Help Me Understand the LM386 Chip Amp

[tedmich]

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

[MarkusBass]

Exactly. I see that you removed such terms like " "Grid leak" and "Current balance" (which were completely wrong). But there would be nothing wrong if you put there "current mirror" which is correct description of the op amp stage. You can learn about it here: https://en.wikipedia.org/wiki/Current_mirror

Mark

[Dave H]

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

It looks like "The Wrath of God" descended on Rob in this thread

[robrob]

Markus, I thought the current mirror acted to balance the two halves of the differential mirror, that's why I had "current balance" on the schematic. Does the current mirror function when only one input is in use like in the Ruby?

[MarkusBass]

I thought the current mirror acted to balance the two halves of the differential mirror, that's why I had "current balance" on the schematic.

This is one of the "not even wrong" statements. There are no two halves and there is not differential mirror here. You shouldn't develop new terms to describe what is widely known. You could write "current equaliser", or "current double terminator" but why would you do that? You do this all the time: you call transistors - "tubes". Yes, somehow a transistor can be compared to a tube but no one says that a transistor is a tube (except you). There must be a reason for this

[R.G.]

@ robrob:

You're suffering from overthinking it. The LM386 is just one modestly specialized version of opamp, and ought to be looked at in that way by everyone who's not actually trying to dig into the transistor-ness of how the insides work.

The specializations are worth understanding.

- the LM386 opamp has been set up so that it can provide lots of output current, suitable for driving a speaker directly
- it has had biasing and gain-changing feedback resistors included inside so that making a few-components speaker amplifier is easy
- it has had its input circuitry tinkered with so that input signals are pulled to ground, and the inputs biased at ground by the included 50K input resistors
- pins were brought out to the outside to provide for attachment of large capacitors where needed, and for external gain modification

Other than that, it's an opamp. The input impedance is low, about the value of the 50K input resistors, so it needs an input buffer to avoid cutting treble from a guitar signal by loading the inductive guitar signal. That's what the input JFET is for. The output resistor/capacitor network doesn't shape frequency response in any audio fashion, it's there to maintain stable operation in the fact of possibly capacitive loads - another consequence if its being an opamp.

I highly recommend you re-cast your explanation in terms of a customized opamp being manipulated by other circuit elements. I can dredge you through the blow-by-blow of what the transistors do, but that's not going to help your readers any.

[Dave H]

The way the output is biased is interesting. The input transistor bases are grounded and their emitters are 2 x Vbe above ground which means there's almost the full supply voltage across the left side's 30k (2 x 15k) emitter resistor. The current mirror forces the right side emitter current to be equal the left side so the right side's single 15k resistor drops half the supply voltage and it's connected to the output so the output must be at half the supply voltage.

[robrob]

Markus, I meant to say "differential amplifier", not "differential mirror".

On the webpage I said each of the 10 transistors in the LM386 are the equivalent of a triode tube. Could we not replicate the LM386 circuit using triode tubes and an output transformer?

[robrob]

Thanks R.G., I'll try to incorporate that into the article.

[MarkusBass]

Could we not replicate the LM386 circuit using triode tubes and an output transformer?

So you don't know it but yet you put this information on your web site. I doubt whether it's possible. Please try to do it and show us results.
I think that the information provided by R.G. is what people are looking for. And no one is interested in the information you are trying to provide. This is something that op amp designers could look at but since the information is highly incorrect, I doubt whether they would like to do so. And they already know all the technical details of this design.

[robrob]

That was a rhetorical question.

[robrob]

I want to thank (most of ) you guys for providing meaningful guidance. I took it to heart.

I took a public flogging but I learned a lot in the process.

[robrob]

In Chrome it's the circular arrow in the upper left corner of the browser.

[Justin Thomas]

Well, you <COULD> probably make a basic op-amp with triodes, but that's how we ended up with a pocket calculator the size of a skyscraper. And the Greenies would go apeshit if you tried.

Justin

[Enzo]

Here:
The Philbrick Archive

The famous Teledyne-Philbrick op-amp modules of the 1950s.


Could you do it in triodes? Well, there are only a few parts, draw it up as you think it would work.