Power Dissipation

Papabravo

Joined Feb 24, 2006
16,105
What is the Base to Collector resistance be of 100 pnp transistors in parallel with total power 36V DC at 2.25A
I don't know how to answer your question.
How are the transistors biased, and how would you measure this resistance, and do you expect it to be constant as a function of collector current.
A schematic of what you are talking about might be helpful, and it only needs to show the first and last legs for example.
I would imagine that since the collector and emitter currents will be large and that the base currents will be relatively small (unless this is a forced beta situation) that the answer would be relatively high.

I am curious as to the reason for your question.
 

BobTPH

Joined Jun 5, 2013
3,308
Don’t know why you want base to collector resistance. Typically no current flows between them.

And nothing in a bipolar transistor acts like a resistor.

Bob
 

Thread Starter

George-O

Joined May 27, 2021
8
All - PNP the The Emitter is more positive than the
Don’t know why you want base to collector resistance. Typically no current flows between them.

And nothing in a bipolar transistor acts like a resistor.

Bob
Emitter has+36 V; 35.3V past the Emitter to base diode voltage the collector is GND the base to collector has a 14 ohm resistor tied into the header of the burn-in board and all of these PNP resistors are in parallel - there are 100 total per burn-in board. I had a design engineer ask me what he resistance between the base and collector would be? Mathematically I would have 1400 ohms in parallel 100 times = 14 but it does not seem correct or what would be correct for the application. Oh Yeah - there's 2.25 A total current - the object was to dissipate 80% of total peak power among 100 devices ~ 800mW per device.
 

Papabravo

Joined Feb 24, 2006
16,105
All - PNP the The Emitter is more positive than the

Emitter has+36 V; 35.3V past the Emitter to base diode voltage the collector is GND the base to collector has a 14 ohm resistor tied into the header of the burn-in board and all of these PNP resistors are in parallel - there are 100 total per burn-in board. I had a design engineer ask me what he resistance between the base and collector would be? Mathematically I would have 1400 ohms in parallel 100 times = 14 but it does not seem correct or what would be correct for the application. Oh Yeah - there's 2.25 A total current - the object was to dissipate 80% of total peak power among 100 devices ~ 800mW per device.
I didn't ask for a paragraph describing a circuit. I don't do paragraphs. I did ask for a schematic diagram.
 

DickCappels

Joined Aug 21, 2008
7,424
The base-to-collector resistance would be quite high. The base-to-collector capacitance would be approximately 100x that of a single transistor biased that way, which is often in the datasheet. The base-to-collector resistance is independent of current.

Did you really me impedance? Or emitter resistance re?
 

BobTPH

Joined Jun 5, 2013
3,308
In the circuit he describes, or I think he describes, current flows is in through the emitter, and out through the base and collector . So how is the “resistance” between the base and collector relevant?

Bob
 

rsjsouza

Joined Apr 21, 2014
309
I suspect the OP is asking for what the load would be when trying to drive the base of 100 PNP BJTs.

That is where the description falls apart. It is hard to visualize "a 14 ohm resistor tied into the header of the burn-in board and all of these PNP resistors are in parallel" - too many ways to interpret what is a "header of the burn-in board", a few other ways to connect a resistor to a BJT (Base? Collector? Base-to-ground? Base to header?), then they are in parallel... So yes, a schematics is critical here.
 

LesJones

Joined Jan 8, 2017
3,172
If we take your question literally then the answer would be 16 ohms BUT as others have told you your question does not make sense. This answer is interpreting your question as you have the base emitter junction of 100 PNP transistors connected in parallel and you are passing a current of 2.25 amps through these junctions in parallel and you measure a voltage of 36 volts across the junctions. The polarity cannot be forward biasing the junction as that would give a voltage of about 0.7 volts. If the junction was reversed biased then I would expect the voltage to be about 5 volts (This is a typical base emitter junction reverse breakdown voltage.) This is the reason members are trying hard to find out what your question REALLY IS.

Les.
 

MrChips

Joined Oct 2, 2009
23,515
If you wish to determine static resistance, determine the current through the load and the voltage across the load.
R = V / I.
 

BobTPH

Joined Jun 5, 2013
3,308
There is no load, the emitters are at +36V and the collectors are grounded. And, if I understand it correctly, all the bases are tied together and grounded through 14 Ohms.

Bob
 

BobTPH

Joined Jun 5, 2013
3,308
His description cannot be correct. If there is 14 Ohms between the base and ground, and the base is at 35.3V, then there is 2.5A flowing through your he resistor. But he says there is 2.25A total.

Bob
 
Top