-5

Again and again I encounter this inverter diagram:

Inefficient yet popular transistor inverter

However, a moment's consideration will tell you that it consumes more power when off than it provides to the load when on. I have tested this, and can confirm that it does.

So my question is, why is it used? Rather than, say, this one?

schematic

Sod Almighty
  • 1,295
  • 13
  • 22
  • I'd guess didactic purposes. It's easy to understand. – Hearth Apr 25 '17 at 12:30
  • 3
    Used where? This circuit snippet is not good or bad outside of any context, only the context makes it good or bad. Not everyone cares about every nW of power spent somewhere. – PlasmaHH Apr 25 '17 at 12:34
  • Your more efficient circuit would be slow if there is load capacitance . – Autistic Apr 25 '17 at 12:34
  • 1
    The answer is, it's not used to power any significant loads, because it's not suited to it. – Neil_UK Apr 25 '17 at 12:37
  • 2
    I don't see that circuit used to drive loads. Usually R is the load. Where do you encounter this circuit again and again? You must have lots and lots of examples that you can easily edit into your question. – TonyM Apr 25 '17 at 12:39
  • The invertor that is supposed to be horrible is popular in colleges .I guess it is easy for tutors to teach .It was never taught as the worlds best circuit .It is Msquared L which is mickey mouse logic . – Autistic Apr 25 '17 at 12:45
  • That inverter, with 3 input resistors of size 10R, is basic NOR gate, produced by Fairchild as UL911 and is the only IC used in Apollo Lunar Lander Radar computer. Thousands used, in that computer. – analogsystemsrf Apr 25 '17 at 16:42
  • @TonyM Your sarcasm is not helpful. – Sod Almighty Apr 25 '17 at 17:42
  • 1
    @SodAlmighty, Grow up: your question has the title 'why is this horrible circuit used'. You have attracted a variety of questioning comments along with mine. But you have still not edited any examples whatsoever into your question. – TonyM Apr 25 '17 at 19:43
  • The other comments are questioning. Yours are borderline rude. Flagged. – Sod Almighty Apr 25 '17 at 20:18
  • Besides, my question doesn't need examples. Your personal requirement for examples could be easily sated, however: a simple Google image search will turn up dozens of variations on this circuit. – Sod Almighty Apr 25 '17 at 20:20
  • 1
    @SodAlmighty, yes, we're all a big gang here, waiting to pounce on the paranoid. Back to reality: the onus is on you to substantiate your question. It is not my task to do research for you. When you ask for help on this site, you are appealing to the goodwill of others, which I reputably give with kindness. Entitle your question sensibly and elaborate your question. Petulant outbursts and accusations are a waste of everyone's time. Maybe you could try harder on the Almighty side of things. – TonyM Apr 25 '17 at 20:28

3 Answers3

6

The problem is in your mindset that the circuit is intended to drive a 'load' of some sort, and also from that your definition of 'on' and 'off'.

It's being presented as a logic-level signal inverter. As such, there is no load and there is no 'on' or 'off'.
There is only a 'high' or 'low' output voltage - which may not even necessarily be able to supply any significant amount of current - it's just a voltage level for signalling.

It's also only a bare-minimum snippet of a circuit and isn't intended to be used for any useful purpose on its own.
It has all kinds of horrible characteristics for a logic circuit like very low input impedance and either very-high output impedance or very high power consumption (depending on the choice of value for 'R').

There's nothing intrinsically 'wrong' with the circuit - it just has a handful of conditions you need to be aware of if you choose to use it as a part of a larger circuit (much like anything else in electronics ...).

brhans
  • 14,373
  • 3
  • 34
  • 49
  • Yes, I'm thinking more of driving a small load (e.g. an LED) than providing a logic state. Would I be much better off using an inverter IC, or should I use my "better" circuit? I want to waste as little power as possible. – Sod Almighty Apr 25 '17 at 18:17
4

Real inverter circuits (even 1960s TTL) are usually push-pull (or totem pole) because any single transistor inverter will have a trade off between power consumption in one of the two states and switching speed with a load capacitance/drive capability. The totem pole TTL output was used because it is not easy to make a good PNP transistor in the bipolar technology of the day- you will notice it uses only NPN transistors.

If you need a single inverter made with a discrete transistor for some reason there is very little inherent reason to prefer one over the other- the best choice will depend on what state it usually sits in, whether the load is grounded or connected to Vcc etc. They are equivalent, just complementary.

Spehro Pefhany
  • 376,485
  • 21
  • 320
  • 842
3

I believe that's the basic bipolar RTL inverter. Like a lot of things, it survives in textbooks while being virtually extinct in the wild.

It dates from an era when NPN transistors were much preferred to PNP ones due to asymmetry of manufacturing, and transistors were generally "expensive".

It has the property that it can be chained - if you try to chain copies of your circuit, what happens? Yours can provide a current source but not a sink.

pjc50
  • 46,540
  • 4
  • 64
  • 126