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VBC of npn transistor in common collector mode

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.

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.

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.

Thanks in advance.

Reply to
John B
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Keerect.

...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

Apply, say, +10 to the collector, and a load resistor from emitter to ground, a conventional emitter follower.

Apply base voltages starting at zero and working up to +10. Ve will follow Vb up, with about the 0.65 volts loss you mention. At Vc=+10, Vb=+10, Ve will be +9.3 or so and beta will still be pretty good, for reasonable load currents.

If you keep increasing Vb above +10, Vce will go to zero, beta will drop like a rock, and at Vb=+10.6 the b-c junction will begin to forward conduct. This is "inverted state" operation. The right base voltage (or current) can make the transistor saturate to precisely zero volts, and more base current can pull the emitter above +10.

You might also sometimes notice strange behavior in the conventional emitter-follower region, such as Ve being more positive than Vb.

2N2222's like to break into wild UHF oscillations when used as emitter followers.

John

Reply to
John Larkin

The more common name for this is an emitter follower.

Absolutely not. Common emitter is inverting though.

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

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

Graham

Reply to
Eeyore

(1) As a transistor, or as a "diode", floating collector (emitter)? (2) Higher bulk resistance in the collector can obscure the difference.

...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

Uh ? There's several big errors in his post !

Graham

Reply to
Eeyore

Some actual numbers for NPN jellybean transistors using multimeter hFE and diode check functions:

hFE rev hFE Vbe Vbc

2N4401 243 1 .640 .629 2SC1815Y 159 2 .702 .696 8050 151 27 .677 .676

So, not really "far less", only about as much as Vbe changes with a

0.5-5°C junction temperature change.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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Reply to
Spehro Pefhany

Nope. Just in your reading of it. The OP has INVERTED the transistor, emitter now has function of collector and vice versa.

He only erred in terminology, calling it "common collector".

...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

How bizarre !

Does *any* current flow that way ? I.e. collector -ve wrt emitter for an npn device ?

Graham

Reply to
Eeyore

Duh! It's NPN viewed from both directions ;-)

...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

Spehro,

Did you curve-trace the parts to determine reverse beta?

...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

The unused lead floating.

I was a bit surprised to see the 30:1 difference in reverse beta between types that are more-or-less interchangable.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

No, as I implied, I just plugged them into my multimeter using the hFE function and reversed* them for the reverse hFE. Quick and dirty numbers.

  • a bit more gymnastics required for the Japanese pinout devices.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

That's "inverted mode." Common collector = emitter follower.

I happen to have measured these yesterday: Vce ~2.8V, i(b) = 10uA

(MOT) MPSA18 #1 412 10.8 MPSA18 #2 420 12 BC547B #1 250 4.4 BC547B #2 222 3.3

(ITT) 2n3904 120 4.7

(PH) BC548 188 4.7

(NSM) 2n5772 83 0.9 PN4250 275 4.0

Best, James Arthur

Reply to
dagmargoodboat

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 !

Graham

Reply to
Eeyore

It's done for the ultra-low saturation voltage. I just measured an MPSA18, inverted, Vsat = 0.5mV.

Cheers, James Arthur

Reply to
dagmargoodboat

It was common in Studer circuits of the 70's for muting audio signals, ugh

martin

Reply to
martin griffith

And you have to be extremely cautious... if that emitter (collector) breaks down during fault conditions, insidious damage slowly eats your device.

...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

Aside from the very low saturation voltage someone else mentioned, you sometimes run into that mode in transient or fault contitions.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Reply to
Spehro Pefhany

I downloaded part of a textbook on this subject. Curve traces are provided for the 2N3903; forward and reverse. BF = 117; BR = 0.9. No comment was made on the forward voltage drops, though.

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?

Reply to
John B

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