mosfet enquiries

A BJT is a current sensitive current controlling device personified by the parameter β, also known as hfe. A MOSFET is a voltage sensitive that has an extremely high input impedance and a tiny capacitor. Because of this, one cannot test a MOSFET outside of a complete circuit configuration like one can by measuring the voltage drop from base to emitter of a BJT. This is because while holding the MOSFET in your hands, the small capacitor across the gate to source will hold an unknown stored charge that establishes the characteristics between drain and source.

The best method to test the parameters of a MOSFET is with a transistor curve tracing instrument. Short of this you can, and many people do, build your own curve tracing instrument. A good web page on the operation of a MOSFET is the Wikipedia page. It is very thorough and thus possibly confusing since a MOSFET can be a baffling part.

Good Luck

are you trying to use it as a switch? if so you need to apply a voltage to the gate higher than Vgt >4V. If you want the swich to turn of fast you need to charge the gate cap with alot of current. if you dont need fast swithcing speeds then you can use little gate driver.

what are you trying to use the fet for? what is your max current and voltage you need?

i would like to use the mosfet to amplify the power of a signal, which is approximately 200kHz. The signal is a squarewave with 12V , i hope to get a 12V and 0.8A output.Is that possible

Yes, a 12V square wave can easily trigger the transistors you've listed with 0.8A drive capability. The critical values you should be concerned with are V_gs, I_d, V_ds, R_ds and the input capacitance C_iss. The output impedance of your 12V square wave source with the input capacitance will determine how quickly you can switch this. Now remember to consider these parts as just voltage controlled switches with an R_ds resistance and your design work will be greatly simplified. Since you're driving this with a square wave, this will also be an appropriate simplification. Just to cover all bases, you will need to provide a power supply.

Mr. Redfred is right. the only other thing i would tell you is to watch out for the load. if you are trying to switch a coil or a relay, you want to use a diode or a snubber so you dont stress the fet. what are you trying to switch? if you give your circuit setup or a guess at what you are thinking, we can tell you if it will work ( like your input drive chip, fet, and load).

http://www.electro-tech-online.com/electronic-

projects-design-ideas-reviews/103771-push-pull-amplifier.html#post846306

actually i would like to use mosfet to replace the transistor in the circuit since i saw some where that mosfet has higher switching frequency and has lower on resistance.can u pls hav a look at the circuit n giv some comment.what do u mean by using a diode so that i would not stress the fet?

Our guest very accurately explained the condition that requires some snubber network requirements. If the load you are driving is an inductive load you can have a condition known as fly-back. An easy way to think about is is that an inductor is a current storage device, just like a capacitor is a voltage storage device. So when one tries to abruptly current from flowing through an inductor, the inductor will manifest whatever voltage that is required to maintain that current the instant after trying to interrupt the circuit. This can be voltages high enough to cause arcing, in the case of motor winding commutation or in the case of a transistor driving a relay, enough voltage to exceed the breakdown voltage of the transistor.

I took a look at the push pull circuit you posted. one of the biggest differences of using a fet inplace of the BJT is the gate voltage is much higher than that of a BJT. this will make your input circuit look different. i guess i probably wouldnt use a circuit like the one the push pull shows, but maybe for a sin wave amplifier.

do you need to source and sink current? it would be very helpful to know your load or maybe what your goal is. I keep thinking you want to make a switch but maybe you want a class ab amp or something else i dont even know about. your power levels seem pretty low so it should make the circuit a little easier. maybe you are just interested in learning about fets and how they work and what applications you can use them in. Please tell a bit more about what you are trying to use this circuit for.

actually i just want to study about fets and how to work them correctly

Then I would recommend first doing a single MOSFET circuit instead of a push pull design. The obvious options will be to fabricate just the top or bottom half of the circuit you propose. By doing this you will be able to attribute all non-linear effects you find to just the one MOSFET you are using. As I mentioned earlier, while a very useful component a MOSFET can be a very confusing component. This is particularly true when trying to use a MOSFET in a linear fashion. A very good application note is MOSFET basics written by Fairchild semiconductors. In the opening paragraphs you'll find that power MOSFETS were designed to permit faster switching of power than what BJT can easily handle. Another confusing thing with a MOSFET is depending on the fabrication making it an enhanced or depleted component, a MOSFET can be ON or OFF without a bias voltage. This is why you must get reference material in the way of an application note or a text book to get a good grasp of these very useful components.