1. The Differential Mode Op-Amp

2. What is the Differential Mode ? The op-amp can be connected up in various ways or modes. What it does depends on how it is connected up. When connected up in the differential mode, it finds the difference between the two input voltages and multiplies it by the gain.

3. The Differential Mode Op-Amp + - V1V1 VoVo RfRf R1R1 0 V +V s -V s The feedback resistor ( R f ) is used to control the gain of the op-amp. The input voltages are fed in through the input resistors ( R 1 and R 2 ). R2R2 R3R3 V2V2 R 3 is always the same size as R f.

4. Revision – The Gain of an Amplifier You may remember from Standard Grade that the gain of an amplifier is the ratio of the output and input voltages. The gain can also be expressed in terms of the resistors used to connect up the op-amp.

5. The Differential Mode Gain Equation

6. Example 1 Calculate the output voltage from the op-amp below. + - 0.3 V VoVo 150kΩ 20kΩ 0 V +V s -V s 0.5V 20kΩ 150kΩ

7. Example 1 - Solution

8. Example 2 What size of feedback resistor would be required to produce an output voltage of – 6 volts ? + - 0.6 V - 6 V RfRf 20kΩ 0 V +V s -V s 0.3V 20kΩ R3R3

9. Example 2 - Solution