Found in an old Radio Electronics, this circuit applies to frequency meters for frequencies up to 100 MHz. The input impedance is in the order of 1 M ohm.
This circuit was obtained from an American 80's documentation. The transistors can be 2N2222 and...
This circuit was found in Motorola documentation from the 1980s. PUTs are not common components...
This circuit is based on an uncommon Texas component today, as found in an American 1980s...