| 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/ |