Posted on Dec 29, 2012

A variable Wien bridge provides frequency tuning from 20 Hz to 20kHz. Gain control comes from the positive temperature coefficient of the lamp. When power is applied, the lamp is at a low resistance value, the gain is high, and oscillation amplitude builds. The lamp`s gain-regulating behavior is flat within 0.25 dB over the 20Hz-20kHz range of the circuit. Distortion is below 0.003%. At low frequencies, the thermal time constant of the small normal-mode lamp begins to introduce distortion levels about 0.01 %.

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This is because of hunting when the oscillator"s frequency approaches the lamp"s thermal time constant. This effect can be eliminated, at the expense of reduced output amplitude and longer amplitude settling time, by switching to the low-frequency, low-distortion mode. The four large lamps give a longer thermal time constant, and distortion is reduced.

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