Vbe (bipolar transistors)

[sina25]

vbe equation

Is necesary a mininum voltage (Vbe) so that this equation be valid?

Ic=Is*e^(Vbe/Vt)

Is necesary Vbe=0.7 Volt (300ºK) so that the equation be valid?
or this equation is valid simply with a Vbe>0?

and why?

thanks

 

[leomecma]

vbe bipolar

I think is true for Vbe > 0, because the rule Vbe > 0.7V is just an aproximation for fast design .... in Art of Eletronics the autor just says that equation is true if three rules are obeyed:

1) The collector must be more positive then emitter
2) The base emitter and base collector behave like diodes
3) And not exceed Ic, Ib, Vce, Ic*Vce and temperature

they works with this equation.

You can find it on pages 62 and 80.

the correct equation is: Ic = Is*[exp(Vbe/Vt)-1]



leomecma

 

[sunsina]

vbe in bipolar

The best refernce is to Look at the Specific BJT Transistor DATASHEET.
In Most of them there is information about the vbe value and there dependst on
or Ge type (vbe≈0.3) silicon type(vbe≈0.7).

Vbe affects from tempreture .

 

[Ahmed Ragab]

vbe 0.7

Is necesary a mininum voltage (Vbe) so that this equation be valid?

Ic=Is*e^(Vbe/Vt)

Is necesary Vbe=0.7 Volt (300ºK) so that the equation be valid?
or this equation is valid simply with a Vbe>0?

and why?

thanks

Well this equation is valid for all Vbe, the bas emitter junction is treated as a forward biased junction and the current passing through it is Ic

Check "Solid state electronic devices" for Streetman if you need any more details on the relations governing the currents and voltages across the transistor.

Salam.

 

[platonas]

vbe volt

Vbe is as described by "sunsina". It is necessary since below this voltage the transistor does not conduct. So you need to include the Vbe.

 

[yjkwon57]

Hi~~

A threshold voltage of a diode or a base-emmitter junction diode is just a parameter in a model. The behavior is just as expressed by the equation you have written.

 

[jc2]

What is a diode or base-emitter junction threshold voltage?

thanks

 

[A.Anand Srinivasan]

this eqn is valid for Vbe>0

 

[Ahmed Ragab]

What is a diode or base-emitter junction threshold voltage?

thanks

The base-emitter junction threshold voltage is the minimum voltage needed to to be placed at that junction in order to switch it ON and start its conduction of current.

It's about 0.7V for Silicon made devices.

 

[VVV]

The equation is valid for Vbe >0.
However, due to the exponential factor, the Vbe voltage will end up being about 0.7V at room temperature (300K), for any collector currents within practical limits. That is why we consider it 0.7V in most calculations. But if the temperature and/ or current variations are large, then you will have to adjust it accordingly.