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Design >> Analog Design >> series and parallel resistors with BJT in hardware design
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Message started by kataria0 on Oct 2^nd, 2009, 7:43am

Title: series and parallel resistors with BJT in hardware design
Post by kataria0 on Oct 2^nd, 2009, 7:43am

Going from high speed analog IC design to low speed hardware design (almost digital) I am noticing that at many spots there is a huge resistance (eg like 10K) in series with BJT's base and even bigger resistance (like 50K) in parallel to Base-Emitter junction. What purpose do these resistors serve? Thanks in advance.

Title: Re: series and parallel resistors with BJT in hardware design
Post by loose-electron on Oct 2^nd, 2009, 9:40am

The resistance in the base serves to limit base current into the transistor, so that you do not burn out the Vbe junction. Without the resistor, you are essentially forcing a voltage onto a diode. Putting 3V or 5V logic swing into a diode makes for lots of current and "poof" fried transistor.

A resistance in parallel with the collector to emitter nodes of the transistor - Depends on the application. Some things may want an output that has a maximum and minimum resistance, depending on state control.

A resistor between the base and the emitter of large value? Thats a little unusual. It would allow current division between the Vbe junction and the resistor, and in the right ratio with the resistor in series with the base, reduce the voltage swing into the base. That would help keep the transistor out of saturation (bipolar version, not MOS, they are different) and allow the bipolar transistor to change state a bit quicker (no longer needs to recover from saturation.)

Title: Re: series and parallel resistors with BJT in hardware design
Post by kataria0 on Oct 2^nd, 2009, 11:16am

Thank you very much for the clear explanation.

Title: Re: series and parallel resistors with BJT in hardware design
Post by loose-electron on Oct 2^nd, 2009, 11:35am

not a problem, happy to help - jerry