Electronics > Beginners

What's the use of OpAmps?

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David Hess:

--- Quote from: rstofer on April 28, 2019, 05:16:15 am ---Maybe they're useful when you put a capacitor in the feedback path and create an integrator.  Analog computing...
Wait!  That's why they were invented in the first place!  Operational Amplifier - performs 'operations' like differentiation and integration or addition and subtraction.
--- End quote ---

Single transistors can do all of these things as well but with much less precision.  The single transistor "Miller integrator" is included inside most operational (and audio) amplifiers where it provides the frequency compensation and usually most of the voltage gain.

A single transistor shunt feedback adder or inverter is also completely feasible but compared to the version using an operational amplifier, it has terrible offset and drift.

rstofer:
James Thomson first discussed solving differential equations with a mechanical ball-disk integrator back in 1876 but it was used earlier by Lord Kelvin in a tide prediction computer in 1872.  Bright brothers!

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

Lord Kelvin came up with the integration method of modeling differential equations and the magic is based on the '=' sign.

Take an equation like Ay'' + By' + Cy + D = 0 - just a simple second order DE with constant coefficients.  Lord Kelvin's contribution was to solve for y'' as y'' = (-By' - Cy - D) / A = (-1/A)*(By'+Cy+D).  Pretend you actually have y'' and integrate once to get y' and again to get y then mix in all the coefficients creating the right hand side of the expression.  Now, if we just HAD y'' we'd be all set.  But we DO have it because of the '=' sign.  All that mess we just created on the right hand side IS y'' so just feed to output back to the input, apply initial conditions and let the thing run.

Somehow I find this exciting!  I really wish I had had access to an analog computer back in college.  Not just for the Differential Equations course but also for Control Systems.

See that MATLAB Simulink schematic I posted earlier to see how this all plays out.

Oh, and accuracy is everything.  We want to be able to read values off a strip chart recorder or oscilloscope.  No drift, no effects from bias current (think 1 uA bias current with a 1 Mohm timing resistor - HUGE error!).  We can overcome some of that by scaling time such that the timing resistor can be 1/10 or 1/100 as large.  Of course, the timing capacitor has to be pretty high quality as well.  The op amps are the least of the problems.

Sometimes we only want the shape of the result and the absolute values don't matter.

tggzzz:

--- Quote from: rstofer on April 28, 2019, 05:25:01 pm ---I really wish I had had access to an analog computer back in college.  Not just for the Differential Equations course but also for Control Systems.

Oh, and accuracy is everything.  We want to be able to read values off a strip chart recorder or oscilloscope.  No drift, no effects from bias current (think 1 uA bias current with a 1 Mohm timing resistor - HUGE error!).  We can overcome some of that by scaling time such that the timing resistor can be 1/10 or 1/100 as large.  Of course, the timing capacitor has to be pretty high quality as well.  The op amps are the least of the problems.

--- End quote ---

The analogue computer I saw in ~1973 at a local university when I was at school, had 100V supplies and a flying lead patchboard. Eventually people learned that one end of the patch cables bit.

My school math teacher had an interesting discussion with him about whether or not digital computers would supplant analogue and hybrid computers.

IDEngineer:

--- Quote from: tggzzz on April 28, 2019, 05:38:22 pm ---My school math teacher had an interesting discussion with him about whether or not digital computers would supplant analogue and hybrid computers.
--- End quote ---
There was a time when folks wondered if this newfangled "electronics" thing would supplant hydraulic circuitry. No, that is not a typo.

Marco:

--- Quote from: Darkwing on April 27, 2019, 09:51:40 pm ---Well, I don't get it ... I've now read a lot of times about OpAmps, but I don't see the use of them

--- End quote ---

High amplification allowing high linearity with feedback, rail to rail operation, auto-zero operation, very low offset with very low drift, very low bias and leakage current.

Most of that require either 100s of transistors to equal an opamp, or might even require very small area monolithic pairs which simply don't exist as discretes.


--- Quote ---Side quest 1: What's this "rail-to-rail" thing?
--- End quote ---

They generally automagically transition between P/N input pairs near the rails so they maintain amplification from rail-to-rail, again requiring tons of transistors.

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