VBC of npn transistor in common collector mode

[John B]

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.

 

[Jim Thompson]

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.

Keerect.

...Jim Thompson

 

[John Larkin]

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.

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

 

[Eeyore]

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.

The more common name for this is an emitter follower.

This is an inverting amplifier...sort of.

Absolutely not. Common emitter is inverting though.

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.

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

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.

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

 

[Jim Thompson]

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

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

...Jim Thompson

 

[Eeyore]

Keerect.

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

Graham

 

[Spehro Pefhany]

Keerect.

...Jim Thompson

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

 

[Jim Thompson]

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

Graham

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

 

[Eeyore]

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".

How bizarre !

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

Graham

 

[Jim Thompson]

How bizarre !

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

Graham

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

...Jim Thompson

 

[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

Spehro,

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

...Jim Thompson

 

[Spehro Pefhany]

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

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

 

[Spehro Pefhany]

Spehro,

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

...Jim Thompson

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

 

[[email protected]]

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

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

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

 

[Eeyore]

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

...Jim Thompson

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

 

[[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 !

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

Cheers,
James Arthur

 

[martin griffith]

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

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


martin

 

[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

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

...Jim Thompson

 

[Spehro Pefhany]

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

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

 

[John B]

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?