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?

 

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.

__________________
Feels good, man.

 

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?

 

Both have advantages, and both have disadvantages - but bipolar are FAR more common than FET's.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

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.

For discrete transistors the situation may be different.

 

Certainly is, and is what we are talking about

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

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!

 

Mosfets can switch at a much higher frequency.

 

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.

__________________
Tanaka Sensei (avatar) says: Please spell it "ridiculous" correctly! Not "rediculous". ^^

 

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.

But you don't need massive resistors, or waste huge amounts of power, you only need to drop a couple of hundred mV.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

thanks for the answers

who is going to show me how to bias a buz11 mosfet as switch
for a small toy motor from a 9V power source?

 

mosfet is temp variation independent.

 

Is it true to say that Mosfets is much less forgiving and less idiot proof?
That they ideally need a mosfet driver?
and they are more prone to static dammages?

These are questions, not affirmations of course!

Alain

 

No, I would not say MOSFETS are less forgiving. In fact they have some characteristics that make them a bit harder to work with, like self latching and their sensitivity to ESD.

The majority of MOSFETS need a 10V source on the gate-source to turn on fully. There are some logic level MOSFETS that will turn on fully at ~4.5V, some even less, but these are for smaller applications. Larger applications like motor controls, power inverters, etc. you will need a driver circuit of some sort to get them to switch fast enough.

They can handle VERY high currents, although it's very hard to find MOSFETS that can handle over 400V or so. At that point its better to go to BJTs (IGBTs for power applications) as they can handle the higher voltages easier.

My knowledge is from the power electronics point of view, fyi.

 

Is any component really temperature variation independent? I don't think so...

MOSFETs have a resistance R(ds)(on), the drain-source resistance when current is passing through the FET. This resistance increases with increasing temperature, usually due to increasing current. This effect is important in DC-DC converters, when high currents are passing through the FET.

I almost always use FETs - probably because that was what we used in school. If you need to use them in low voltage applications, check the threshold voltage (voltage at which the FET turns on). My favorite NFET for 3V applications is the BSS138 - cheap, and with a very low threshold voltage.