This circuit was found in a Popular Electronics from the 90s. The transistors are general purpose, and the gain depends on the ratio between R2 and R1.
Based on FET operational amplifiers, this circuit was found in a manufactirer documentation. The...
This circuit, obtained from a 1965 magazine, can still be assembled using some modern equivalent...
This circuit uses an operational amplifier that is still common today. The indication is made by...