[HN Gopher] A Practical Discrete 386
___________________________________________________________________
A Practical Discrete 386
Author : Tomte
Score : 23 points
Date : 2022-11-27 12:35 UTC (10 hours ago)
(HTM) web link (hackaday.com)
(TXT) w3m dump (hackaday.com)
| kwantam wrote:
| 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/
| jazzyjackson wrote:
| non blogspam title:
|
| A Homebrew LM386 -- Does Anyone Want to Build It? Help Save Us
| All from the Indignity of ICs!
| johndoe0815 wrote:
| The article is about a discrete implementation of the LM386 op
| amp, not an 80386 processor :).
| antiquark wrote:
| I, too, was disappointed!
|
| Was hoping for something like this:
|
| https://monster6502.com/
| Max-q wrote:
| 6502: 3500 transistors. 80386: 275000 transistors. It would
| have been a big board ;)
| dvh wrote:
| That's only 520x520 transistors. Using sot23 it would be
| maybe 2x2m.
| pengaru wrote:
| The i386 had 100x the number of transistors, it'd cover an
| entire wall.
| duskwuff wrote:
| That just sounds like a challenge. :)
| coupdejarnac wrote:
| 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...
| emeraldd wrote:
| Those are fun little chips if you need a small audio power amp.
| Had the same thought when I pulled up the article.
| Starwatcher2001 wrote:
| 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.
| bitwize wrote:
| Aww man. Was hoping we could rebuild Strong Bad's old computer.
| userbinator wrote:
| 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.
___________________________________________________________________
(page generated 2022-11-27 23:00 UTC)