VBC of npn transistor in common collector mode

[John B]

The more common name for this is an emitter follower.

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.

Absolutely not. Common emitter is inverting though.

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.

Granted.

No - wrong. Beta hasn't changed and this configuration does indeed have current
gain.

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

Not much difference really. I can't ever recall seeing it specced in this
configuration. The c-b junction is always reverse biased anyway.

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

 

[John B]

Thank you!
This answers my question, directly.

"Spehro Pefhany"

 

[Jim Thompson]

[snip]

So if you don't like my terminology of "common collector," then what would
you call it, "common emitter, backwards"?

What, then, is (npn) common collector? Collector tied to AC ground...? As
in the upper rail?

[snip]

...Jim Thompson

"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

 

[Eeyore]

I disagree. Emitter follower has a different topology. There is nothing
"below" the collector, of my flipped transistor, other than an attached
ground.

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

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.

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

Graham

 

[John B]

So...then it's "common emitter, inverted"?
Sounds about right to me.
Thanks for your astute replies!

[snip]

So if you don't like my terminology of "common collector," then what would
you call it, "common emitter, backwards"?

What, then, is (npn) common collector? Collector tied to AC ground...? As
in the upper rail?

[snip]

...Jim Thompson

"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

 

[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.

 

[Jim Thompson]

So...then it's "common emitter, inverted"?
Sounds about right to me.
Thanks for your astute replies!

[snip]

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

...Jim Thompson

 

[Eeyore]

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.

That would seem to be correct !

Graham

 

[Eeyore]

So...then it's "common emitter, inverted"?
Sounds about right to me.
Thanks for your astute replies!

[snip]

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

...Jim Thompson

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

Graham

 

[[email protected]]

In the days of MAT100s I could see that working but it somewhat surprised me to
see it work with a modern device.

I can't see the point of doing it though !

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.

 

[Fred Bloggs]

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.

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

 

[[email protected]]

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

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

 

[Jim Thompson]

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

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

...Jim Thompson

 

[Rich Grise]

For a typical npn bipolar transistor, such as the 2N2222A, we can expect a
DC voltage drop of about 0.65 V, from base to emitter ("V BE"). Such is the
case of the ubiquitous "common emitter" configuration.

Suppose we flip the transistor, and use it "backwards." Let's call this,
"common collector." Correct me if I'm wrong, but that sounds like the
accurate description of the configuration, because we are grounding the
collector, driving the base with a controlling current, and attaching a
pulled-up load to the emitter. This is an inverting amplifier...sort of.

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.

Beta can be expected to be "miserable," with a value of less than 1, instead
of roughly 100. So amplification is "in the eye of the beholder." Granted.

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

Can anyone advise on the forward voltage drop of between base, and
ground? (That is, "V BC") I expect that the collector is very lightly
doped, compared to the emitter, so the barrier should be far less,
leading to a lower threshold voltage to overcome.

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

Cheers!
Rich

 

[[email protected]]

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

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).

 

[John B]

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

 

[Jim Thompson]

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

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).

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

 

[Tim Williams]

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

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

 

[[email protected]]

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



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.

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.

 

[[email protected]]

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

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.