Bias Resistor Transistor

[tshankar501]

Why is "Bias Resistor Transistor" is used? Please google the word and you can see lot of "Bias resistor transistors" are being sold by different companies?

Please tell what is the advantage of this "Bias resistor transistor" holds when compared to the ordinary BJT? In what way is the resistor between base and emitter helps? I find that in all the application notes listed by different companies, replacing the "Bias Resistor Transistor" with this ordianry BJT works fine...

Also in all the datasheets, it is explicitly said that these "Bias resistor Transistors" are used for digital signals. For digital operation, we dont need to bias the input at all right?

 

[rowokii]

When you use a transistor as a switch to turn loads on and off, you typically put resistor on the base to limit current and you pull down the base to ground using another resistor to get better noise immunity. It looks like these "bias resistor transistor" integrated these resistor in the same package as the transistor to save cost and board space.

 

[A.Anand Srinivasan]

bias resistor transistor is to prevent the false triggering of the transistor due to leakage.... suppose the transistor is connected to the emitter of say a power MOSFET and there is a possibility that this transistor could turn on due to the power MOSFETs leakage... that bias resistor is selected such that the leakage current wont be enough to make the voltage across the bias resistor to be around 0.7V... so the transistor cannot turn on due to lack of enough current...

it is used often in digital circuits because digital circuits are more prone to kickback noise and glitch due to switching and hence we need protection from false triggering of transistor.... also we are dealing only with 2 levels and hence the presence of the bias resistor is not going to affect the operation of the transistor much....

 

[tshankar501]

thanks guys.....for your answers......

and from the parasitics point of view, do you think whether it helps in eliminating parasitics. If so, how?

 

[barath_87]

I think bias resistor only helps in fixing the dc operating point and has no significance on the parasitics of the device...

 

[A.Anand Srinivasan]

ya it helps in lowering the effect of the base emitter capacitance effect on the output since it is in parallel to that capacitance and hence reduces current through that parasitic capacitance....

 

[spikeyang]

Bias resistor transistor is usually for enable-signal or inverter purpose. YOu can use it in open-collector occasion, which source more current than normal logic-gate.
It saves room for separate resistors.