Wien-Bridge Oscillator

Posted on Feb 6, 2014

The classic Wien-Bridge oscillator, with incandescent lamp nonlinear stabilization, provides a low distortion sine wave source. A thorough and clear discussion of the Wien-Bridge oscillator is provided in Malvino`s Electronic Principles. Using an inexpensive opamp with sufficient bandwidth and slew-rate, it is possible to nudge the oscillation fre

Wien-Bridge Oscillator
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quency of this classic circuit into the MHz range. The following simple circuit demonstrates a 1. 27 MHz Wien-Bridge oscillator. Although intended for use in ultra low noise/distortion premium audio applications, the National LM4562 opamp has sufficient bandwidth, slew rate (GBW: 55 MHz, SLR: 20 V/us) and output drive capability to provide decent MHz signal oscillation. At 1 MHz, in order to stay within the slew-rate limit, the circuit must be designed with an oscillation amplitude of less than 2Vp. This can be achieved by proper selection of the incandescent lamp and feedback resistor. At the oscillation frequency, determined by the lead-lag network feedback transmission peak, the lamp resistance must be exactly half the feedback resistance. The lamp resistance, which increases with voltage, is determined by the RMS voltage across the lamp, so the voltage feedback network must satisfy this condition. (As a guideline, it is useful to measure the DC resistance versus voltage of the lamp). Also the values must be within the current drive capability of the selected opamp. In addition, the output signal peak amplitude must be within the voltage range capability for the available Vcc/Vee supply. Fortunately, these conditions are easily satisfied with a little planning. In this example a #1819 lamp, nominally 28 V @ 40 mA was used. Using standard resistor values, an Rf of 180 ohm provides output oscillation at 1. 27 MHz with 1....

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