After the recent pretty disappointing results with a transformer-based Component Tester, I’d like to try and generate a ± 10 V sine wave at approximately 50 Hz in some other way. Using as few components as possible.
This is where we enter, eh, squarely into the analog electronics domain. Yes, we could generate it with an ATmega, but frankly that sounds like a bit of overkill, would require a fair amount of filtering to remove residual switching effects, and besides we’d still have to amplify it up to 10 Vpp.
Time to introduce some new circuitry!
One of the most incredible electronic building blocks invented in the second world-war era was the Operational Amplifier, or “op-amp” in short.
There’s way too much to say about this amazingly universal circuit, which even has its own schematic symbol:
A positive and negative power supply pin, a positive and a negative input, and an output pin. That’s it.
I’ve only just started exploring op-amps, really – one superb resource on the web comes in the form of a free eBook from 2002 on the Texas Instruments site, titled “Op Amps For Everyone”, by Ron Mancini.
In his chapter on Sine Wave Oscillator, he mentions a “Quadrature Oscillator” built from two op-amps:
It uses very few components. This one was dimensioned for about 1.6 KHz, so I started with capacitors ten times as large, i.e. 0.1 µF, to lower the oscillation frequency. Here’s the result, using a TLV2472 dual op-amp:
Powered by a supply of ±2.5V (i.e. 0 / 2.5V / 5V), I see this result on the scope, when attached to the sine output:
Yeah, right. Clipping like crazy, i.e. overshooting into the limiting 0V and 5V power lines. The FFT shows it’s not anywhere near a pure sine wave, even though the shape vaguely resembles one:
A pure sine wave would have a single peak at the oscillating frequency.
Here’s the cosine output, again showing that it’s running way outside its linear range:
So yeah, we’re generating a 160 Hz signal, but it’s no sine wave and it would be useless as Component Tester.
Oh well, it was still an interesting first trial!