inductors & op amps (2023 Update)

Actual inductors are pretty crappy compared with capacitors, so the mathematical symmetry is mot that useful in op amp circuits.

In other situations, they can be pretty useful. For instance, I often drive TE coolers using the BJT 'sinulated inductor' with an actual inductor replacing the emitter resistor. (It's inside an op amp loop to preserve bias stability.)

Cheers

Phil Hobbs

RichD wrote: ===========

** Was once done all the time to create "graphic equalisers" and notch filters.

BTW an active ( op-amp) inductor is known as a "gyrator".

Of course, real inductors are everywhere in switch mode PSU regulators.

..... Phil

Wah-wah pedal:

Well, no, that's not the whole story. Inductors work on dI/dt, and at low currents, they get non-ideal behaviors. You usually use op-amps with low currents and voltages. Multilayer ceramic capacitors are cheap to mass-produce for surface mount, so they'll typically underprice fixed inductors, except tapped coils and transformers.

An ultrasound machine I worked on had 512 channels with SM inductors for impedance matching, and individual transformers to drive each channel... all surface mount, and it seemed practical to me.

For individually-tuned VHF television channels, it was common practice to trim dozens of inductors (by mashing the coils with a computerized pushstick...). Have you priced a trimmer capacitor lately?

What happens at low currents?

Good question. Anyway, there's nothing "bulky" about inductors in the RF sphere.

Some big iron-core inductors get lossy at low currents. That's an issue in current transformers.

Signal-level inductors can melt or saturate at high currents. I was asking what they might do at low currents.

Whit?

They are great for peakers and filters. I'd rather buy a tiny 8 cent inductor instead of a big $3 fast opamp.

But he draws the line at designing one for a particular application and getting it wound. That will cost more than 8 cents, but it can still be cost-effective.

yet, he has build and sold far more devices with custom wound inductors and transformers than you have in the last few decades ...

Coilcraft is just now making us four samples of a 300 watt planar power transformer, this series:

We need a non-standard turns ratio (for a 48v to 400v dc/dc converter) so they will add a 2-turn aux winding. Coilcraft is great.

How about this one?

This was fun too:

wound on a specially-designed tool. A Sharpie pen, actually.

I don't understand why Sloman keeps inventing obviously absurd insults that just insult himself. I think he's not well.

Bill seems to think that tapping gratuitous insults on his keyboard will keep what passes for his brain in fair fettle, but I believe that alone is not sufficient exercise.

Two things: first, magnetic materials have coercivity, and that reduces the impedance (measured value for high current is higher inductance than measured value for low current). Second, a low DC current can walk the core magnetization up near saturation, causing distortion; higher AC current can re-center the magnetization to zero, with some associated heating.