Opamp?? FET vs Bipolar

[evo9]

Hey guy's please shine some light on this for me. I have red somewhere that you should not swap out FET opamps with bipolar's. But I have never been able to find, or read the reasons why this is not a good practice. I have seen on other forums, where someone modded a sound card going form FET to bipolar. They said the modd has improved the sound card.









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[jan.didden]

Originally posted by evo9
Hey guy's please shine some light on this for me. I have red somewhere that you should not swap out FET opamps with bipolar's. But I have never been able to find, or read the reasons why this is not a good practice. I have seen on other forums, where someone modded a sound card going form FET to bipolar. They said the modd has improved the sound card.

There is at least one technical reason: most bipolar opamps have orders of magnitude more input bias current than FET input opamps. Depending on the circuit details, the increased bias current can cause appreciable DC offset at the output. Again, depending on the circuit, this may or may not upset or even damage your system.

It should be clear that indiscrimiate changing of device types without having a good look to the consequences is a bit like playing Russian Roulette.

Jan Didden

[AndrewT]

Originally posted by janneman
It should be clear that indiscrimiate changing of device types without having a good look to the consequences is a bit like playing Russian Roulette.

agreed, informed decisions are usually better than guesswork.
eg. filters generally assume zero source impedance and infinite load impedance.
FET input stage gets closer to this than BJT for the load.
BJT output stage might be better than FET output stage for the source side. But are any opamps built with FET output stage?

[peranders]

Andrew, no to your last question if you'll exclude CMOS opamps.

[peranders]

You have a drawback with JFET's, higher distortion under certain conditions.

Walt Jung has written some about it.
http://www.analog.com/library/analog...inal_PtB_F.pdf
Page 5.108

What Walt Jung writes here is unknown to most people I should think. I hadn't heard it before I read it in Walts book.

Compare this when folks are tweaking and it got soooo much better. Sometimes maybe the distortion higher with a better opamp?

[evo9]

Thanks for answering guy's! I was about to swap out some opa2134ua with ad8599arz. Then I remember they were FET & Bipolar. I got some ad8066arz, I will use instead. I am trying to improve the sound of my pioneer car stereo.

[AndrewT]

if you equate zero distortion with sounding better than some distortion/colouration then putting the Pioneer in the bin is the best improvement you can use.

[nelsonvandal]

Originally posted by evo9
I was about to swap out some opa2134ua with ad8599arz. Then I remember they were FET & Bipolar. I got some ad8066arz, I will use instead. I am trying to improve the sound of my pioneer car stereo.

AD8599 is a very good opamp. Much better than OPA2134 to my ears. I don't think you're getting in to any trouble just swapping them.

[evo9]

Originally posted by AndrewT
if you equate zero distortion with sounding better than some distortion/colouration then putting the Pioneer in the bin is the best improvement you can use.

You may have a point there! But its already paid for, may as well do what I can with it. There is always the Alpine F1 if I want to.