Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

[SOLVED] fet amplifiers instead of transistor

Status
Not open for further replies.

jeffrey samuel

Advanced Member level 4
Joined
Jul 23, 2012
Messages
1,094
Helped
107
Reputation
214
Reaction score
107
Trophy points
1,363
Location
chennai,india
Activity points
6,376
can the basic fet amplifier be more stable than their transistor counterparts

or they also lack stability
 

DeboraHarry

Full Member level 5
Joined
Dec 3, 2011
Messages
275
Helped
48
Reputation
94
Reaction score
43
Trophy points
1,308
Location
UK
Activity points
3,905
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

Super Moderator
Staff member
Joined
Jan 22, 2008
Messages
48,369
Helped
14,245
Reputation
28,751
Reaction score
12,931
Trophy points
1,393
Location
Bochum, Germany
Activity points
279,978
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

Advanced Member level 4
Joined
Jul 23, 2012
Messages
1,094
Helped
107
Reputation
214
Reaction score
107
Trophy points
1,363
Location
chennai,india
Activity points
6,376
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

Super Moderator
Staff member
Joined
Jan 22, 2008
Messages
48,369
Helped
14,245
Reputation
28,751
Reaction score
12,931
Trophy points
1,393
Location
Bochum, Germany
Activity points
279,978
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

Advanced Member level 5
Joined
Apr 18, 2012
Messages
1,981
Helped
632
Reputation
1,266
Reaction score
624
Trophy points
1,393
Activity points
12,776
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

Advanced Member level 4
Joined
Jul 23, 2012
Messages
1,094
Helped
107
Reputation
214
Reaction score
107
Trophy points
1,363
Location
chennai,india
Activity points
6,376
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

Newbie level 4
Joined
Sep 14, 2010
Messages
5
Helped
1
Reputation
2
Reaction score
1
Trophy points
1,283
Activity points
1,318
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

Super Moderator
Staff member
Joined
Jan 22, 2008
Messages
48,369
Helped
14,245
Reputation
28,751
Reaction score
12,931
Trophy points
1,393
Location
Bochum, Germany
Activity points
279,978
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.
 

Status
Not open for further replies.

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top