A low frequency or audio frequency generator is an almost indispensable instrument in which it operates in the field of amplifiers and surroundings. The scheme we are going to present is sold in kits by the British firm AMTRON. But, as it does not present its products in Brazil, we do, venia venia, the presentation of the scheme to elucidate our readers. The range of the audio frequency generator is from 15 to 22,000 Hz, in three ranges selectable by the two-pole, three MN positions. The maximum output is 2 effective volts, with a linearity of 1.5 dB and a maximum distortion of 1% at all frequencies. The circuit is classic of a Wien bridge, whose basic principle can be seen in figure 1. This bridge derives from the Wheatstone bridge, which is used for the measurement of capacitors and resistors. We will not give here the theoretical treatment of the functioning of the Wien Bridge, because our interest is to divulge an easy circuit of simple construction, for the bench man or experimenter who wants to build his own instruments. All values are in the diagram in figure 2. The only necessary adjustment, before using the audio-frequency generator, is made through a 220 ohms P3. A voltmeter is placed at the output of the generator, on the scale that allows observing between 1.5 and 3 volts. The selector switch MN is placed in position b and the potentiometer P1-P2 is placed in the position that is equivalent to 1,000 Hz and P3 is adjusted so that the voltage is 2 V.
