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VBC of npn transistor in common collector mode

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this,

I disagree. Emitter follower has a different topology. There is nothing "below" the collector, of my flipped transistor, other than an attached ground. If this npn transistor were to be suddenly replaced by a pnp, then I see an emitter follower. Of course, the controlling signal at the base would then have to be referenced to the upper rail, instead of to ground, as my npn is.

It inverts voltage. When the base voltage rises, more current flows down into the emitter from the upper rail, through a load, and out through the collector to ground. The voltage at the emitter pin goes down, not up.

An emitter follower is non-inverting. This is not an emitter follower.

instead

Granted.

current

It's Beta reverse...as attested to by so many other posters in this thread. It's drastically different from Beta forward.

ground?

Not to be argumentative, but a c-b junction is forward biased in a saturated circuit that employs common emitter. Very common situation.

John B

Reply to
John B
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Thank you! This answers my question, directly.

"Spehro Pefhany"

Reply to
John B
[snip]
[snip]

"Common" (in configuration sense) means AC ground, thus "common collector" IS an emitter follower.

Collector to DC ground is an INVERTED transistor. See the references on the S.E.D/Schematics page of my website.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.
Reply to
Jim Thompson

Emitter follower and common collector are the same thing. You were mistaken to call it common collector.

Since supply and ground are typically the same thing for a.c. coupled signals that difference is largely academic.

Graham

Reply to
Eeyore

would

As

Reply to
John B

OK. I hear you, and I hear Mr. Thompson. I agree that emitter follower and common collector are one and the same. I agree that I erred in calling my circuit "common collector." As such, it is not emitter follower, either.

I'll go with Thompson's description, of "inverted transistor." And since I believe the topology of the circuit has to be one of three choices, I'll now choose "common emitter." INVERTED, of course.

Thank you for your replies.

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is.

signals

Reply to
John B

[snip]

The collector has become an "emitter" because the base-collector is now the forward-biased junction.

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

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| 1962 | I love to cook with wine. Sometimes I even put it in the food.

Reply to
Jim Thompson

That would seem to be correct ! :-)

Graham

Reply to
Eeyore

It would be more obvious with an old alloy diffused type. Is that the right terminology ?

Graham

Reply to
Eeyore

As others have mentioned, the saturation voltage can be very low.

I used a pair of saturated switches - the first one operated normally to sink the bulk of the current and the second one inverted, for low saturation voltage - as the core of a precise (better than 0.1%) pulse-width modulated multiplier back in 1974. It was a slow as a wet week - which was fine for process control - but cheap.

Not my idea, but I had to rework it a bit to get the accuracy the boss had expected - the switches were a bit slow, so timing errors were comparable with the voltage offset errors. Very cute - I suspect that Peter Baxandall might have had the original idea, granting where my boss did his apprenticeship.

--
Bill Sloman, Nijmegen
Reply to
bill.sloman

You know the NG has gone completely to hell when the Sloman's posts become incomprehensible.

Reply to
Fred Bloggs

Sorry Fred. I'll post a circuit diagram on my web-site in a day or two.

--
Bill Sloman, Nijmegen
Reply to
bill.sloman

Two switches in parallel? Sounds like BS (Bill Sloman) to me ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.
Reply to
Jim Thompson

This configuration is not called "common collector" - in that one, the bias is normal, but the output comes from the emitter, so it's also called an emitter follower, which is clearly not what you have.

You're just running the transistor upside down - it's so uncommon, I don't even know if it has a "standard" name, but it is doable.

I can't imagine a beta less than one, but I haven't really looked at upside-down transistor circuits all that much.

This one, I don't know - why not slap one in the protoboard, and tell us? :-)

Cheers! Rich

Reply to
Rich Grise

Apparently Jim isn't tuned in to that sort of circuit.

In fact it was two switches in series. We had a 0V to 10V signal range,

Vin -- 3k3 --+-- 3k3 --+-- 3k3 ----250R pot --- 10k -----+ | | | | NPN NPN(inverted) virtual earth op amp out.

and the first switch sunk the bulk of the current (3mA worst case) when it was turned on, leaving the inverted transistor only some 100uA to deal with.

The voltage divider also protected the base-emitter junction of the inverted transistor. With 15V rails, we could be sure that the reversed base-emmitter junction didn't ever see more than 5V - Vbeo was 6V for the part that we used (BC184 IIRR).

--
Bill Sloman, Nijmegen
Reply to
bill.sloman

This I cannot follow. Can you provide directions to your website? Thanks.

Reply to
John B

From the sketch I don't see two devices in *series*. I see a regular device switching to ground, then a resistor before the inverted device switching to ground.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.
Reply to
Jim Thompson

What font was that in? It doesn't line up, and the bottom line wraps!

Courier, or Courier New, is a pretty standard fixed-width font I can recommend.

Tim

--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
Reply to
Tim Williams

There is a resistor in front of the regular device, so that it only has to sink a predictable amount of current, and a resistor after the inverted device, so that the summing junction is still looking at a well defined impedance when the switch is closed/on.

To me, this looks like a series of resistors and switches along the signal path, but I'd be interested in hearing how you would prefer to see it described.

--
Bill Sloman, Nijmegen
Reply to
bill.sloman

I'll post directions once I've got the circuit diagram drawn and up-loaded. I've got some other fish to fry so it may take a day or two.

--
Bill Sloman, Nijmegen
Reply to
bill.sloman

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