This circuit was found in an article in Electronics Now magazine from March 1997. The transistors are general purpose, and the indicator is a 1 mA full scale analog type.
This circuit, which presents a certain danger and should be used with caution, was found in the...
This circuit with variable capacitance diode (varicap) is from a publication from the 90s. The...
This circuit, shown in the figure, consists of a variable gain amplifier with differential input...