16th May 2008 12:36 PM
Mosfet vs BJT
i know both of them from school but there is something i dont understand!!!
please tell me when we have to use a BJT and when a mosfet! is it bad idea to use a buz11 as switch for a small fan?
16th May 2008 12:51 PM
Very time-sensitive switching applications benefit from the BJT's lesser capacitance, but MOSFETs have more thermal stability and can act as precision resistors. That's all I can think of, I'm not too experienced as of yet.
16th May 2008 01:06 PM
i think mosfets require voltage and not current to become open but how much voltage? also a mosfet as switch controls better a device that needs much current or much voltage?
16th May 2008 01:11 PM
Both have advantages, and both have disadvantages - but bipolar are FAR more common than FET's.
16th May 2008 01:53 PM
Depends how you classify this and in what applications. Nowdays because of the simplicity and ubiquity of the fabrication process for MOSFETs and the miniturisation capabilities of the MOSFET (particularly when compared to BJTs), particularly in ICs and MSI, LSI and VLSI systems, MOSFET is the device of choice. Given MOSFETs in VLSI systems might total several hundred million on a single chip I would suggest that MOSFETs are by far more common than BJTs in both numbers and application diversity.
Originally Posted by Nigel Goodwin
For discrete transistors the situation may be different.
16th May 2008 02:23 PM
Certainly is, and is what we are talking about
Originally Posted by The Rat
16th May 2008 04:30 PM
Here's some of when/why I use them:
1) Need a switch to be fully-on fully-off and carry lots of current -MOSFET
2) Need a switch that needs to have lowish capacitance - BIPOLAR
3) Need a cheap, dirty 2 or 3 component current source - BIPOLAR
4) Need a low voltage noise amplifier - BIPOLAR
5) Need an amplifier with VERY low input/bias current - MOSFET
6) Need a low noise AND low input current amplifier - JFET
7) Need a one component current source - JFET
8) Need switch or amp that must cost almost nothing - BIPOLAR
9) Need multiple transistor package that has matching - BIPOLOAR
10) Need switch that may be over-voltaged - MOSFET
11) need switch/amp that sits in nasty RF environment - MOSFET (BIPOLARS rectify & cause offsets)
There are alsways exceptions.. but there ya go!
16th May 2008 06:25 PM
Mosfets can switch at a much higher frequency.
16th May 2008 06:29 PM
As a switch...
MOSFETs can generally switch faster (they certainly require less complex and less pwoer to drive their gates). But if I'm not mistaken, BJTs designed for the task can switch very very fast since they have no gate capacitance to charge and can also operate in quasi-saturation mode for even faster switching at the expense of conduction efficiency. MOSFETs have less losses when used as a switch at "lower" voltages (lower as in industry's definition which is <~200V).
MOSFETs act like a resistor when on while BJTs act more like diodes. The resistance can be modified by changing the "dimensions" of the MOSFET while the BJT's "diode voltage drop" can't be changed so easily unless the materials are changed. THis tends to make MOSFETs have less losses at the lower voltages but also means MOSFETs can be paralleled since current imbalances will cancel out.
With parallel BJTs, the best BJT will hog the current from the other "not so good BJTs" and burn out and the cycle repeats with the remaining BJTs until they are all burned. This is similar to parallel diodes. You can correct for imbalances by manually tuning resistors in series with each BJT, but for power applications that's needing massive resistors and wasting lots of power.
Last edited by dknguyen; 16th May 2008 at 06:35 PM.
16th May 2008 08:04 PM
You don't need to 'manually tune' the resistors, you just use identical values in order to make them share the current equally. This is also essential on some types of audio output FET's as well - essentially it's because of the different temperature coefficients of the types of devices.
Originally Posted by dknguyen
But you don't need massive resistors, or waste huge amounts of power, you only need to drop a couple of hundred mV.
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