what component is this? - Page 1
Forum section: electronics / beginners

author: Dmeads
author_rank: Regular Contributor
timestamp: February 27, 2019, 07:32:53 pm
content: Hello!I am thinking about making a 741 op amp with discrete components just for the heck of it. I found this great article where someone looked at the chip inside the IC and created a schematic based on that chip.http://www.righto.com/2015/10/inside-ubiquitous-741-op-amp-circuits.htmlWondering what the component circled in blue is tho? It looks like a transistor with two collectors?Also if I did want to build a 741, could I use general purpose transistors (2N2222)? or would I need a special kind?Thanks!

author: jmelson
author_rank: Super Contributor
timestamp: February 27, 2019, 07:52:40 pm
content: Quote from: Dmeads on February 27, 2019, 07:32:53 pmYes, exactly. You can't buy these at the store, but you can make them on an integrated circuit. Likely, two well-matched transistors with their base and emitters tied together would work about the same, especially if thermally coupled.QuoteWell, 2N2222s are power switching transistors, there might be better ones to use, especially for the input stage. The transistor you circled is a PNP, by the way, and there are some more PNP's in that circuit, too.Jon

author: magic
author_rank: Super Contributor
timestamp: February 27, 2019, 09:17:03 pm
content: Just because it's a multi collector device doesn't mean the collectors are designed to be identical and their betas to be equal.Though in this case they seem visually identical, if I'm looking at the right transistor (and I spent maybe 3 seconds on that, I may be wrong).This tranny seems to provide bias currents for the VAS and the bias spreader. I doubt these currents need to track each other, neither 1:1 nor in any other proportion. They probably used a dual tranny for space saving.More importantly though, integrated circuit trannies are individually designed to reach specific parameters (power capacity, beta, capacitance, ...) and simply substituting one type of transistor for all of them is unlikely to give good results without at least adjustments to most passive components.You are probably better off starting with some simple audio amp circuits. They are very similar to basic opamps. Lots of people build such stuff and schematics are everywhere. Whole books are written on amplifier design, like Bob Cordell and Douglas Self.

author: Richard Crowley
author_rank: Super Contributor
timestamp: February 27, 2019, 10:24:30 pm
content: The problem is even more complex than your cited article reveals. Creating circuits, especially "analog/linear" circuits integrated onto silicon (or germanium or whatever) offers a much wider array of options than soldering discrete components together. Virtually all integrated circuits down to the most primitive and simple take advantage of these design options. When you see a schematic for an integrated circuit, note very carefully that it is not an ACTUAL circuit schematic diagram, it is only an "equivalent" circuit. It means only to "suggest" the operation of the circuit if you had implemented it with discrete transistors, resistors, etc. It doesn't mean that you can actually create a functionally equivalent circuit out of discrete components.OTOH, there ARE discrete-component op-amp circuits available which ARE designed to construct from discrete components. Of course all op-amps were originally implemented with fire-bottles ("tubes" or "valves") And then with discrete transistors, resistors, diodes, etc. In the audiophool world there are still people who make op-amps from discrete components. Some are constructed so that they will plug into an 8=pin DIP socket:And there are the modular-size opamps that API borrowed from analog computing back in the early 1960s, their famous 2520. Some of these are still available from people who continue to clone these vintage parts, in kit form, or assembled and tested.And there are more recent circuits such as the popular Jensen 990 op-amp which continues to be favored by people in search of that particular sound of discrete opamps and $100 transformers.Note that even the discrete-component versions "cheat" and use monolithic/integrated matched-pair input transistors. A device that was abandoned first by National Semiconductor, and then by Texas Instruments (who bought out National and Burr-Brown), and then by Analog Devices (who bought-out SolidState Devices). So it is harder and harder to create a discrete op-amp with performance anywhere equivalent to modern monolithic ICs at a tiny fraction of the price.

author: ebastler
author_rank: Super Contributor
timestamp: February 28, 2019, 12:13:14 am
content: These guys have done a nice job, at least cosmetically:https://www.evilmadscientist.com/2014/the-xl741/

author: Nitrousoxide
author_rank: Regular Contributor
timestamp: February 28, 2019, 06:05:00 am
content: If you feel like something CMOS flavoured:Not the most stellar configuration. But low component count!