# Phototransistors - question - not really homework

#### Wolframore

Joined Jan 21, 2019
2,541
Concerning Phototransistors... is there an actual base current since light activates the base? The reason why I ask is that with standard transistors:

Ie = Ic + Ib. Is the collector current the same as emitter current or is there an actual flow of electrons through the optical base? Am I the only one confused by this?

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#### Wolframore

Joined Jan 21, 2019
2,541
Thanks @ericgibbs this is very descriptive but still not sure if I have more current in the emitter vs collector like it is in a standard BJT. My guess is the current through the emitter is the same as collector current but it’s not clear.

#### SamR

Joined Mar 19, 2019
4,195
Absent a PDF I would use 0.7V?

#### SamR

Joined Mar 19, 2019
4,195

#### nsaspook

Joined Aug 27, 2009
9,738
Thanks @ericgibbs this is very descriptive but still not sure if I have more current in the emitter vs collector like it is in a standard BJT. My guess is the current through the emitter is the same as collector current but it’s not clear.
The bias current path for base current is from the collector when photonic energy is absorbed by the base and creates a leakage (resistive) path on the reversed CB junction by creating extra conduction holes.

#### Wolframore

Joined Jan 21, 2019
2,541
The bias current path for base current is from the collector when photonic energy is absorbed by the base and creates a leakage (resistive) path on the reversed CB junction by creating extra conduction holes.
So Ic = Ie in this case! But the concern is that there's actually a base current separate from the collector current (induced by photons) and Kirchhoff says the sum=0...

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#### dl324

Joined Mar 30, 2015
14,475
So Ic = Ie in this case! But the concern is that there's actually a base current separate from the collector current (induced by photons) and Kirchhoff says the sum=0...
There is a base current. It's from the photons striking the collector-base junction.

From your first post:
Ie = Ic x Ib.
I know you meant '+' instead of 'x', but you might want to correct it before you have more peer corrections.

#### Wolframore

Joined Jan 21, 2019
2,541
ugh... you're right... edited. and so the emitter current must be the sum of both... my textbook is wrong again! hah... instructor says I get no extra points for pointing out errors in the textbook but it's a little frustrating at times.

#### dl324

Joined Mar 30, 2015
14,475
so the emitter current must be the sum of both... my textbook is wrong again!
Can you post the section in question?
hah... instructor says I get no extra points for pointing out errors in the textbook but it's a little frustrating at times.
You can contact the author so s/he can correct it in a future revision. And your instructor could make note of it so s/he can point it out to students who might otherwise be confused.

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#### Wolframore

Joined Jan 21, 2019
2,541
Let me try to get that one together... it's actually in two areas... first describing Collector current is the product of Beta and base current... but later he asks what the emitter current is and says it's the Collector current.

Another revision means another $200 for text books for students... I caught another one where a battery did not specify value and asked such as: This one the answer is that the current would increase... my answer was as long as the battery is enough to forward bias D1, I (D2) would increase. #### dl324 Joined Mar 30, 2015 14,475 Another revision means another$200 for text books for students...
If you already have the book, you can just mark the correction. I've had instructors point out errors in textbooks so we could make the corrections ourselves.

Some books have the same errors revision after revision. AOE is one of them. I think version 2 had an error about the beta to use for operation a transistor in saturation mode. Version 3 has the same error; though slightly less wrong with a more elaborate (but still incorrect) discussion.

#### Wolframore

Joined Jan 21, 2019
2,541
There are two versions of this book. One for conventional current and the other electron flow. I hate how it goes back and forth at times. It’s ok as long as you are driven to find the correct answers. To be totally honest I didn’t even buy the book. I tried to find the author but his contact isn’t listed.

#### crutschow

Joined Mar 14, 2008
29,808
Current can't be created from nothing so obviously the collector and emitter currents must be equal if there is no external base connection.
The photoelectron "base" current is generated at the collector-base junction and is part of the collector and emitter current.
So technically a small portion of the collector current becomes the base current.
Thus I believe the book is correct.

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#### Wolframore

Joined Jan 21, 2019
2,541
Current can't be created from nothing so obviously the collector and emitter currents must be equal, if there is no external base connection.
The photoelectron "base" current is generated at the collector-base junction and is part of the collector and emitter current.
So technically a small portion of the collector current becomes the base current.
Thus I believe the book is correct.
Book states: (this is all it states on this matter)

But later states:

implying Ie and Ic are the same. My original calculation added the 50 uA to the collector current. I'm not entirely certain if this is the case. It's not a large current but this is something we would account for with standard BJT with base current.

#### dl324

Joined Mar 30, 2015
14,475
implying Ie and Ic are the same.
For sufficiently high values of beta, Ib is small enough that ignoring it doesn't have a significant effect on calculations. It would have been better if the equations showed that Ie was approximately equal to Ic. But it's less than 1% error in this case.

#### Wolframore

Joined Jan 21, 2019
2,541
At active region with beta of 100 it's approx 1% but during saturation you can get higher error: the emitter and base current difference shows even in this simple simulation... I can't find one for a phototransistor... not even sure if you can model one which needs a light source.

#### dl324

Joined Mar 30, 2015
14,475
At active region with beta of 100 it's approx 1% but during saturation you can get higher error
That's true, but the example in question isn't for saturation mode.

#### crutschow

Joined Mar 14, 2008
29,808
My original calculation added the 50 uA to the collector current.
Okay, but where is this base current coming from?
Re-read my post #14.
As I stated, you can't generate a current from nothing, so it must be part of the collector current if there is no external base connection (and most phototransistor have none).
What about that is difficult to understand?