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T.C. of transistor and diode

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Manchested

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Hi,

I have a few questions regarding the temperature coefficient of transistors and diodes. Hope anyone could help me out.

1. Do VBe of a Bjt and a regular diode have the same T.C?

2. Do they mean the same? The resistances inside the components changes as the temperature increases.

3. Say If VBE of a Bjt has t.c of -2mV/C, and a diode has the same t.c.
If the diode is being used as a reference in an amplifier circuit, as the temperature increases, will the Ic remain approx. constant? I think that Voltage drop across the diode decreases, as temp increases, but since VBe will be more sensitive, as temp increases, so Ic will have no temperature dependence. am I correct?

4. If a diode has a positive T.C, does it mean the voltage drop across it increases as temp increases?
 
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Miguel Gaspar

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For 1 the construction of Diodoe and transistors are diferent. The Volume of P and N in a diode are the same, but in the transistor they aren't the same. The quantity of dope materials in P and N in a diode are the same but the N in emitter are very much than in the base P. So the T.C. are not the same.

For 2 Yes they mean the same.

3. If you use a BJT as a diode you ensure the same T.C. and your cosiderationas are good.

4. Yes it increases.
 

Manchested

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the T.C. are not the same.
Could you please tell me the T.C for 1N4148? I tried googling it but couldn't find any datasheet that has the diode's T.C.

3. If you use a BJT as a diode you ensure the same T.C.
Does the Bjt make a good voltage reference? When using bjt as a diode, do I just use collector as positive, emitter as negative, and connect base to collector?

Is there any other option to ensure the same T.C and make a good voltage reference? How about using 2 diodes in series?
 

Miguel Gaspar

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For the TC of a diode try using a electronic simulator to determine the TC in especial for the diode 1N4148.
For a voltage reference the best is using a zener diode temperature compenzate. Refers to Semiconductor and Integrated Circuit Devices

Zener Diodes where you can find zeners for this special purpouse
 

FvM

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The -1.9 mV/K T.C. of forward voltage is common to all silicone semiconductors, thus 1N4148 has basically an identical characteristic. Besides this effect, you'll also find temperature dependant resistance effects, that show at higher currents.

Due to the systematic forward voltage T.C., silicone diode or tranistors aren't suited as temperature independant voltage reference. In a bandgap refrence circuit, the systematic T.C. is cancelled to generate a constant reference voltage. See: Bandgap voltage reference - Wikipedia, the free encyclopedia
 

BradtheRad

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If you're looking for an adjustable precision voltage reference which is temperature compensated over a wide range,
you might want to look at the LM431 Adjustable Precision Zener Shunt Regulator.

LM431 - Adjustable Precision Zener Shunt Regulator

From the webpage:

Features

Average temperature coefficient 50 ppm/°C
Temperature compensated for operation over the full temperature range
Programmable output voltage
Fast turn-on response
Low output noise
Description

The LM431 is a 3-terminal adjustable shunt regulator with guaranteed temperature stability over the entire temperature range of operation. The output voltage may be set at any level greater than 2.5V (VREF) up to 36V merely by selecting two external resistors that act as a voltage divided network. Due to the sharp turn-on characteristics this device is an excellent replacement for many zener diode applications.
 

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