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24th July 2014, 11:33 PM  #1 
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Join Date: Sep 2010
Location: North Queensland.

What normally determines the voltage 'gain' in a simple op amp circuit?
G'day all, another simple op amp question please.
In a simple non inverting op amp stage, what component(s) 'mostly' determines the overall voltage gain? Is it the resistor from the inverting input to earth, or the impedance (resistance) of the feedback network, or both? Regards, Felix. 
24th July 2014, 11:35 PM  #2 
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The gain is 1 + R1/R2 where R1 is the resistor from the output to ve in and R2 is from ve in to earth.
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25th July 2014, 12:07 AM  #3 
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Join Date: Sep 2010
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G'day mate, thanks for that general formula.
A bit of a background to this. I have built a number of simple full feedback phono stages of similar design, but with different (but equivalent) RIAA networks and some of them produce considerably higher overall voltage output with the same value inverting input resistor to earth. Regards, Felix. 
25th July 2014, 12:08 AM  #4 
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The Wikipedia article on opamps seems pretty informative...
Operational amplifier  Wikipedia, the free encyclopedia ~Tom
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25th July 2014, 12:32 AM  #5 
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Join Date: Dec 2013
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Without seeing specific examples of your circuitry it is difficult to say with any certainty, but it could be due to different impedances for your 'similar but different' RIAA networks altering the gain of your opamp circuit (as you suggest they are in the feedback path).

25th July 2014, 12:45 AM  #6 
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25th July 2014, 01:02 AM  #7  
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Quote:
The equivalence may be in the RIAA response, but to be completely equivalent they would have to have the same impedance, which they are not, hence the gain variation. Lower impedance RIAA networks have lower noise, with the right lower impedance inverting input resistor, but can be a far worse load on the opamp output stage than the next stage, so a balance of noise/distortion. rgds, sreten. NE5534 is a great opamp for a phono stage. Last edited by sreten; 25th July 2014 at 01:06 AM. 

25th July 2014, 01:06 AM  #8 
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Yep it looks right  you have to plug in Xc as an imaginary number as its in quadrature with the resistive impedance.
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25th July 2014, 03:03 AM  #9 
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Location: North Queensland.

G'day all, I am using the figure 2a of this AN 346 Application note in one phono stage http://www.ti.com/lit/an/snoa586d/snoa586d.pdf and this RIAA network in here in another Opamp Based RIAA Phono Preamp for MM and MC Phono Cartridges and I obtain somewhat higher output with this RIAA network. Regards, Felix.
Last edited by fap; 25th July 2014 at 03:05 AM. Reason: grammar. 
25th July 2014, 05:51 AM  #10 
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Location: close to Basel

Hi,
the impedances in both branches, the backfeeding as well as the gndshunting, determine the gain, added by +1 in the noninverting configuration. As a note, all values regarding Phono stages are referenced to a frequency of 1kHz. Now You just need to calculate the impedance values at 1kHz. For caps You calculate Zc=1/(2pi x f x C) This gives ~159155/C (C in nF) Now C1 is paralleled with R1 and C2 is paralleled with R2. You now use the formula for paralleled Resistances which is: 1/R = 1/Rx +1/Rcx Use the calculated impedance value of the cap as Rcx. As the paralleled Rs and Cs are connected in series, the calculated ´Resistances´ add up to the complete feedback resistance at 1kHz. Now after the noninverting OPAmps gain formula the gain at 1kHz calculates to: Av = 1+ Rfb/R0 (with R0= shunt resistor to gnd). So its not very hard or complicated to get an idea about the gain of the circuit. jauu Calvin
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