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Old 30th October 2013, 04:17 AM   #1
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Default connect two inverting op amps together

i did some reading on wikipedia and apparently an inverting feedback amplifier layout has lower distortion compared to a non inverting feedback loop, so assume i make something like the design below, will it work? (picture only shows 1 channel)
Click the image to open in full size.

the first op amp will invert the signal and handle the gain where gain = 1 + R2/R1
the second op amp will have no gain and just invert the signal again

assume that the thing above actually works, do i really need R3?
following the above question, is the reduced distortion actually worth doing? will using 2 inverting feedback loops actually have more distortion compared to just 1 non inverting feedback loop?

im kind of lost here as i am quite new to diy audio, please help, any help will be appreciated
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Old 30th October 2013, 07:09 AM   #2
Johno is offline Johno  Australia
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That config will not work (go back to your inverting op amp gain equation to see why)

Better if the gain is spread across 2 inverting config op amps more or less equally.

Anyway, does it really matter if the output is inverted?

Last edited by Johno; 30th October 2013 at 07:11 AM.
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Old 30th October 2013, 07:23 AM   #3
Mooly is online now Mooly  United Kingdom
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Quote:
Originally Posted by thelegendaryblah View Post
the first op amp will invert the signal and handle the gain where gain = 1 + R2/R1
That equation is incorrect. The gain is -R2/R1 or if you prefer R2/R1 *-1
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Old 30th October 2013, 05:57 PM   #4
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thanks, i just realised what's wrong with my schematics, so basically the second op amp inverts the inverted signal but Vout would be 0 since 0/R3 = 0

so what if i do something like the schematic below?
Click the image to open in full size.
where:
R2 = R4
R1 = R3 = R2/2
gain would be 4 if i didnt mess things up right?

Quote:
Anyway, does it really matter if the output is inverted?
i dont know, i used search around the forums as well as on google and there are no conclusive answers, people did say that a lot of preamps and CD players are inverting however

sidenote, what happens if i use resistors with larger resistances but remain proportional for all the numbers, like
R2 = 1000, R1 = 500
compared to
R2 = 10, R1 = 5
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Old 30th October 2013, 06:11 PM   #5
Mooly is online now Mooly  United Kingdom
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You calculate the gain for each stage. Lets say R1 is 68k, R2 is 390k, R3 is 1.5k and R4 is 4.7k.

With R1 at 68k and R2 at 390k then the gain of that stage is -5.7
With R3 at 1.5k and R4 at 4.7k then the gain of the second stage is -3.1

Multiply the two together and you get the overall gain which is +17.67

Yes
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Old 30th October 2013, 06:13 PM   #6
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Remember that R1 and R3 define the input impedance for the relevant stage. You shouldn't go to low that the opamp can not drive R3 fully. Usually around 2K would be as low as you would normally go for many common opamps.
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Old 4th November 2013, 10:46 PM   #7
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sorry for being away for quite some time (was busy dealing with midterm)

i see that using 10k ohms is popular (as various cmoy designs use 10k ohms) for R1 and R3

i remember reading somewhere that an ideal op amp should have infinite input impedance, so since higher is better, when is it too high (say i go crazy and get something like 2200k ohms resistors)

i see from the datasheet for a OPA2227 that it has an input voltage range of + or - 2V, is this the limiting factor preventing ultra high input impedance in op amps?
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Old 4th November 2013, 11:29 PM   #8
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Do not confuse op amp input impedance with the input resistors or with the stage input impedance. They are 3 different quantities.
- op amp input impedance depends on the device you choose
- input resistor is a value you must choose
- stage input impedance is the parallel combination of input and feedback resistances but of course is dominated by the input resistor which is usually the smaller.

Op Amps are not perfect, they have a small but significant input current (that is they do not have infinite input impedance).

So this current flowing through your input resistor creates an error voltage that is also amplified along with the signal and appears at the output.

Therefore it is wise to keep the input resistance a low as practical, that is somewhere between about 2k and 10k. If you required a stage input impedance to be a certain value, to suit the signal source for example, you will need to calculate it.

Just as importantly and for the same reasons, the non-inverting input to the op amp should also have an input resistor, even though it goes to ground. The value of this resistor should equal to the parallel combination of the inverting input and the feedback resistors. This value allows the input current generated voltages at the inputs to be equal. However, at the output the 2 offsets are self cancelling (the inverted error cancels the non-inverted error).

Hope this helps
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Old 5th November 2013, 06:22 AM   #9
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Quote:
Originally Posted by thelegendaryblah View Post
sorry for being away for quite some time (was busy dealing with midterm)

i see that using 10k ohms is popular (as various cmoy designs use 10k ohms) for R1 and R3

i remember reading somewhere that an ideal op amp should have infinite input impedance, so since higher is better, when is it too high (say i go crazy and get something like 2200k ohms resistors)

i see from the datasheet for a OPA2227 that it has an input voltage range of + or - 2V, is this the limiting factor preventing ultra high input impedance in op amps?
If you go to high on the resistors for your inverting amp drawn above then you will run into problems with loss of bandwidth and increased noise.

Not sure what you are looking at to interpret the "input voltage range" of the OPA2227. I think the -/+2 volts you mean is how near to each rail you can take the input voltage, and that's measured "open loop" with no feedback.
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Old 6th November 2013, 12:29 AM   #10
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i see, but since resistors come in certain values only (and i only have certain values only), when i place a resistor connected to the positive input and ground and assuming i do not have the exact value for the parallel combination of R1 and R2, then should i overshoot and use a bigger value? or should i actually use the closest value possible to get the best noise canceling effect?

the example below should illustrate what i meant nicely
Click the image to open in full size.
assume:

R1 = R3 = 5.7k ohm
R2 = R4 = 10k ohm
gain = approximately 3.1

if i use the layout on the top of the image, then
ideally the resistors to positive input should be
R5 = R6 = 57/15.7 = 3.63k ohms
but i only have 3k and 3.9k resistors, i also have 680 ohms resistors so should i use the 3.9k resistor or should i put two resistors in series for R5 and R6 so that
R5 = R6 = 3.68k ohm which is really close to 3.63k ohms?

or alternatively, could i follow the layout on the bottom of the image where
R7 = R6/2
which ideally should be 1.315k
then should i use a single 1.5k resistor or combine a 1k resistor to a 330 ohm resistor?

sidenote: is the noise generated by the resistors really that significant? and would trimpots work better since they are trimmable?

thanks in advance

EDIT: just realised i forgot about the resistors having an error rating of + or - 1%, so i guess i could actually manually pair all the resistors with a multimeter and get to the ideal value as close as possible

Last edited by thelegendaryblah; 6th November 2013 at 12:38 AM.
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