Preamplifier stage (CB8934E)

This circuit is from a 90s Popular Electronics. It appears elsewhere on this site. Q1 can be any general purpose transistor.

200 nA constant current source (CB8243E)
This adjustable circuit from 0 to 200 nA was found in a 90's EDN. The components can still be...
Ultra Low Frequency Generator 555 (CB13246E)
We found this circuit in a technical documentation from the 70s. The MC1555 is nothing more than...
Absolute Value Comparator (CB21598E)
Based on common operational amplifiers, this circuit was found in Texas Instruments documentation. The power...