Design Ideas: February 15, 1996

The circuit beneath the dashed line in Figure 1 is the well-known Wien-bridge oscillator. The lamp is a common tungsten incandescent bulb. Its resistance forms a substantial part of the gain-setting network. For the oscillator to function properly, the increase in lamp resistance accruing from the ac signal must bring the open-loop gain to unity while the op amp is still in its linear range. At that point, R_A=2(R_B+R_LAMP), and the output is a low-distortion sinusoid. It's difficult to achieve stable operation at high frequencies, because the output decays rapidly if the open-loop gain falls below unity. The lamp does not have enough time to respond, and "squegging" develops.

The additional circuitry above the dashed line prevents squegging and allows for amplitude adjustment. In the modified circuit, R_A and R_B must be large enough to reduce the ac signal through the lamp in order to prevent it from warming up due to the ac power. Instead, the lamp derives its heat from the dc current that flows through R_C. This current is proportional to the ac output level. You select R_C to be as large as possible without causing saturation of Q₁. The output-signal amplitude is approximately 2V above the dc level at the base of Q₁. Therefore, the op amp must be able to deliver 13 mA without clipping.

A pulse of current flows through Q₁ every time the output signal rises 0.7V above the dc level at the base of Q₁. C_A smooths out the current pulses and is just large enough to limit the ripple at the collector to a few millivolts. The dc current warms up the lamp filament until the circuit meets the following condition: The sum of R_B and the parallel combination of R_C and R_LAMP equals ½R_A. The output level is adjustable between 4 and 8V p-p. The circuit requires approximately 2 sec to settle after power-up. (DI #1828)