Op-amp With same Rf/Ri ratio. But why resistor value multiply by 10

Thread Starter

Romie77

Joined Jan 4, 2020
23
I have two boards. One of them is higher version of another board.
In one board, 500k and 3.32k ohm resistor. While in another board, value of both resistor are multiplied by 10. The thing is ratio will remain the same. Then why did resistor value are multiplied by 10 (in higher version)?
For same input voltages to both circuit, output will be same. What could be the reason to change resistor value?
 

andrewmm

Joined Feb 25, 2011
535
A good question , home work ?

Think about the amplifier, and where the 1 - Ra/Rb comes from. It assumes that the amplifier is perfect, zero output impedance, infinite input impedance, infinite gain, not affected by capacitance.

So what happens if you put say 100M and 10M as opposed to say 10K and 1K or 100 ohm and 10 ohm ?

what changes from the eyes of the op amp, and how is that different to the perfect model ?
 

Thread Starter

Romie77

Joined Jan 4, 2020
23
Well, from the eyes of amplifier the ratio of feedback resistor and input resistor remains the same. Is there any other points which I had missed?

I'm sorry, may be I didn't get your point.
 

Delta prime

Joined Nov 15, 2019
430
Well, from the eyes of amplifier the ratio of feedback resistor and input resistor remains the same. Is there any other points which I had missed?
Hello there :) The feedback network is connected in parallel of the actual "load" and wastes part of the opamp output stage driving capability). Unless there are valid motivations to do otherwise, it is recommended that the current flowing through the feedback network is negligible with respect to the current flowing through the intended load
The "inverting amplifier", unlike the "non-inverting" op-amp-based amplifier is not a "true" voltage amplifier, since its input impedance is not (ideally) infinite, but is given by R1. As a consequence, the value that you choose for R1 is also the input impedance of the amplifier! If you expect that the circuit behaves like an "inverting voltage amplifier", this input impedance needs to be significantly larger than the internal resistance of the source providing the input signal.
the impact of 1pF parasitic capacitance in parallel to a 100Ohm resistor is negligible up to more than 1GHz, while the impact of the same 1pF parasitic in parallel to a 1MOhm resistor could be relevant at less than 200kHz), and hence better high-frequency behavior. Moreover in integrated designs, low resistance values also mean a smaller foot print.
 
Last edited:

andrewmm

Joined Feb 25, 2011
535
Well, from the eyes of amplifier the ratio of feedback resistor and input resistor remains the same. Is there any other points which I had missed?

I'm sorry, may be I didn't get your point.

Your assuming the amplifier is perfect then.

Change the values of the resistors,

the current in / out of the input / outputs change, (amplifier does not have infinite input impedance, or zero output impedance )
the effect of capacitance on the circuit changes, Assuming capacitance is constant, a 10M resistance resistance causes a much different RC filter than a 10K resistance.

Offset voltage, is cause by ( to a fist degree ) current going into / out of the input pin,
a large resistance causes much larger offset voltage, which the amplifier has to compensate for .

input noise, a higher resistor value has a higher noise than a low resistance one, https://en.wikipedia.org/wiki/Johnson–Nyquist_noise#:~:text=Thermal noise in an ideal,a nearly Gaussian amplitude distribution.

A small resistance system is in parallel with the load and / or input,
so the amplifier has to have more current gain to drive the higher load,


Read up on the op amp circuits , they are wonderful systems,
made designing so much easier back in the 70's,
but they are so good people forget the assumptions made by the 1 - Ra / Rb rule.

if you look at sites with plenty of op amps, such as analog devices, you will see many op amps,
each with a specific twist / advantage in one area,

90 % of the time , you will use a general op amp, and general resistors in the Kohm range , but as you delve deeper, you need to read the data sheets and the app notes,

https://www.analog.com/media/en/news-marketing-collateral/product-selection-guide/Op_Amps_SG_2011-12_SG_Specs.pdf

https://www.analog.com/en/education/education-library/op-amp-applications-handbook.html#
 
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