Bias Resistor Transistor

[Rahul Gavade]

Hi,
I wonder why a Bias Resistor Transistor (BRT) datasheet never gives you base to emitter saturation voltage.
You find only the input voltage required to turn it on.
If i want to find out the emitter current given the input voltage that powers the base, i need to assume that the Vbe_sat is 0.7V. Why doesn't the datasheet give this value as it would have given in a normal transistor datasheet.:thinker::thinker:

 

[betwixt]

I would guess for two reasons:
1. it can't be used or measured outside the device
2. that kind of device isn't usually used in situations where you would rely on that parameter.

The data sheets usually tell you the ratio of series and shunt resistors so you could make an assumption about input voltage needed to produce 0.7v at the internal base junction if you take into account Ib.

Brian.

 

[Rahul Gavade]

In case, I am required to find out the exact base current going into the base of an NPN transistor which is at saturation, and I know the series resistor, then the voltage across shunt resistor is overshadowed by base to emitter forward biased junction. In this case, I must assume a base to emitter voltage. Or is there another way to find the base current.

 

[Audioguru]

Most silicon transistor datasheets specify a base current of 1/10th the collector current to guarantee a fairly good saturation.
You do not assume a base-emitter voltage because it is different for each transistor (unless they are monolithic on an IC) and it depends on the amount of base current. It is from 0.6V to maybe 0.8V for an ordinary little transistor and it changes when the temperature changes.

 

[crutschow]

Base-emitter voltages don't vary much between transistors of similar current ratings. So just pick one that appears similar and take the base-emitter voltage from its data sheet.

 

[Rahul Gavade]

I would guess for two reasons:
1. it can't be used or measured outside the device
2. that kind of device isn't usually used in situations where you would rely on that parameter.

The data sheets usually tell you the ratio of series and shunt resistors so you could make an assumption about input voltage needed to produce 0.7v at the internal base junction if you take into account Ib.

Brian.

Thanks Brian.. !

 

[Audioguru]

Base-emitter voltages don't vary much between transistors of similar current ratings. So just pick one that appears similar and take the base-emitter voltage from its data sheet.

Pick a number, almost any number.
Each transistor part number has different transistors and even in one part number there are different transistors since that cannot make them the same.
2N3904 datasheet says, "Base-Emitter Saturation Voltage= 0.65V minimum to 0.85V maximum, IC @ 10mA, IB@ 1mA. When IC @ 50mA and IB @ 5mA then Vbe= 0.95V max."