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Quetion:

Design a noninverting amplifier with a gain of 2. At the maximum output voltage of 10 V the current in the voltage divider is to be 10 μA.

I can easily understand the first part of the problem . enter image description here

Here, gain = 1+\$\frac{R_f}{R_1}\$ , To make gain = 2, \$R_1\$ should be same as \$R_f\$
But what is meant by current in the voltage divider in the second part of the question?

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2 Answers 2

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But what is meant by current in the voltage divider in the second part of the question?

It's a hint as to what value resistors to use. With 10 volts and 10 uA, the impedance is 1 million ohms hence, R1 = Rf = 500 kohm.

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But what is meant by current in the voltage divider in the second part of the question?

This question is to determine sensible resistor values for Rf and R1. For a gain of 2 they could both be 100 Ω or both be 1 MΩ but different currents will result and it is unlikely that your op-amp could maintain 10 V output in the 100 Ω case. In the 1 MΩ case it might be a bit sensitive to noise or bias currents may cause too much of an offset.

  • You know the output voltage (10 V) and you know that the left side of R1 is connected to ground (although the schematic is missing a connection dot).
  • You know the current.
  • Now calculate the total resistance (Rf + R1).

You should be able to figure it out from there.

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