Op Amps - Key Facts |
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Ideal Op Amp |
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Gain |
Infinity |
Input resistance |
Infinity |
Output resistance |
Zero |
Gain Bandwidth Product (GBP) |
Infinity Gain x Frequency = GBP |
Comparator |
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Gain |
Infinity |
Output |
Always saturated high or low |
Input Resistance |
Infinity |
Comparisons |
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Inverting Amplifier |
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Gain |
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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
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Gain |
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Input resistance |
The input resistance of the op amp which is roughly infinity. |
Summing Amplifier |
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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 |
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Calculate Vout |
Vout = ( V+ - V- ) x ( Rf / R1 ) |
Input Resistance |
2 x R1 |
Voltage Follower |
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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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