Op Amps  Key Facts 

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Ideal Op Amp 

Gain 
Infinity 
Input resistance 
Infinity 
Output resistance 
Zero 
Gain Bandwidth Product (GBP) 
Infinity Gain x Frequency = GBP 
Comparator 

Gain 
Infinity 
Output 
Always saturated high or low 
Input Resistance 
Infinity 
Comparisons 

Inverting Amplifier 

Gain 

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. 
Non Inverting Amplifier  Two Ways to Draw the Diagram


Gain 

Input resistance 
The input resistance of the op amp which is roughly infinity. 
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. 
Difference Amplifier 

Calculate Vout 
Vout = ( V+  V ) x ( Rf / R1 ) 
Input Resistance 
2 x R1 
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. 
Schmitt Trigger

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