In this note on page 22, it says,
In practice we should however avoid extremely large resistance values in the feedback circuit.
It says so after designing this circuit:
The requirement of the circuit is:
The solutions it gives is:
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1\$\begingroup\$ So what is your question? The use of high-value resistors, the theory of the amplifier configuration, or the proposed solution set? \$\endgroup\$– MOSFETCommented 2 days ago
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\$\begingroup\$ For 20 dB of voltage gain there's no need for very large resistance values. Using very large values is done sometimes in transimpedance amplifiers, but then may require guard rings and PCB cutouts to manage unwanted trace currents. In extreme cases, IC dice and wire bonding may be required to avoid leakage between pins in epoxy packaging, though that is rare and more for delicate experimental systems. In your case, there's nothing achieved by large values to even consider the idea. \$\endgroup\$– periblepsisCommented 2 days ago
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3 Answers
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If you use too large of values for resistors (such as over 1MΩ), you may exceed the input bias current for the opamp and you will have signal degradation\instability issues.
Larger resistances cut back feedback current, feedback current reduces the ability to drive loads. Feedback current also wastes energy at heat.
But large resistance comes at a cost of thermal noise. In some low level analog designs (such as those that measure nV) large resistors cannot be used because they will create too much noise.
Source: https://www.rfcafe.com/references/electrical/noise-power.htm
answered 2 days ago
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Low values use more current which comes from op-amp output and that come from the power supply.
There is one more thing.The feedback resistors effect the stability of the circuit also.The below statement is taken from an Analog Devices application note:
An amplifier’s internal input capacitance, found in the specification table of the data sheet, interacts with \$R_F\$ to form a pole in the transfer function. If \$R_F\$ is exceedingly large, this pole will affect stability. If the pole occurs at a frequency much larger than the crossover frequency, it will not affect stability. However, if the location of the pole as determined by \$f = 1/(2\pi R_FC_\text{in,amp})\$ occurs near the crossover frequency, the phase margin will be reduced leading to potential instability.
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Aside from the Johnson noise that the resistor would cause, large resistors in the feedback loop are associated with high gains.
Many of the "ideal" op-amp equations we're all used to are derived using the assumption that the gain of the circuit is much less than the open loop gain of the op amp. Once your circuit gain and the open loop gain are of the same order of magnitude, you're much more likely to find that the output of the circuit doesn't match your design equations.
