Michael Fisch
EDN
In 1940, William Hewlett and David Packard launched a product from a garage. The product was a Wien-bridge oscillator. It consisted of a single-pole highpass filter in series with a single-pole lowpass filter. To keep the gain constant, the circuit used an incandescent pilot light to provide AGC (automatic gain control). As is true for all incandescent bulbs, the pilot light has nonlinear resistance. When you turn on the circuit, the cold lamp's resistance is low, resulting in high gain. As the gain increases, the resistance of the warming lamp increases. Thus, the lamp provides an AGC function. The circuit has been in use for more than 60 years and is still in use. The only problem with the Wien-bridge oscillator is that below unity gain it does not function.
When working for a telephone company, I had to develop a 20-Hz, high-voltage sine-wave ringer circuit. The circuit had to be adjustable from 20 to 200 V p-p. The most difficult part was that I had to adjust the oscillator's gain to a value below unity.
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| Figure 1. | This Wien-bridge oscillator is, according to the author, the first oscillator that can function at gains below unity. |
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The basic oscillator had to have gain slightly greater than unity to make the positive-feedback network oscillate. It also needed an AGC loop to control the greater-than-unity gain. So, I added a third loop around the oscillator and dubbed it a voltage-controlled, regulated-output feedback circuit. The end result (Figure 1) is a simple push-pull circuit. By adding D1, a zener diode, to another feedback loop, I maintained the amplitude even when I adjusted the gain to a value lower than unity. The 5.2 V zener diode maintains the gain. The result is that when the gain falls below unity, the amplitude tries to decrease but cannot do so, because the zener-diode voltage pulls it back up. IC2, the LT1056AN amplifier serves as a driver for the two LEDs, which alternately turn on and off at the frequency of the oscillator. IC1, an LT1012AN amplifier has very low offset voltage. IC3, a high-voltage TI/Burr-Brown (ti.com) amplifier, produces the final output of 20 to 200 V. The final result – a major breakthrough – is a Wien-bridge sine-wave oscillator that you can adjust below unity gain, and the circuit still maintains its AGC.
