Can we use FET instead of BJT in differential amplifier?

[graciousparul]

differential amplifier

Hi,

I am reading a book on OPAMP, All the circuits in the book expalained using BJT. I am wondering, can we use FET instead of BJT in differential amplifier? if no, Is there any particular reason for it?

Thanks in advance

 

[Eugen_E]

Re: differential amplifier

Yes, you can use FET in a differential amplifier, and this helps minimizing the input current on the inputs. There many opamps with FETs, either full CMOS opamps or only JFET input, like TL082.

 

[a_tek7]

Re: differential amplifier

BJTs are more noisy than FETs
but gain : BJT ≥FET

 

[graciousparul]

Re: differential amplifier

Hi,

thanks.....
but can u say about the effect of resistance as well loading effects which arises?

and
let me know
WHAT ARE REPLACEMENTS DONE IN USING FETs for DIFFERENTIAL AMPLIFIER CONFIGURATION(dual input,balanced output)?

regards,

 

[a_tek7]

Re: differential amplifier

Replacement of configuration is dependet to applications sometimes you need a LOW imepdance,high current,improvment of harmonic distortion,sinking and driving current...in input sometimes you need a range of ρA input bias current so we must FETs but in communication circuit you need that your first stage have a noise figure around 1 because the noise figure of first stage is added directly to noise figure of system.I think this book is good for you (analysis and design of analog integrated circuits GRAY MEYER HURST LEWIS (chapter5))