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BJT for a diode question

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ljy4468

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Dear all,

When npn bjt is used for pn diode,
collector is shorted to base. (diode connected TR)

It ensures reverse bias of base & collector.

Then How situation happens when collector is connected to vdd?(most positive voltage) so it also ensures reverse bias?

But I have not seen diode connected TR like that.
Anyone gives me further explanation??

Regards,
Thanks
 

when u r connecting it to a more positive supply you are effectively increasing the depletion region in the base collector junction thereby reducing the effective base width and this would effectively allow put the transistor in saturation region or active region... so it wont behave like a diode...
 

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    ljy4468

    Points: 2
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Thanks, but I have another question.

collector is shorted to base. (diode connected TR)

It ensures reverse bias of base & collector.

Ok, that is right.

Then In diode connected bjt,

all current flows from base(p) to emitter(n)?

because collector(n) & base(p) is reverse biased??

I'm a bit confused.
 

what is there to be confused in this... since the base collector junction is reverse biased there is gonna be just the leakage current from that.... other than that the base emitter path for the current is gonna be there...

now tell what your confusion is all about a little clearly....
 

Ok, I totally understood what you are saying.

but, here is a material which confuses.

this picture is from the book.
'fundamentals of microelectronics', razavi, wiley

According to attatched, base current is just Iref/beta.
That means the rest of the current must flow to collector.
Isn't it?

i.e., for diode-connected bjt,
If ib=1, ic=99, ie=100.

But As you mentioned, collector current is only leakage current.
So I'm confused.

Would you explain further please?

For diode connected bjt, all current flows from base to emitter?

Regards,
Thanks.
 

sorry man i got a little confused... actually here the collector has a voltage greater than emitter... enough to be in operation and hence there would be current through the collector also.....

i think the transistor in such configuration is called diode connected because it has only two terminals.... when you connect it to a positive supply you can't call it so....
 

Thanks for your kindly answer!

A.Anand Srinivasan said:
sorry man i got a little confused... actually here the collector has a voltage greater than emitter... enough to be in operation and hence there would be current through the collector also.....

i think the transistor in such configuration is called diode connected because it has only two terminals.... when you connect it to a positive supply you can't call it so....
Click to expand...

Collector has a voltage greater than emitter, as you said, That is OK.

Then, how is the current flow from collector to base region?
Because Collector(NW) and Base(PW) has same voltage(shorted), there cannot be a current flow.
If collector is higher than base voltage, then Current can flow through E-field. isn't it?

But there are no E-field cause they(B,C) are shorted.

I cannot understand that Base-Collector current is related to Collector-emitter.
voltage.

Here is the image for convenience.

Thanks,
Regards.
 

k... look at this, this way... there is a potential difference between emitter and base which is too small and due to the potential difference between base and emitter charge carriers are being swept into base but since the base is very small it is just able to recombine some of the charge carriers which arrive and the remaining due to their energy cross the base region and move into the collector region.... in the collector there is very less chance of them recombining with holes and hence we get collector current...
 

I know what you're saying.

Electrons swept into collector when there is E-field between base & collector.

My question is
In above case, there is no voltage difference between B & C. Then, E-filed induced?
 

A.Anand Srinivasan said:
it is due to E-Field induced between E & C(B)....
Click to expand...

Thanks for your kindly answer, again.

Here is my last question for confirmation.
E-field is induced like below picture?
 

it is not like that.... it is from the p region of B and n region of C to n region of E.... u are directing it from a lot of directions...
 

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    ljy4468

    Points: 2
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