4N25 with signal off the emitter?

Hi all,

The circuits I’ve seen for the 4N25 and similar opto devices, all show open collector mode with the collector pulled up and the signal taken from the collector, as shown attached from the Vijay datasheet.

That has the effect of inverting the output wrt the input, and yes I know that’s easy to handle in code. But is there a compelling reason not to pull the emitter down and take a non-inverted signal from there?

4n25 connection.PNG

But then I found this which does show a common collector opto-isolator so they are indeed used that way.

I'm thinking though, that in the Arduino world, the use of the internal pullup and yes then having to handle the negative logic in code, is easier than buggering about with an external pulldown even though that logic is the right way up.

(Especially in a world where active low is common anyway.)

Short version: I don't see a good reason why you shouldn't take the output of an optocoupler from the emitter. There may be a penalty in speed, or in heat dissipation.

Here's the tl;dr:

Here's a paper from Vishay, who ought to know their way around optoelectronics: http://www.vishay.com/docs/83590/fastswit.pdf. It shows both common emitter and common collector configurations, and promises to discuss how each is appropriate for different applications. A cursory reading doesn't find that discussion, but it appears that a common collector setup will switch faster.

If we were using a standard transistor, we'd want to use it in a common emitter configuration, so that we could drive it all the way to saturation. In the common collector configuration, we can't get the emitter voltage higher than about 0.7V below the base voltage, so the transistor will always be in the linear operating region, and always making a bit of heat, when it's on. But, lot of our circuits will dissipate the same heat whether or nor we saturate the transistor, and changing the terminal from which we take the output will just move that heat from the load resistor to the transistor. I think that it often won't matter much.

I don't know how a phototransistor operates in common collector, but, I suspect that the base current arises from electrons liberated in the base region by incident photons, and they still have to cross the base-emitter junction, so I expect that the minimum voltage across the transistor in that configuration will still be about 0.7V. I look forward to reading what you find.

[Edit: grammar, spelling, punctuation]

Thanks tmd3, great find.

I won't pursue it further, I was just curious. Certainly in an Arduino world with built-in pullups, common emitter is much easier.

[As a philosophical aside, with an opto it's not strictly true to talk of common emitter and collector imo, since even the grounds may be galvanically separated and thus not really common.]

The word “common” in the term "common emitter is specific to the transistor and in no way relates to the
input side of an opto coupler since it is a term derived from the classic transistor configurations , having
nothing to do with optos.Common Emitter

In this circuit the base terminal of the transistor serves as the input, the collector is the output, and the emitter is common to both

(despite the fact that the transistor is a photo transistor , since the output of the photo sensor is connected
to the base which is common to the output. Although, I have to concede that the base is isolated from the
input of an opto so one might claim (as you are apparently) , that it is not truly common. I think that’s
splitting electrons so to speak…

I think that's splitting electrons so to speak..

Which is exactly why I put it in [parens] and called it a "philosophical aside", "aside" being the operative word there.

With the opto output NPN config’d as CE, no other components are required - if you’re confident in the “internal pullup” arrangement.

The proper expression would have been "I think that's splitting 'common' electrons so to speak...".

Emitter followers tend to be very fast, because the output impedance is tiny, about Re/beta. Whereas the 'common emitter' (ie, inverter) ckt has a passive pull up, Rc, which will produce MUCH slower low-to-high transitions. Way back when, this is why they went to active-pullup output stages in TTL logic.

This will only be worse if using the internal pull up rather than a 2.7K - 4.7K external pull up. And the biggest problem with optoisolators like 4N25 is, they're already slow to start with.

So in short (or not ...), the photo-transistor in an opto-coupler is a "black box" component with two terminals - it makes absolutely no electrical difference where you connect it in a circuit, it just switches. Whilst is is generally a NPN transistor, without the base terminal you would never know whether it was in fact actually a PNP. As such, terms like "common emitter" have no meaning because there is nothing to "common" it with.

The only possible use of the base connection is where you wish to reduce the sensitivity by connecting it through a resistor to the emitter, or slowing it down with a capacitor. These would be rather extraordinary situations.

Except that, back in the real world, the 4N25 schematic shows it as being an NPN, so all the previous comments apply.