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Op Amps - Key Facts


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Summary PDF    Summary DOCX

Everyone Ideal Op Amp

Summary-op-amp.gif

Gain

Infinity

Input resistance

Infinity

Output resistance

Zero

Gain Bandwidth Product (GBP)

Infinity

Gain x Frequency = GBP

Everyone Comparator

Summary-comparator.gif

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

Summary-Inverting.gif

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

Summary-Non-Inverting.gif        Non Inverting - Alternative 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

Summary-Summing-Inverting.gif

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

Summary-difference-amplifier.gif

Calculate Vout

Vout = ( V+ - V- ) x ( Rf / R1 )

Input Resistance

2 x R1

AS A2 Voltage Follower

Summary-voltage-follower.gif

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

Op Amp Inverting-Schmitt-Trig.gif

  • 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.

 

 

 

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