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Tuesday, May 12, 2020

An Understandable Chip: The LM386


Over the years I have made my distaste for integrated circuits very clear.  I've presented them as mysterious little black boxes that don't really belong in our homebrew rigs,  rigs that we are building, after all,  because we want to avoid the use of MYSTERIOUS BLACK BOXES.  

So, I don't use voltage regulator chips -- I use Zener diodes.   I prefer analog LC oscillators to AD9850s or Si5351s.  And I have repeatedly built discrete component audio amplifiers when most normal people just put an IC AF amplifier in the circuit. 

When I built the Q-31 Shortwave AM receiver, I kind of ran out of gas at the end.  I wanted to get the receiver going and I didn't want to build yet another discrete AF amp.  So I used an LM386.  

I rationalized this deviation from cherished values by noting that the discrete AF amplifier circuitry that I was using was remarkably similar to what exists inside the LM386.  So that little chip is just as understandable as my discrete component creations.  It wasn't REALLY a mysterious black box... 

Around this time I found a web site that made me feel mush better about all this.  It explains very nicely how the little LM386 does such a great job.  It really packs a lot of amplification into a very small package.   Here is the web site: 

https://www.electrosmash.com/lm386-analysis

Shortly after my transgression,  Jenny List over at Hack-A-Day posted a nice piece looking at the inner workings of Op-Amps.  Who knows, I may seen be using 741 chips too! 

Here is the Hack-A-Day piece: 

https://hackaday.com/2020/05/09/an-op-amp-from-the-ground-up/

5 comments:

  1. I'm all for using IC's in any case where it's the right tool for the job, but only if you understand *why* you're using the IC. Otherwise, it is indeed just a Mysterious Black Box.

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  2. W1FB published many DC receivers with ICs, it was the latest thing at the time. I don't see it as evil, but with leaded parts going away, I am running out ICs I want to use. I do have an NE602 in my DC RX, from W1FB's circuit. I don't find it objectionable, I rarely experience the malady's that many claim for it. Ed KC8SBV

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  3. The 386 , as Farhan uses it in the BITX series, has only one flaw - and that only occasionally evident: it doesn' like the input pin direct to Ground. This is the default condition with volume control at absolute minimum, at which point it is liable to become unstable, giving opbjectionable has to the discerning ear - particularly when using headphones. There is a simple fix for this: a resistor from volume control wiper to input pin - any value from 1k to 10k has been used successfully.
    This is not documented in any data sheets AFAIK, but its cause may be found by a deep-dive into the atomic structure of semiconductor junctions, particularly transistor base-emitter at 'full-idle' 0V. They're not totally inert!

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