Design Ideas: November 10, 1994
The Wien-bridge sine-wave oscillator uses a light bulb to stabilize its amplitude. The circuit in Fig 1 doesn't have a light bulb; it sports several enhancements that lower its distortion and generate a test signal pure enough for testing modern op amps and high-resolution A/D converters.
ICand associated components form the Wien-bridge and function as a bandpass filter. IC1's output goes to the voltage-controlled amplifier (VCA), IC3. ICacts as a "smart resistor" whose value the circuit continuously adjusts via IC4. ICadds the outputs of ICand the VCA and feeds the result into the bridge. These two op-amp inverters eliminate any common-mode signal that might limit performance.
The circuit's AGC loop begins with diodes Dand D2. These diodes half-wave rectify the outputs of ICand IC2. These outputs are 180ø out of phase; so ICsees a full-wave rectified signal through Rthat is proportional to the output signal's amplitude.
Integrator ICcompares the average value of the rectified current to a constant current through R2. Any imbalance in these currents causes ICto output a correction signal, changing the gain of VCA IC3. The VCA's gain adjusts the oscillation's amplitude until IC4's input currents are equal. Rand Cfurther filter the correction signal to remove harmonic components that would manifest themselves as distortion at the circuit's output. Dminimizes damage to Cand Cin the event of reverse polarization.
The ac performance of Cand Cis critical to this design. I recommend polystyrene or polypropylene film types; and make sure you connect the outside plate as Fig 1 indicates. Mylar capacitors can degrade the circuit's performance by 6 dB. Cand Care peculiar to ICand IC2. They eliminate distortion arising from Vnonlinearities in the op amps' output stages.
The large ratio of output signal to distortion and noise floor makes verifying the performance of this circuit with standard test equipment difficult. Therefore, I used the tunable, buffered-output, twin-T filter in Fig 2 to reduce the fundamental (1 kHz) in the output by 70 dB. Spectral analysis of the filter's output permits calculation of THD.
When properly tuned, the filter reduces the second and third harmonics by about 10 and 5 dB, respectively. Harmonic-distortion calculations must take this reduction into account. Harmonic-distortion calculations must factor in the gain of IC(Fig 2). Be sure to use the same high-performance capacitors used for Cand Cin Fig 1 for C1, C2, and C in Fig 2.
To tune the filter:
Note: You may need to make small adjustments to Rfor successful power up. (DI #1617)
