We found this micro amplifier in a technical documentation from National Semiconductor. The circuit can be used on an experimental phone, being powered even by alternative power sources.
With this circuit, from an old operational datasheet, it is possible to measure the CMRR,...
Found in a Popular Electronics from the 90s, this circuit can be adapted to use LEDs. The power...
The circuits shown in the figure, that of an infrared video signal transmitter and receiver, were...