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9th November 2018, 12:03 #1
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DC Amplification using OpAmp
I have a small signal of 10 mV. I need to amplify it to 4 V which need an amplification factor of 400. I would like to use inverting amplifier whose gain is R2/R1, where R2 is a feedback resistor. I am wondering what is the best practice to get the amplification by a factor of 400.
1 Use one OpAmp to get gain 400. R1 = 100 , R2 = 40 K.
2 Use two OpAmp in series. Each with a gain of 20. R1 = 500, R2 = 10 K.

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9th November 2018, 12:20 #2
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Re: DC Amplification using OpAmp
Hi,
One Opamp. Because it´s DC. A second OPAMP just adds more errors.
The two OPAMP solution is good for AC, when you get problems with to the gainbandwidth limit of a single OPAMP.
****
your calculation is true if the source resistance is way below 100 Ohms.
For exact overall gain calculation you need to add the source_resistance to the input_resistance.
Example:
If your source resistance is 10 Ohms, then combined with your 100R and 40k this gives just a gain of 40k/(10R + 100R) = 363.
If you want a gain of 400 then you need to subtract the source resistance from the (ideally) calculated input resistance. 100R  10R = 90R
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9th November 2018, 12:31 #3
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Re: DC Amplification using OpAmp
As an alternative, you have mentioned two inverting opamps in serties. From this, I derive that the sign of the amplification (plus or mins) does not matter, correct?
In this case, why not use a single noninvering opamp stage?
The advantage: Because of the very high input resistance the internal resistance of the signal source does not matter at all.

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9th November 2018, 16:20 #4
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9th November 2018, 16:48 #5

9th November 2018, 22:54 #6
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Re: DC Amplification using OpAmp
Hi,
I am not sure if I understood source resistance. I guess it is the resistance of the source, for example signal generator, right ? Usually the standard signal generator has 50 Ohm output characteristic impedance. Is this what you mean source resistance ?
Is it true for all OpAmps that the input impedance is high and the output impedance is low ?

10th November 2018, 01:17 #7
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Re: DC Amplification using OpAmp
Hi,
Is this what you mean source resistance ?
Is it true for all OpAmps that the input impedance is high and the output impedance is low ?
But not for the circuit. Especially the inverting circuit. It's input resistance is the 100 ohms resistor.
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10th November 2018, 04:52 #8
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