Chips vs. discrete

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In yesterday’s post, I explained the difference in terminology between a discrete component op-amp and an integrated circuit op-amp was the way they were manufactured and packaged. The circuit itself could be identical. But they are typically not.

An op-amp is a functional amplifier that always has two defining blocks: an input differential pair and an output gain stage. There’s often a third block added, an output buffer. Let’s look at yesterday’s drawing.

The Diff pair is the input and the feedback point. Any signal put into one or both of these two inputs will be amplified if there is a difference between the two. The gain stage takes the output of the diff pair and does two things: it amplifies the AC signal and gets the DC where we want it. (The DC at R1 is very high, nearly at +V. We want this taken back down to halfway between +V and -V—which is where started at the input). Once the signal has passed through the diff pair and gain stage—larger now for AC signals and the DC is equal on input and output—we can then wrap the output signal back to the – in through a few resistors to set the gain. That is how we apply negative feedback.

Of course, there is much more to learn about op-amps but these are the basics of its operation.

Understanding these basics, Stan and I realized a few things about the limitations of the integrated version of this circuit. The first is rather obvious. Because the IC op-amp is encased in plastic we could not choose what types of devices were used, nor what the values of the resistors and other parts were. But worse, and this was a big deal for us, we could not change the + and – voltage. IC op-amps have limits which are typically +15 and -15 volts—parameters that could not be changed in an integrated package.

Tomorrow I’ll explain why we needed more voltage and what we did.