We found this circuit in the April 1993 Electronics Now magazine. Almost any general-purpose JFET can be used. Power is supplied by a battery (2 x 9V).
Found in Motorola documentation from the 90's, this circuit has a high input impedance. The power...
This circuit from a National Semiconductor manual from the 1980s shows how to use the LM388, a...
This circuit was obtained in a 1977 Radio Handbook. The transistor is from the era and the coils...