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The instability of Collector current(Ic) of a transistor depends on three factors viz. Icbo, Vbe & Beta. Can you explain why Vbe decreases(by 2.5 mV/°C) with increase in temperature?

Abhishek
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2 Answers2

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The diode current has the formula

Idiode = Is * e ^ [Q * Vdiode / (K * T)]

WIth the T being temperature, and a common voltage at 1milliAmp being about 0.6 volts, we can simply divide that 0.6 volts by room temperature of 290 degree K, and we have about 2.1 milliVolts change per degree. This however requires the WRONG POLARITY of voltage change.

Digging further, wikipedia says I am wrong, and that the change of Is will be the far greater (and opposite polarity) effect. Check out Wikipedia "diode equation".

examining "reverse saturation current" on Wikipedia, we see the useful statement "a rule of thumb is that the current doubles for every 10 ° C". We can use that, along with 18 milliVolts being adequate to cause Idiode to double, and see this predicts 18/10 == 1.8 milliVolts decrease.

analogsystemsrf
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"Can you explain why Vbe decreases(by 2.5 mV/°C) with increase in temperature?"

To me, this is not the correct wording and can lead to misunderstandings.

No - Vbe does not decrease. The other way round: When the temperature goes up, the collector current increases. However, if you want to keep the collector current constant you must (externally) decrease the Vbe voltage by 2.5mV/K.

This tempco is attributet to the temperature sensitivity of Icbo only.

This link leads you to a note written by one of the best-known electronic engineers (Robert A. Pease): https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=9&ved=2ahUKEwiyz5qugKfpAhVmShUIHdoDBM0QFjAIegQIAxAB&url=ftp%3A%2F%2Ftelescript.denayer.wenk.be%2Fpub%2FCD-Microcontrollers%2FIIW%2FABA%2FCD_analoog_e%2Fbijlagen%2Ftransistoren%2FVbe_stuff.pdf&usg=AOvVaw2Z5cjXa6fAcZPkYwGkO7F3

LvW
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  • "However, if you want to keep the collector current constant you must (externally) decrease the Vbe voltage". Well, but if the collector current is kept constant, like it happens in a PTAT thermometer, Vbe will decrease without external intervention. On page 3, Pease shows how to control Vbe by means of Ic: "Another useful way to look at it, is that any time you change the current by a factor of 10 at room temp (about +27 degrees C), the VBE will shift by 60 mV, up or down, as appropriate." Just saying :-) – Sredni Vashtar May 09 '20 at 16:44
  • @Sredni Vashtar...In all his considerations Pease presupposes that Ic is determined by the voltage Vbe. It is therefore clear - when he speaks of a Vbe change - that he means the "necessary" change. Anything else would be a misinterpretation of the exponential relation Ic=f(Vbe). How should a change in Ic automatically (internally in the transistor) affect the voltage Vbe? And this is, of course, also not the case with the PTAT sensor - after all, the base receives the necessary voltage externally via the connection to the collector (voltage divider principle). – LvW May 10 '20 at 09:04
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    Ic is imposed by a constant current source. You can imagine a solar cell under a constant flux of photons.You change the flux of photons, you change Ic, and Vbe changes accordingly. You do not even need to know what is the value of Vbe, in fact you have to measure it in order to tell the temp in a PTAT thermometer (well, you read two Vbes). You need to jump through hoops to justify Ic=f(Vbe) as a causal relationship. The relationship is there, but causality is not implied. – Sredni Vashtar May 10 '20 at 18:07
  • Your "example" (flux of photons) results from a mistake in thinking (because photons cause an imbalance of charged carriers within the base region); and the sentence "...VBE changes accordingly" is a very vague statement. Does it change automatically ? Are the terminals floating? No - of course not. Are you really satisfied with these "examples"? I really do not know what you are trying to proove? – LvW May 11 '20 at 07:30
  • Sredni Vashtar...I am afraid, your misunderstandings result from the fact that you still believe that the BJTs collector current would not be determined and controlled by the base-emitter voltage. I remember a corresponding discussion between us some month (or years ?) ago. – LvW May 11 '20 at 07:53
  • You are still building a strawman. I have zero problem in accepting voltage control, but unlike you I do not exclude current control because "voltage causes current". If you believe that, you have to jump through hoops to justify the behavior of certain circuits, like this one. I confess I was expecting you to correct my statement that the collector current is constant, because it isn't: Ie is. I wonder if you did not point it out because otherwise you would have had to consider the role of Ib. But let's consider a simple diode connected in series with a solar cell under constant photon flux – Sredni Vashtar May 13 '20 at 20:03
  • the current is imposed by how many photons hit the cell's junction. You vary the photon flux, you vary Id. And being Id linked to Vd (because they are two sides of the same coin), Vd will vary accordingly. Changing the current in the diode resulted in a change in Vd. If Id is held constant by illuminating the cell with a constant photon flux, when the temperature of the diode junction changes, Vd will change accordingly - without any 'external' intervention. Requiring 'external' action on Vd in order to justify an alleged 'cause-effect' relationship between Vd and Id is jumping through hoops. – Sredni Vashtar May 13 '20 at 20:09
  • I am very sorry , but I must admit that I do not intend to discuss the results of a photon flux upon a diode - in resonse to a question regarding the tempco of a bipolar transistor. I am afraid this would not help too much....I really do not understand what you are trying to tell me. Do you really deny that - in order to keep the Ic constant - we must reduce the B-E voltage by app. 2mV/K ? This information can be found in many, many good textbooks. Please, tell me what - in your view - is wrong with this statement? – LvW May 13 '20 at 20:34
  • Sredni Vashtar - when you try to analyze and understand the stabilization effect of the emitter resistor RE, you will see that RE does exactly what the theory requires: When a rising temperature would cause an increase of the coll. current, the increasing voltage across RE will REDUCE the base-emitter voltage in order to keep Ic nearly constant. What do need else to be convinced? – LvW May 13 '20 at 20:46
  • The solar cell is a separate component from the diode. It represent a constant current source. You keep bringing up the Re degeneration to justify 'voltage control is the only way', but Re degeneration is a kind of feedback based on voltage comparison, so it's no wonder it is better explained by voltage control. Try to explain how Vd changes in the solar-cell + diode series circuit. You have to jump through hoops to do that. (You still can, of course, since V and I are the two sides of the same coin). Just wanted to point out that. – Sredni Vashtar May 13 '20 at 20:58
  • As mentioned earlier - I would like to stay on topic (temperature properties of the BJT) and NOT explain solar cell functions. I recommend again to stop here. – LvW May 14 '20 at 06:38