We found this circuit in the Linear Applications Handbook by National Semiconductor. It uses the LM3900, an operational transconductance amplifier as a base. Q1 can be a 2N2222 or similar.
This circuit was found in an American documentation from the 90's. The operational amplifier is the...
This detector circuit was found in documentation from the 1980s. The diode is a high frequency detector...
For those who like electronic games, have good experience in the design of printed circuit boards...