You may have gathered from my last-but-one post on bit-banging the AD9833 DDS module that I have been playing with audio oscillator designs. As part of this process, I have tried a few Wien bridge oscillator designs that purport to not require a proper non-linear device (such as a miniature lamp or a RA53 thermistor). I could not get any of these designs to work in a stable, predictable and reliable fashion. Eventually, I turned to the classic Bill Hewlett-inspired miniature lamp with op-amp design. Here’s the basic schematic from a Linear Technology App. Note…


…although why it gives the frequency for values given as 1.000kHz to three decimal places, when the math clearly indicates that the frequency for values given is 995Hz is anybody’s guess.

One of the things about this design is that the current consumption is much lower than you might imagine. The #327 lamp is rated at 28V 1.12W giving a nominal current of 40mA. But the above circuit runs at just a few mA – the lamp is most certainly not being used as a lamp and is by no means glowing.

I wanted a battery-powered oscillator, so I came up with the following design (click for a larger version):

197SVThe TLE2426 splits the 9V from the battery into +/- 4.5V and theMCP111 provides a low-battery-voltage indication. PR1 should be adjusted to ensure that the output is stable (it also adjusts the headline output voltage). You can try almost any jelly-bean op-amp (e.g. TL072, RC4558) in place of the LM833, but bear in mind that many op-amp outputs don’t go anywhere near the rails so you shouldn’t be shooting for more than about 1V r.m.s. output voltage. D1,D2 and R8 protect the op-amp output and C9/R9 make an attempt to isolate the circuit from noisy grounds. The circuit as shown with PR1 adjusted for a nominal 1V r.m.s. output at pin 1 of the LM833 consumes about 7mA of current.  svfavicon.png



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