Single Pentode Wien Bridge Oscillator

His oscillator sought to use resistors and capacitors instead of bulky low frequency LC tank circuits to generate audio frequencies. He obtained low distortion (<0. 5%) with the use of a two stage amplifier with enough negative feedback to set it`s gain to just 3, as required for oscillation of the Wien Bridge circuit. He used a light bulb to serve as amplitude detector and linear gain setting resistor for the fed-back amplifier
Single Pentode Wien Bridge Oscillator - schematic

. Hewlett credits L. A. Meacham with the use of the light bulb for automatic oscillator gain stabilization as published in the Bell System Technical Journal Vol 17, p. 574, 0ctober 1938. The Wien Bridge network was invented by Max Wien in 1891. This excerpt from Hewlett`s thesis shows half the bridge. At the resonant frequency with equal resistors and capacitors, the attenuation is 1/3 of the input voltage with zero phase shift. This means that if an exact gain of 3 can be provided from the output of the network to the input, we have a sinusoidal oscillator. The simplified circuit on the right shows the lamp that stabilizes the non-inverting gain of the fed-back amplifier to exactly 3x. The lamp resistance increases when a larger signal is applied to it. This means that if oscillations grow too large, the lamp resistance increases to reduce oscillation amplitude until a stable equilibrium is reached. At this equilibrium point, the RC-CR Wien network forces a gain of 3x at the fed-back amplifier, which is to say, it forces the lamp resistance to a value that yields an amplifier gain of 3x. In turn, the Lamp forces the Wien network oscillation amplitude. This lamp stabilized Wien bridge oscillator differs fundamentally from RF LC oscillators commonly used in Radio circuits in that the oscillation currents and voltages in the Wien oscillator always remain nearly perfectly sinusoidal. Typical RF oscillators with an LC tank...

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