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Audio signal (sinewave) generators

2015-01-01 00:00  
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There are dozens, probably hundreds of circuits for audio sinewave generators but certain 'old chestnuts' are still favourite and the basic circuits haven't changed significantly since they were built with valves.


The Wien Circuit


There is one 'favourite' circuit for making an audio oscillator the Wien bridge. It is a bridge, i.e. a circuit having four 'legs'. The circuit below shows the basic idea.


Circuit: /files/Images/Article/Circuit_Diagram/wein.gif

The Rx, 2Rx feedback divider establishes a gain of 3 in the op amp. Positive feedback is via the Wien network, R and C (two of each). The characteristic of this network is that the network's output (at the ve input to the opamp) has a maximum at a frequency Fo, as shown on the diagram. The mathematical analysis of any network tends to simplify itself if resistors/capacitors are related to each other by some simple ratio: with the Wien network the maths come out best if the two resistors are equal and so are the two capacitors! Hence the simple formula for Fo.

So, for 1kHz put, 1000 into the formula in place of Fo, choose a sensible value for R - let's chose 100K. swap the formula round to read C= 1/(2 x PI x 1000 x 100,000) and C works out to be 1.59nF: This is not a particularly sensible value for a capacitor but if we increase the capacitor we need to decrease the resistor and vice versa, so we can now chose a sensible capacitor: how about 4n7?

Putting 4n7 in the formula and rearranging gives R=1/(2 x PI x 1000 x 4.7 x 10-9). We get 33.9K for the resistors. At this point you need to decide how accurately you want 1kHz. Most Wien bridge circuits tend to be variable frequency, so we simply calibrate the scale to suit!

Because the Wien circuit attenuates the input by 3 times, we need a gain of 3 in the amplifier to make it oscillate. This was why we chose the 2Rx/Rx negative feedback.

There is one slight problem: is the feedback resistors define a gain of slightly less than 3, then the circuit won't oscillate. It the gain is slightly more than 3 the circuit will oscillate and the amplitude will only be determined by the IC's output hitting the supply rails and clipping - not a sinusoidal waveform. So how do we keep the gain at exactly the right level to just sustain a sinewave? What we need is some component which has a resistance proportional to the signal level: if the level increases, so does the resistance, putting down the gain. Similarly if the output drops, the gain will increase, maintaining stability. Fortunately there are several ways of doing this.


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