# Interfacing with Transistors

I was reading the chapter of The Art of Electronics about bipolar transistors (chapter 2), and I came across this paragraph:

But why should I use transistors, specifically, for interfacing? Why not op-amps? Are FET transistors also good for interfacing?

Could you give concrete examples (circuits) using transistors for interfacing?

• Powerful because you can do almost anything with transistors, including building a custom op amp (if nothing exists that meets your needs). Maybe? – Matt Aug 4 '18 at 3:31
• It is important to understand that IC's are made for the benefit of IC manufacturers. Not customers. They have to be apparently useful enough to customers to get bought. But like novel pharmaceuticals, the customer is not the beneficiary. "The latest IC" is often not the cheapest or even best solution - just a profitable one for the seller. A BC847, at way less than 1 cent in volume, has no marketing department pushing you to use it. Like Aspririn. – Henry Crun Aug 4 '18 at 3:57
• In bulk, BJT's are less than 1 US penny each. If you can do something with one or two transistors, that will usually be the cheapest and best way to do it. – mkeith Aug 4 '18 at 6:19
• @mkeith I buy them at about 0.3 to 0.4 cents each. Usually in lots of about 20k (10 boxes of 2000 apiece, for a total of about $60-$80.) Dirt cheap. I give them away by the dozens to students interested in playing. I've not yet found a source for mosfets anywhere close to that pricing, though. Sadly. – jonk Aug 4 '18 at 6:36
• @mkeith 3 cents would be as low (or perhaps slightly lower) as anything I've seen. And that's 10X more than BJTs I can readily find. So BJTs it is. – jonk Aug 4 '18 at 7:12

Using discrete transistors has benefits:

• They are cheap - basic ones are well below 1c in volume.
• They have high voltage ratings. A $1 IC today is often 5V rated, and a 1C transistor can switch 80V • They have high current ratings. A 1c transistor can drive 1/2 an amp. A$1 transistor can drive 50amps. Your micro/opamp/whatever can drive 20mA.
• They are really really small in SOT963 packages, and come in ones.
• The heat making part of the circuit can be put in away from the rest of the circuit
• They can be inherently more robust, since they are quite a big lump of silicon, not a few micrometers in the corner of a chip
• They are in stock. If you have been in manufacturing for a while, you will suffer PTLD (Post Traumatic Leadtime Depression), a known IC condition leading to acute hair loss.
• They are good enough, and good enough is enough
• Just to add: (1) They are the exact opposite of boutique and are often widely multi-sourced. (2) While they last, you can get BJTs used for horizontal flyback (old TV) circuits that can handle HUGE voltages and HUGE rates of change of voltage that NO IC comes anywhere close to achieving. (Sadly, I'm pretty sure that is going away forever, never to be had again.) Anyway, +1. – jonk Aug 4 '18 at 6:42
• @Jonk give us a part number... – Henry Crun Aug 4 '18 at 8:23
• In general, just look up "horizontal output transistor." (Aka "HOT".) The turn-off time is critical because of the heat generated, otherwise. But these are in general high voltage BJTs (1500-2000V is typical.) Their construction is interesting, varying from "comb" and "stripe" to a variety of "mesh" types. The fastest ones I've personally used (replaced, I mean) were inside a Tektronix monitor that supported a 200MHz pixel rate, long time back. – jonk Aug 4 '18 at 19:47

why should I use transistors, specifically, for interfacing? Why not op-amps?

You should use transistors because they are useful. So are op-amps, use those too. One can NOT achieve optimum results without considering all the options.

So, learn how to use bipolar transistors: they are the basis for op-amps, after all. They are also the basis for gates, latches, timers, SCRs, thermometers, photosensors, etc.

Interfacing is always a mixed-up task, where rules of input are different from rules of output, and where solutions fall outside the well-understood common functions that an IC catalog will support. A knowledge of transistors fills many gaps.

Would you like an amplifier, to run on

simulate this circuit – Schematic created using CircuitLab

0.6 volts and 100 nanoAmps, to provide gain of TWO.

[simulates: 154nA, VG=1.2; VG=2@178nA,R1=2.7M,BC547C. (H.C.)]

• Next you'll be trying to tell me its cheap too – Henry Crun Aug 4 '18 at 4:00
• Some years back I thought thru an entire "class" of below 0.6 volt VDD transistor circuits. That 1nanoAmp Idd suggests the Vbe is 0.6v(for 1mA) - 6decades * 0.058v = 0.252 volts, is a wonderful idea. – analogsystemsrf Aug 4 '18 at 4:12
• I'd expect ~ unity gain but be surprised if it was more than 1.5 – Tony Stewart EE75 Aug 4 '18 at 5:08
• @TonyEErocketscientist Come on Tony, run the simulator, then give him crap about it. BTW, I am assuming its something a bit hotter than a 2n3904. What transistor is it ASR? – Henry Crun Aug 4 '18 at 8:21
• G=1.2, Beer for Tony... – Henry Crun Aug 4 '18 at 9:43