Voltage to frequency converter (CB8928E)

Found in ancient American documentation, this circuit uses an operational amplifier of the time, which is uncommon today.

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Active inductor (CB20748E)
This circuit simulates an inductor. Or operational amplifier of old type, but a configuration is...
Voltage reference with FET (CB21534E)
We found this circuit in a 1967 Electronic Design. The circuit uses contemporary transistors that...
Relay Control shield or interface (CB15483E)
To control relays safely with double insulation, we suggest the circuit in the figure. The optical...