Why BJT and FET are called current/voltage controlled voltage/current devices?

hi friends plz help me iam confusing why bjt is called current controlled voltage device and fet is called voltage controlled current device
i know in bjt by varying base current we r measuring collecter current so it is current controlled similarly in fet but my query is why it is voltage device and fet is current device

difference between fet and bjt

in fet the gate voltage controls the drain current

bjt internal resistance

in fet gate voltage conrols the drain current so it is voltage control
see the equation of relationship between Id and Vgs

and in bjt u see the equation Ic=(beta)*Ib so it is current controlled

differences between bjt and fet

When working in active region (in the BJT case) and the saturation region (in the FET case) the ouput current of each device is controlled by a variation of an input parameter. In the BJT the collector current would be Ic=BETA*Ib, where Ib is the base current (which would be the input control parameter). In FETs the Drain current is Id=kn*(vgs-Vth)^2 where kn and Vth are constant values, so the output current is control by Vgs which is a voltage.

Obviously all these equations are very aproximate relations between one parameter and another, but they serve as an example for the current/voltage controlled devices definition.

diemilio

difference between bjt and fet?

Although EE's will shoot me for this...

An easy way to look at it is this.

In an FET, the gate is insulated from both the source and the drain by a very large resistance. Hence, there is no current flow into the gate. Therefore, it must be a voltage-controlled device. Specifically, the channel resistance is a function of the gate to source voltage. Incidentally, that high resistance looking into the gate makes for a nasty capacitor to charge, hence the need for FET drivers on occasion. However, you also get essentially an open circuit for a load for the source driving the FET's gate.

In a BJT, the base is connected to the emitter via an internal resistance. The resistance is in the low ohms. Therefore, there is a means for current flow. Hence, the BJT is a current-controlled device. Specifically, the current through the device (collector to emitter -- assuming an NPN) is a function of the current into the base. And, since you have current flow through the device and some Vce, like the FET's drain to source, you have a resistance, collector to emitter. Since there is a low value resistor looking into the base of an on transistor, you end up with a rather low input impedance, an impedance that can be increased via increasing the emitter resistor.

I know it's a very simplistic view, but it works for me.

One other thing I think is neato about FET's is, since it is a true resistance between the drain and the source and not just a current through a voltage drop, you can literally measure an FET's resistance via an ohmmeter connected d-s or s-d. Just apply a voltage across the g-s leads and then measure the resistance between d and s. Hence, analog switches are more doable with an FET.

difference of fet and bjt

friends you are giving answers for why bjt is called current controlled device but my problem is (bjt is current controlled voltage device)wht it is called only voltage device,
simlarly for fet is is current device..
thanks for ur replies.........

difference between jfet and bjt

toram16 said:

friends you are giving answers for why bjt is called current controlled device but my problem is (bjt is current controlled voltage device)wht it is called only voltage device,
simlarly for fet is is current device..
thanks for ur replies.........

Click to expand...

Sorry, but I'm not following your question

the difference between the bjt and jfet

toram16 said:

friends you are giving answers for why bjt is called current controlled device but my problem is (bjt is current controlled voltage device)wht it is called only voltage device,
simlarly for fet is is current device..
thanks for ur replies.........

Click to expand...

I dont understand this either.

differences between fet and bjt

toram16 said:

friends you are giving answers for why bjt is called current controlled device but my problem is (bjt is current controlled voltage device)wht it is called only voltage device,
simlarly for fet is is current device..
thanks for ur replies.........

Click to expand...

Yep, me too. I do not understand what you're asking.

difference between bjts and fets txt

Toram16,

A major difference between the operation of any MOSFET and the bipolar junction transistor (BJT) is that the FET is voltage-controlled, whereas the BJT is current-controlled. To control current passing between the drain and the source of a FET one uses a control voltage at its gate. With the BJT, a combination of base voltage and base current is involved.

Please consider websites below for some basic comparision:

**broken link removed**

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Regards,

bjt current device fet voltage device

thanks

bjt collector connected to gate jfet



Picture will do the work...

differences fet and bjt

BJT is current controlled becuase u r varying base current.
But in mosfet u have vary gate voltage inorder to make the drain current to flow in themosfet.

the difference between bjt and jfet

FET works on reverse voltage while BJT works on forwerd voltage..... drain, source, gate are terminals of FET while its emmitor base and collector in BJT.......