Crystal variable oscillator (CB9506E)

Found in Radio Electronics from the 90s, this circuit can vary the frequency in small values around the crystal frequency.

Auto Bias for FET (CB6125E)
This configuration is from a 1964 US documentation valid for any FET. The range of power voltages...
Rectangular Generator (CB13240E)
The operational amplifier used in this circuit from a 1970s publication is no longer easily found...
Precision rectifier with AC DC converter (CB9603E)
This circuit was found in an old American documentation. It can operate with signals from 40 Hz to...