Basic Concepts about BJT,FET and MOSFET

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muhammad11

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Can any one tell the basic difference between BJT, FET and MOSFET in term of their applications like amplification, switching etc.
 

A BJT has a fairly low input resistance that needs a bias current. A Jfet and a Mosfet have very high input resistance that need a bias voltage.
BJTs are small and large and can operate at low or high currents. Most Jfets are small and operate at low currents. Many Mosfets are large and can operate at very high currents.
They can all amplify and switch. Some are good for high frequencies and can switch very fast.
 
Basically , all are constructed for the main purpose "AMPLIFICATION",

but

mainly there are some parameters differentiating them.

1.Gain: For BJT you will get a good gain, but the problem is,it consumes the current from the source causes you are obtaining gain by loosing some thing (power).
But in MOSFET the same gain you will get by zero input current.

2.Physical parameters: Your BJT area is not a matter for amplification (of course you won't consider it), but in MOS area of MOS is the main parameter .

3.MOS is better immune to noise than BJT. BJT mostly suffered by thernal noise.
 

Audioguru said:
A BJT has a fairly low input resistance that needs a bias current.
Click to expand...

Because the questioner (Muhammad11) seems to be a newcomer: I think, it would be beneficial to learn from the beginning the correct explanation:
The BJT requires a DC bias VOLTAGE (of app. VBE=0.65...0.7 volts) for opening the B-E pn junction - but a corresponding bias current IB cannot be avoided.
 

Because of this thread and talking of amplifiers, in the high frequency model of a MOSFET simplified, which looks like this:


How do I find the r0 corresponding to Early effect in a datasheet? for example in this datasheet how do I find that r0 ?
http://www.irf.com/product-info/datasheets/data/irf510.pdf

Or is the same as Rds(on) ?
 
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phanikiran said:
But in MOSFET the same gain you will get by zero input current..
Click to expand...

Both switch and amplifier depends on changing the gate voltage (or base current). Changing gate voltage needs some current because the gate is associated with some capacitance. Power mosfets have significant capacitance and may need considerable current to turn on and off. For example, in the CPU you are running (say intel Desktop CPU) has about a billion transistors and the switching them on and off is the single major cause of production of heat in the CPU (many watts) that need a hefty heatsink.

It is simply not possible to have amplification (gain) without using power.
 

Yes, you are right , but here input current is gate current(from input voltage source) , ofcourse it needs some Id (drain current or supply current) to give the required gain.
 

The gate-source of a Jfet is a reverse-biased diode that draws no Dc current.
The gate-source of a Mosfet does not conduct dc current because it is like a capacitor.
The base-emitter of a bipolar transistor is a forward-biased diode that draws DC current.
The grid-cathode of a vacuum tube .... forget it.
 

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