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8th December 2010, 01:50 PM  #1 
diyAudio Member
Join Date: Dec 2010

non inverting op amp input impedance help
i have been given the task of designing an non inverting op amp, the criteria is
gain of 2 and input impedance of 9 if anyone can help me out in how it should look and what is the best way to design it. the op amp i designed i was told by a freind it was wrong so i am here to see if anyone can help p.s. i am new to this forum so apologise if i have posted in the wrong place. 
8th December 2010, 02:05 PM  #2 
diyAudio Moderator

Is this a class homework assignment? If not, what is this amp supposed to be doing? What's the function in the device in which it's used? 9 what?
I'm moving the question over to a more appropriate place.
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8th December 2010, 02:15 PM  #3 
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

Hi,
look up Wiki or other site and find out what inverting and non inverting schematics look like. Find out what unity gain stable means. Find out how to set the gain of an opamp circuit. Find out what determines the input impedance of an opamp gain block. Download the datasheet of a few "old" opamps. ne5534, tl071, lm353 etc. Look at the schematics for what you need and then alter to meet your assignment specification.
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8th December 2010, 04:58 PM  #4 
diyAudio Member
Join Date: Nov 2010
Location: Milan

Sure is open loop?
First of all, In my way of understanding, NonInv depends on the topology of connection, not on the device. The same about the gain and the IN impedance, if we are speaking about CLOSEDLOOP (i.e. circuit, amplifier, NOT device). So design or use ANY High Open loop gain amplifier with differntial imput (i.e. o.l. gain =1000).
With such premises, you need to put a feedback resistor EQUAL to input resistor, so your gain will be Vg=1+ Rf/Ri=2. I hope it helps effebi 
8th December 2010, 07:00 PM  #5 
diyAudio Member
Join Date: May 2007

It might save time if your tutor posted the actual question here, then told all his students to simply copy our answer. Not good for education, but great for gaining marks. We could split the teaching fee among ourselves, or donate it to the website.

8th December 2010, 07:11 PM  #6 
diyAudio Member
Join Date: Nov 2010
Location: N.E. PA

In a noninverting configuration, the input impedance is a function of the input bias current. In even cheap opamps, this impedance is typically >1000 megohms. Very, very high. In most configurations it is desirable to have an established impedance, for signal matching, or to reduce stray current noise or other effects (in other circuits it is desirable to have very high impedances to not load certain sources). A simple resistor, or the desired impedance value, can be simply connected from the noninverting input in order to establish the desired value.
paul 
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