Radio with amplifier (CB20486E)

We found this circuit in a publication of 1991. The audio amplifier is an operational that admits equivalent.

Differential Amplifier For Positive Edge LM3900 (CB4038E)
This circuit, with an operational amplifier, was found in the Motorola Dataar Linear of 1990. The...
Current to voltage converter (CB5283E)
This circuit is from a 2005 Linear Technology publication. It converts current into load sensing...
Frequency meter (CB7383E)
Found in a Popular Electronics from the 90s, this circuit reaches 100 kHz and battery power is...