Need Reference Recomendation on Design Using Strictly NPNs - Page 1
Forum section: electronics / projects

author: TheRuler8510
author_rank: Regular Contributor
timestamp: December 21, 2013, 06:42:06 pm
content: Hello Group,Does anyone have a book or paper recommendation on designing with strictly NPNs? I want design a 741 operational amp equivalent using only 2N2222A or similar NPN. Those pesky PNPs have a way of polluting every design.Thanks in advance!

author: SeanB
author_rank: Super Contributor
timestamp: December 21, 2013, 06:55:45 pm
content: Even a 741 has lateral PNP transistors in it. Going to be hard to find any opamp without PNP somewhere in it.

author: tszaboo
author_rank: Super Contributor
timestamp: December 21, 2013, 06:56:01 pm
content: also, if you find an amplifier design, which only contains resistors, please post it here.

author: SeanB
author_rank: Super Contributor
timestamp: December 21, 2013, 06:59:05 pm
content: Only way is to look up valve instrument amplifiers like the Philbrick ones, as valves only came in one variety.

author: c4757p
author_rank: Super Contributor
timestamp: December 21, 2013, 07:01:39 pm
content: Quote from: TheRuler8510 on December 21, 2013, 06:42:06 pmDid a PNP transistor kick your dog?Edit: yes, you can indeed design an op amp withonly NPN transistors... just need to fuck up some 2N2222s first...But even here it performs significantly better if you allow just one PNP...Probably don't want to let these get warm...

author: TheRuler8510
author_rank: Regular Contributor
timestamp: December 21, 2013, 07:03:49 pm
content: Quote from: SeanB on December 21, 2013, 06:55:45 pmTo be more concise...I want to design the opamp itself, not "with" the opamp.Thanks,

author: TheRuler8510
author_rank: Regular Contributor
timestamp: December 21, 2013, 07:07:19 pm
content: Quote from: SeanB on December 21, 2013, 06:59:05 pmGood idea...I look those up.Regards,

author: dannyf
author_rank: Super Contributor
timestamp: December 21, 2013, 09:53:41 pm
content: QuoteFairly easy. All you need to remember is that an opamp is essentially a block with very high gain - which can be easily done with npn exclusively.

author: free_electron
author_rank: Super Contributor
timestamp: December 22, 2013, 12:05:16 am
content: Quote from: TheRuler8510 on December 21, 2013, 06:42:06 pmyou are either-bored-have too much free time on your hands-stark raving mad- all or of some of the above.

author: Unknown
author_rank: Unknown
timestamp: December 22, 2013, 01:25:59 am
content: It's also a fairly common lab / homework project in some EE courses.

author: TheRuler8510
author_rank: Regular Contributor
timestamp: December 22, 2013, 02:24:07 am
content: Quote from: free_electron on December 22, 2013, 12:05:16 amfree_electron,You forgot one more option and that is, PNPs may not be available and Digikey does not deliver to where I am at.Maybe when Amazon gets that drone delivery system going...

author: TheRuler8510
author_rank: Regular Contributor
timestamp: December 22, 2013, 02:27:36 am
content: Quote from: Tac Eht Xilef on December 22, 2013, 01:25:59 amIt would be excellent if you could post an example...It seems like it should be an easy answer, until you start looking for them.

author: TheRuler8510
author_rank: Regular Contributor
timestamp: December 22, 2013, 02:33:48 am
content: Quote from: c4757p on December 21, 2013, 07:01:39 pmThanks for the schematic--I'll check that out closer.

author: Unknown
author_rank: Unknown
timestamp: December 22, 2013, 04:02:43 am
content: Quote from: TheRuler8510 on December 22, 2013, 02:27:36 amSorry, I only know that from sitting in on some EE lectures during my (non-EE) undergrad, and discussing the pracs/homework with other EE students over the years.My suggestion to them was the same as SeanB's - look at valve amplifier circuits (especially differential / push-pull stages with negative feedback) and consider what makes an op-amp an op-amp...

author: c4757p
author_rank: Super Contributor
timestamp: December 22, 2013, 04:11:21 am
content: Quote from: TheRuler8510 on December 22, 2013, 02:33:48 amI was having a bit of fun dicking around in SPICE, after I wondered whether an op amp could be done with literallyonlyBJTs (it's easily accomplished with only MOSFETs, as in any CMOS op amp, but AFAIK you can't design a "bipolar resistor" the way you can design a MOS one). I doubt they'll work as anything but a demo of thermal runaway. A quick temp sweep shows the quiescent current having a corner at around 45°C, and going well into multiple amps by 100. That = molten silicon shot at your eyes within a second of turning on the power.Of course, they could be a starting point for a real design, if you put in resistors instead of abused, leaky NPNs, but it really is just a basic differential amp -> buffer, nothing special that you won't see in any op amp datasheet.The output swing of the first (100% PNP-free) only goes down to -4V, but that's just because I couldn't think of a way to shift the output of the long-tailed pair down near the negative rail with only leaky transistors. Level shifting is easily accomplished with a transistor and a few resistors, so that could easily be fixed.Quote from: Tac Eht Xilef on December 22, 2013, 04:02:43 amThis is the key. The "magical" part of the op amp is the differential pair, which only needs to be done with one type of transistor (it often has a complimentary pair of transistors as an active load, but they are unnecessary; you can just use resistors). After that you just need voltage gain and current gain. It's convenient to have both types when designing a circuit like an op amp because each stage tends to invert the signal offset. An NPN differential pair has an output near the positive rail, and a PNP gain stage following that has an input near the positive rail, no level shifting required. But it's just convenience.

author: dannyf
author_rank: Super Contributor
timestamp: December 22, 2013, 01:20:23 pm
content: QuoteSpecifically, Williamson amplifier.The grand daddy of the JLH1969.QuoteMore specifically, the margical part of a Linn 3-stage op amp is the differential pair.There is no law, however, that says an opamp has to use the Linn topology. That will open up your design universe considerably.

author: GK
author_rank: Super Contributor
timestamp: December 22, 2013, 02:32:07 pm
content: Quote from: dannyf on December 22, 2013, 01:20:23 pmThe original audio amplifier by Lin had a "singleton" (in almost* all respects technically inferior to the LTP) input stage, a single transistor VAS and a quasi-complementary power output stage. In no way can the modern 3-stage op-amp with a LTP input stage be attributed to Lin.The term "Linn topology" was invented, erroneously applied and popularized by a selection of idiots and a few self-appointed gurus over at Diyaudio. This has been pointed out numerous times to no avail. Historical revisionism as a side serving to quasi-technical mumbo jumbo.*All else being equal, the "singleton" input stage (in isolation) can be made, theoretically, 3dB quieter than an LTP input stage.EDIT:I needed to refresh my own memory - the Lin amplifier wasn't even as sophisticated as I for some reason remembered it to be (no VAS as in distinct from the input stage):http://www.semiconductormuseum.com/Transistors/RCA/OralHistories/Lin/Lin_Page7.htm