Calculate Amplitude of Sine Wave Generator

Hi Nikhil,

I took a look at SNOA839 and see that the circuit consists of two stages that result in a sine output. The first stage is a square wave generator that is followed by a second-order, low-pass active filter second stage. Basically, the active low-pass is used to attenuate the dominant odd-order harmonics present in a square wave, and any even-order harmonics that occur due to imperfections in the square wave.

The first stage output swings from as close to the positive supply rail and ground as it can. How close it swings to the supply rail depends on the load current it must deliver to the second stage. It appears that from the DSO image that output swings close to 3 V (Voh) on the positive end, and about 0.25 V on the negative end (Vol). So the square wave output is about 2.75 Vp-p. If you look at this in terms of a gain perspective, then it would be about 2.75 V/ 3 V, which would be 0.92 V/V. Note that with a different op amp, even of the same model, having a different output swing Voh - Vol range its "gain" could be different. 

The second stage is a Sallen-Key active filter having a pass-band gain of +1V/V. That is the gain well below the -3 dB cutoff frequency. The gain of the fundamental frequency, or desired sine wave, will be a function of where the filter cutoff is set. The author states that the fundamental sine wave has a peak amplitude of about 87 % of the peak square wave amplitude. So at best the gain of the second stage is 0.87 V/V if the filter isn't adding attenuation of the fundamental.

Multiplying the first and second stage gains, 0.92 V/V times 0.87 V/V, the overall gain is 0.80 V/V. Do note that this is a general purpose sine wave generator and not one that provides an ultra clean output having very low distortion, or exacting amplitude. Apply accordingly!

Regards, Thomas

PA - Linear Applications Engineering