Op Amp

https://en.wikipedia.org/wiki/Operational_amplifier_applications

• http://4.bp.blogspot.com/-J97VeAMfTS8/VarSHHscsOI/AAAAAAAAI4A/nuwYvkHMt2Y/s1600/1044843_565709233486797_1067282875_n.jpg
• https://mysite.du.edu/~etuttle/electron/circ23.gif

Comparator

Simplest op amp circuit

Non-Inverting amplifier

Amplifies the input voltage. Cannot go below unity.

For high power, output can go through a transistor.

Schmitt Trigger

Uses positive feedback.

Current Sensor

I had to do the math analysis to understand this. Configuration is a differential amplifier.

Real Op Amps

I have a 741 and the lowest it will go is to 1.9V, but it can go all the way up to V+. Which kind of sucks, because it can't easily drive N fets. The quad op amps I have are ??? and they go down just a few mV.

I tested this on a 2n7000 and the active region is really around a Gate-Source voltage of 1.2v, so the 741 doesn't go low enough.

I came up with a few solutions. One is to send the output of the op amp through a voltage divider. Even better was a diode (an LED worked well because it has a large voltage drop). This drops the output voltage to below the 1.2V of the N-FET base. The other solution was to drive the N-FET with a PNP BJT. However, since the op amp doesn't go all the way to the top rail, I can't turn the BJT all the way off so I have to drop the voltage to the emitter. Again a diode works, but if an op amp can be spared, it can be pegged high to provide a reference to the voltage drop.