Tutorial 8B - The Non-Inverting Amplifier

In this circuit the input voltage is applied to the non-inverting input. Notice that:

• This amplifier uses negative feedback to the inverting input.

• There is no difference in the voltage between the inverting and non-inverting inputs, so we can say that the voltage at the non-inverting and the inverting input is the same.  So we can say that the voltage at P is Vin.

• Since no current is drawn by the inverting input, the current in Ra is the same as the current in Rf.  So we can treat the two resistors as a potential divider and apply the potential divider equation.

We can therefore write: Rearranging gives: The term Vout/Vin is the gain.

The term: can be rewritten as: Therefore: If we look at the input, we see that there is no feedback resistor in the input, therefore we can say that the input resistance is that of the op-amp.  The input resistance is very high indeed, and very little current is taken.

 Why can we say that the voltage at P is Vin? Extension

The problem with the inverting amplifier used as a voltage follower is that the output is at 180o out of phase with the input.  A voltage follower can be based on the non inverting circuit with 100 % negative feedback to the inverting input, and input resistance is very high indeed. The voltage gain of the op-amp in this configuration is about 1.  This because of  the feedback factor (the fraction fed back), given the code b (beta, a Greek letter ‘b’) is 1.

We can show this by considering the open loop gain A0.  The actual gain A is given by: If b is 1, and A0 is very large, we can say that A is approximately 1.

The main use of the voltage follower is as a buffer amplifier, which matches a high input impedance with a low input load.  You would come across such a circuit in the input stage of a digital multimeter, which has a very high input impedance, allowing the voltage read to be the same as the voltage that should be there.

 What is the gain of a non-inverting amplifier with a feedback resistor of 10000 ohms and Ra value of 100 ohms? 