This circuit generates a good sine wave of 1 kHz adopting the inverted Wien bridge configuration (C1-C2-R3 and R4). It has a variable output, low distortion and low output impedance in order to obtain good overload capability. A small filament bulb ensures a stable long-term output waveform amplitude.
Notes:
- The bulb must be a low current type (12V 40-50mA or 6V 50mA) in order to obtain good long term stability and low distortion.
- Distortion @ 1V RMS output is 0.15% using a 12V 40mA bulb, raising to 0.5% with a 12V 100mA one.
- Using a bulb differing from specifications may require a change of R6 value to 220 or 150 Ohms to ensure proper circuit's oscillation.
- Set R5 to read 1V RMS on an Audio Millivoltmeter connected to the output with R7 rotated fully clockwise, or to view a sinewave of 2.828V Peak-to-Peak amplitude on the oscilloscope.
- With C1, C2 = 100nF the frequency generated is 100Hz and with C1, C2 = 1nF frequency is 10KHz but R5 requires adjustment.
- High gain transistors are preferred for better performance.
