The op amp as an amplifier

For the op amp to work as a amplifier the difference between the two input voltages V₂ and V₁ must be less than 150 m. (More generally it must be less than V_S/A_oV.) This is achieved in different ways by the non-inverting and the inverting voltage amplifiers.

The inverting voltage amplifier

In this use the input is applied to the inverting input and the non- inverting input is earthed (Figure 1). So that the value of (V₂ – V₁) is less than 150 mV a resistor R₁ is placed in the input circuit. Hence V₂ is 0 and V₁ is less than 150 μV. A resistor R₂ forms a feedback loop and since the inverting input is used the feedback is out of phase with the input and is therefore negative feedback.

The maximum value of V₁ at P will be 150 m when the output is V_S and so we can consider V₁ to be virtually zero. P is said to be a virtual earth.


The potential difference across R₁ is therefore virtually V and the potential difference across R₂ is virtually V_o (but reversed in sign). Since almost no current flows through the op amp, the current (I) in R₂ is equal to that in R₁ and so we can write:

I = V_i/R₁ = - V_o/R₂




The voltage gain of the amplifier (A) is known as the closed loop gain and is given by the formula:

Notice that this depends only on the values of the two external resistors.