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.
This amplifier uses the LM358, this operational amplifier with excelents characteristics for this...
The outputs of each switch position are determined by the zener voltage plus the drive voltage of each...
This circuit is found in many LM339 manufacturer's manuals. This is from the SGS manual of...