What normally determines the voltage 'gain' in a simple op amp circuit? - diyAudio
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Old 25th July 2014, 12:33 AM   #1
fap is offline fap  Australia
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Default 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.
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Old 25th July 2014, 12:35 AM   #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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Old 25th July 2014, 01:07 AM   #3
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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.
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Old 25th July 2014, 01:08 AM   #4
tomchr is offline tomchr  Canada
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The Wikipedia article on op-amps seems pretty informative...

Operational amplifier - Wikipedia, the free encyclopedia

~Tom
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Old 25th July 2014, 01:32 AM   #5
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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 op-amp circuit (as you suggest they are in the feedback path).
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Old 25th July 2014, 01:45 AM   #6
RJF is offline RJF  Australia
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So if I pinch a circuit from the first site I can find on RIAA networks, that includes a schematic:

Click the image to open in full size.
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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Old 25th July 2014, 02:02 AM   #7
sreten is offline sreten  United Kingdom
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Quote:
Originally Posted by fap View Post
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 02:06 AM.
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Old 25th July 2014, 02:06 AM   #8
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Quote:
Originally Posted by RJF View Post
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.
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Old 25th July 2014, 04:03 AM   #9
fap is offline fap  Australia
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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 04:05 AM. Reason: grammar.
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Old 25th July 2014, 06:51 AM   #10
Calvin is online now Calvin  Germany
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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
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