Topic 5  The Inverting Amplifier

  Negative feedback is achieved by bringing a fraction of the output signal to the inverting input of the op-amp. 

The photograph shows an op-amp with negative feedback to give an inverting amplifier.

Negative feedback also has the following effects:

• The gain is reduced

•  The bandwidth (frequency response) is increased

• The stability is improved.

  This allows the op-amp to be used in many applications including audio.

  The diagram shows a typical negative feedback arrangement:

In this arrangement the non-inverting input is connected to the 0V line.  Since there is very little voltage difference between the non-inverting and the inverting input, we can also say that the voltage at the inverting input is almost at 0V as well. So we say that the inverting input is at a virtual earth.

Question 1. Do you know what is meant by a virtual earth?  Can you explain in terms of the difference between the inputs of the amplifier?   ANSWER                                                                          

Let us suppose there is a current I1 flows through Ra and a current I2 flows through Rf.  Ohm’s Law allows us to say:

• Current through Ra, I1 = Vin/Ra

• Current through Rf, I2 = Vout/Rf

Since the resistance at the inverting terminal is very high, no current can flow through the inverting input.  At point X (according to Kirchhoff I) the current in is the same as the current out.  Therefore the current is the same through Ra and Rf.

• Kirchhoff I: I₁ + I₂ = 0

• Substituting:            Vin/R_a  +  Vout/Rf= 0

• Rearranging:           V_in /Ra = - Vout/Rf

• Therefore:                 V_out / V_in = - Rf/Ra

Since point X is at a virtual earth and the voltage of X is zero, and therefore the input resistance is the same as the input resistor.  So the input resistance of this amplifier is 1000 W.  This will have implications for load matching for an input device such as a microphone.  The output in all inverting amplifiers relates to the input as shown below:

Note the input and output are 180 degrees out of phase.

Question 2. If  Ra = 1000 W and Rf = 100000 W, what is the gain of the inverting amplifier?  ANSWER

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