One of the considerations of a FET op-amp is the stability. The offset drift due the temperature is much greater in FETs than in bipolar devices. So if stability is an issue, then the choice of fet-input needs to be weighted in the trade-offs. Application usually dictates all. ----- Original Message ----- From: "Mike Hord" To: Sent: Friday, June 11, 2004 10:13 AM Subject: Re: [EE] OPAMP use > It always makes me vaguely uneasy when people start asking questions > about the use of op-amps...I worry that I don't know as much as I > thought I did...So as a check on myself, let me propose this as a > solution to the gain < 1 non-inverting op-amp: > > Unity gain voltage follower into a high impedence voltage divider, with > another voltage follower lowering the output impedence of said > divider. If the output impedence of the signal source is low enough, > the first voltage follower can be omitted. > > Choose resistor values based on input bias current of op-amp; with a > good FET input op-amp and resistors in the 10's of kOhms, the input > current can be ignored (hundreds of uA of signal current vs. 10's of > pA of input bias current). > > Now...does anyone have anything to say about my solution? > Preferably not too abusive, but enlightening me as to any... > shortcomings in my understanding of op-amps? > > Mike H. > > >Regarding "straining the op amp" in the non-inverting mode, I believe the > >writer was refering to the fact that in non-inverting mode, there is a > >large common mode voltage on the input in non-inverting mode, while the > >common mode voltage in inverting mode is zero. > > > >For some notes on using what I call "the theory of the happy op amp" > >(differential input voltage ideally zero), see > >http://kauko.hallikainen.org/cuesta/et113/InstrumentationAmpAnalysis.pdf . > >This is an analysis of an instrumentation amplifier using this approach. > > > >Harold > > _________________________________________________________________ > Stop worrying about overloading your inbox - get MSN Hotmail Extra Storage! > http://join.msn.click-url.com/go/onm00200362ave/direct/01/ > > -- > http://www.piclist.com hint: The list server can filter out subtopics > (like ads or off topics) for you. See http://www.piclist.com/#topics > -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.