[SOLVED] fet amplifiers instead of transistor

[jeffrey samuel]

can the basic fet amplifier be more stable than their transistor counterparts

or they also lack stability

 

[DeboraHarry]

FET stands for Field Effect Transistor. So since a FET is a transistor, how can a FET be more or less stable than a transistor?

Both FETs and bipolar transistors can be used as oscillators, so both can be made to oscillate.

 

[FvM]

Presumed you are talking about FET (MOSFET or JFET?) versus BJT, the question is meaningless without specifying a frequency range and application. Also the term stability can mean a lot of different things. Using ambiguous terms is a perfect way to get people talking at cross purposes.

 

[jeffrey samuel]

Presumed you are talking about FET (MOSFET or JFET?) versus BJT, the question is meaningless without specifying a frequency range and application. Also the term stability can mean a lot of different things. Using ambiguous terms is a perfect way to get people talking at cross purposes.

sorry sir

i am designing a audio frequency amplifier the fact is that i used ce configuration in transistor to design my circuit

but when it comes to practical application there is large change in the practical and theoretical values

and so i thought i have to scrap my model and design a new one using jfet but i have never used jfet amplifiers so i had a question

whether this would increase my efficiency

 

[FvM]

I'm concluding, that your post is effectively a follow up to your previous thread:
https://www.edaboard.com/threads/260717/

There's no reason to assume that FET amplifiers have a more stable operation point. In contrast, bipolar transistor are showing less parameter variation, mainly a certain current gain range which will be tolerated by suitably designed amplifier circuits.

FETs have both IDSS and UTH variation in a larger range, and it's difficult to compensate it with manual adjustment.

but when it comes to practical application there is large change in the practical and theoretical values

A problem of bad considered circuits, I assume.

 

[godfreyl]

but when it comes to practical application there is large change in the practical and theoretical values

That sounds like a bad design. I have seen some classroom exercises where you are told to assume current gain = 100. That is a very bad idea.

A typical small transistor such as BC547B may have current gain anywhere between about 200 and 500. You have to design your circuit so that this does not matter.

Similarly, VBE is not exactly 0.7V. It is a good idea to design your circuit so it doesn't make much difference whether VBE is 0.6V or 0.8V.

...and so i thought i have to scrap my model and design a new one using jfet....

They're worse. The parameters are more variable than with BJTs.

 

[jeffrey samuel]

That sounds like a bad design. I have seen some classroom exercises where you are told to assume current gain = 100. That is a very bad idea.

A typical small transistor such as BC547B may have current gain anywhere between about 200 and 500. You have to design your circuit so that this does not matter.

Similarly, VBE is not exactly 0.7V. It is a good idea to design your circuit so it doesn't make much difference whether VBE is 0.6V or 0.8V.


They're worse. The parameters are more variable than with BJTs.

your statement was my problem i assumed the emitter resistor value arbitarily

never expected it to create a lot of trouble
thanks for pointing the disadvantage of using jfet

 

[KRIS255]

If you are playing around with small FETS (such as the 2N7000) you had better take proper precautions to prevent static electricity from damaging the devices. I have lost a lot of FET's due to ESD and found that blowing ionized air across the bench is the most effective way to protect your devices.

 

[FvM]

If you are playing around with small FETS (such as the 2N7000) you had better take proper precautions to prevent static electricity from damaging the devices. I have lost a lot of FET's due to ESD and found that blowing ionized air across the bench is the most effective way to protect your devices.

Particularly a problem with unprotected MOSFETs (like 2N7000). JFETs have a more regular level of ESD succeptibility. Some protection means are suggested in any case.