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# Op Amps - Key Facts

This is the AQA version closing after June 2019. Visit the the version for Eduqas instead.

 Everyone Ideal Op Amp Gain Infinity Input resistance Infinity Output resistance Zero Gain Bandwidth Product (GBP) Infinity Gain x Frequency = GBP Everyone Comparator Gain Infinity Output Always saturated high or low Input Resistance Infinity Comparisons If V+ is greater than V- then the output is high. If V+ is less than V- then the output is low. AS A2 Inverting Amplifier Gain Gv = -Rf / R1 Gv = Vout / Vin Input resistance R1 Virtual Earth Point P Op Amps have a very high open loop gain so if the output is a few volts, the inverting input voltage will be a few microvolts. This is so close to zero, it is called a virtual earth. AS A2 Non Inverting Amplifier - Two Ways to Draw the Diagram  Gain Gv = 1 + Rf / R1 Gv = Vout / Vin Input resistance The input resistance of the op amp which is roughly infinity. AS A2 Summing Amplifier Calculate Vout Vout = - Rf (V1/R1 + V2/R2 + V3/R3) Input resistance It's R1 for the V1 input and R2 for the V2 input etc. Virtual Earth The point P is the virtual earth. AS A2 Difference Amplifier Calculate Vout Vout = ( V+ - V- ) x ( Rf / R1 ) Input Resistance 2 x R1 AS A2 Voltage Follower Voltage Gain The voltage gain is ONE. Power and Current Gain This can be very large. Input resistance Infinity Output resistance Small so usefully large currents can be provided. A2 Schmitt Trigger On the left, a simple version with positive feedback giving different on and off reference voltages. On the right is a comparator with one extra resistor giving positive feedback to give different on and off reference voltages.