# Gas station without pumps

## 2014 May 2

### First op amp lecture

Filed under: Circuits course,Uncategorized — gasstationwithoutpumps @ 19:06
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Today’s lecture was the first introduction of op amps to the students.  I managed to present

• The basic notion of a differential amplifier: $V_{out} = A (V_{pa} - V_{ma})$.
• Generic negative feedback amplifier.

Generic negative-feedback amplifier design using op amps.

• Formula for voltage divider when none of the nodes are ground. (I had the students derive this.)
• Formula for negative feedback with effectively infinite open-loop gain (deriving $V_{out}= (V_{p}-V_{m}) Z_{f}/Z_{i} + V_{p}$)
• Inverting, non-inverting, and unity-gain configurations.

I even had the students figure out how to make an amplifier with a gain of 0.5. The design they came up with was a unity-gain buffer followed by a voltage divider, though they started with a voltage-divider followed with a unity-gain buffer.  If we had had more time, I would have gotten them (eventually) to come up with a pair of inverting amplifiers.

We’ve talked about the voltage limits on Vout, and about Vp having an extremely large input impedance, but we’ve not talked about the input impedance of inverting amplifiers, about current limitations of the output nor output impedance, nor about slew rate or gain-bandwith product.

We will  talk about current limitations on Monday, since those do matter for the audio amplifier design they’ll be doing next week.