50 (FET) Field Effect Transistor Projects - Battery Portable

Any audio or general purpose FET can be fitted. Sensitivity toweak audio signals can be raised by adding source bias this canbe a 2.2k resistor with 47uF parallel capacitor. Over-loading orexcess volume with powerful signals can be avoided by replacingR1 with a 500k potentiometer, taking the gate to its wiper. Thewhole unit can conveniently be made in a small box, to hold inthe hand, with output socket for headphones or a single earpiece.Audio OscillatorThe circuit in Figure 18 may be used to generate an audiotone for audio circuit tests, or may be used instead of TR2 inFigure 16. An output type audio transformer is used, of the typeintended for ½ watt and similar output stages. The Radio-sparesT/T7, which has a ratio of 9.2 (centre tapped): 1 will be foundsuitable.Tone and output can be altered by changing any of thecornponent values, or by including a source resistor of 1k to 2.7kor so.