A Practical Discrete 386 | Hacker News Reader

A Practical Discrete 386

hackaday.com

30 pointsTomte15 comments

The article is about a discrete implementation of the LM386 op amp, not an 80386 processor :).

Those are fun little chips if you need a small audio power amp. Had the same thought when I pulled up the article.

Not what I expected, but that brought back memories. The LM380 (the 386's cousin) was the first analog chip I ever used, somewhere around 1976. I built a small headphone amp with the chip and a handful of discrete components on a piece of Vero board. I think the circuit came from the chip's implementation notes in the old RS Catalog.

I, too, was disappointed!

Was hoping for something like this:

https://monster6502.com/

The i386 had 100x the number of transistors, it'd cover an entire wall.

That just sounds like a challenge. :)

I found this tool for converting HDL to PCB layout with discrete logic. It should make the dream of a wall sized computer much easier lol. https://hackaday.com/2021/11/13/using-vhdl-to-generate-discr...

6502: 3500 transistors. 80386: 275000 transistors. It would have been a big board ;)

That's only 520x520 transistors. Using sot23 it would be maybe 2x2m.

520x520x1 is 130x130x16. 2x2m is a bit optimistic, but a wardrobe-sized cabinet should be possible.

Indeed, that's what I was expecting too. The 80386 is largely microcode-driven so I thought someone had managed to condense the datapath and sequencer enough to make it implementable with discrete components.

Aww man. Was hoping we could rebuild Strong Bad's old computer.

Clickbait title. I suggest making the obvious change to "LM386".

non blogspam title:

A Homebrew LM386 -- Does Anyone Want to Build It? Help Save Us All from the Indignity of ICs!

This is definitely very cool!

But the circuit topology here can't meaningfully be called a discrete implementation of an LM386, unless by LM386 we just mean "a two-stage op-amp capable of driving a small speaker." The two circuits may have the same goals, but they're completely different designs (and the LM386 is a much better design---no shame in that, considering it's professional versus hobbyist).

The simplified diagram of the LM386 shown on Hackaday's website (which appears to be reproduced from the TI datasheet [1]) shows a few distinctive features that the discrete circuit lacks, and which result in better performance; and we can guess at some other details that are probably lost in the simplification.

- The LM386 input devices are a differential pair of PNP Darlingtons [2]. The Darlingtons are there partly because PNPs have worse current gain than NPNs, but they're also a conscious choice that allows the amplifier to be powered from a single supply while taking in a DC-coupled signal that's referenced to ground: the signal inputs (pins 2 and 3) can safely go below ground by roughly a diode drop, by design.

- In contrast, the linked discrete implementation uses NPN transistors for the input diff pair (Q1 and Q2), which means that it requires a separate R-C biasing network (C2, R7, R11, R12) and also means that the bias current in the diff pair depends on the power supply voltage (making power supply rejection tough).

- The LM386 uses a pair of NPNs in the first stage to effect a differential to single-ended conversion. The simplified diagram doesn't show it, but I'd bet that the real circuit matches the bias current in the output stage (the current source above the two diodes in the middle-right of the simplified schematic) with the current through the input stage, in order to hold the collector of the right NPN transistor at roughly the same DC voltage as the collector of the diode-connected NPN [3] on the left. This minimizes mismatch due to finite output impedance of the NPN transistors [4].

- In contrast, there's no differential to single-ended conversion in the discrete circuit. Q2's gain is essentially thrown away, the signal is taken from R4, and there's no attempt to match the collector voltages of Q1 and Q2 (adding a diode-connected device between Q2's collector and the supply would improve matching).

Again: I love when folks build and release their discrete circuit designs! I've done the same multiple times (here's a fun one [5]). It would have been better for the author (and Hackaday) to leave the LM386 out of this.

Finally, it seems like the author didn't actually build the device, just simulated it. I probably wouldn't build the discrete circuit as described. It's going to have some performance issues (poor power supply rejection is one) and will be quite sensitive to temperature, device characteristics, etc.---all things that one should try to minimize in a design.

[1] https://www.ti.com/lit/ds/symlink/lm386.pdf

[2] https://en.wikipedia.org/wiki/Darlington_transistor

[3] https://en.wikipedia.org/wiki/Diode-connected_transistor

[4] https://en.wikipedia.org/wiki/Early_effect

[5] https://web.jfet.org/gilberd/

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