What normally determines the voltage 'gain' in a simple op amp circuit? - diyAudio
 What normally determines the voltage 'gain' in a simple op amp circuit?
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 24th July 2014, 11:33 PM #1 diyAudio Member   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 diyAudio Member     Join Date: Sep 2007 Location: Hangzhou - Marco Polo's 'most beautiful city'. 700yrs is a long time though... Blog Entries: 143 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. __________________ The heart ... first dictates the conclusion, then commands the head to provide the reasoning that will defend it. Anthony de Mello
 25th July 2014, 12:07 AM #3 diyAudio Member   Join Date: Sep 2010 Location: North Queensland. 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 diyAudio Member     Join Date: Feb 2009 Location: Calgary The Wikipedia article on op-amps seems pretty informative... Operational amplifier - Wikipedia, the free encyclopedia ~Tom __________________ Modulus-86: 40W/8Ω @ 0.000061% THD. HP-1: 3W/20Ω, 450mW/300Ω @ 0.000032% THD. DG300B and Other Tube Circuits. Neurochrome : : Audio - www.neurochrome.com - Engineering : : Done : : Right
 25th July 2014, 12:32 AM #5 diyAudio Member   Join Date: Dec 2013 Location: Sydney, 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 op-amp circuit (as you suggest they are in the feedback path).
 25th July 2014, 12:45 AM #6 diyAudio Member   Join Date: Dec 2013 Location: Sydney, So if I pinch a circuit from the first site I can find on RIAA networks, that includes a schematic: the gain of this circuit is: 1+(R4+(Xc2//R2)+(Xc1//R1)) / R3 where Xc is the reactance (impedance) of the capacitor. and where Xc1//R1 = 1/(1/Xc1 + 1/R1) etc....
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Quote:
 Originally Posted by fap 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.
Hi,

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 op-amp for a phono stage.

Last edited by sreten; 25th July 2014 at 01:06 AM.

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Location: Hangzhou - Marco Polo's 'most beautiful city'. 700yrs is a long time though...
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Quote:
 Originally Posted by RJF the gain of this circuit is: 1+(R4+(Xc2//R2)+(Xc1//R1)) / R3 where Xc is the reactance (impedance) of the capacitor. and where Xc1//R1 = 1/(1/Xc1 + 1/R1) etc....
Yep it looks right - you have to plug in Xc as an imaginary number as its in quadrature with the resistive impedance.
__________________
The heart ... first dictates the conclusion, then commands the head to provide the reasoning that will defend it. Anthony de Mello

 25th July 2014, 03:03 AM #9 diyAudio Member   Join Date: Sep 2010 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 Op-amp 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 diyAudio Member     Join Date: Nov 2004 Location: close to Basel Hi, the impedances in both branches, the backfeeding as well as the gnd-shunting, 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 __________________ http://calvins-audio-page.jimdo.com

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