidal oscillations at a frequency w = 1/RC.