Op Amps

Question 1

What is an Op Amp?

Answer 1

A high gain voltage amplifier with a differential input and a single output.
A linear device that achieves nearly ideal DC amplification.

Question 2

What are Op Amps used for?

Answer 2

Signal conditioning, filtering and mathematical operations.

Question 3

What are the working principles of an Op Amp?

Answer 3

Produce output that is much larger than that across the inputs.

Question 4

What are the differences between an Op Amp and a transistor?

Answer 4

Op Amps are cheaper and easier to manufacture and they also have better characteristics.

Question 5

What is the pinout of an Op Amp IC?

Answer 5

1. Offset Null
2. Inverting input
3. Non inverting input
4. Power - ve
5. Offset Null
6. Output
7. Power +ve
8. Not connected

Question 6

What are the types of Op Amps?

Answer 6

1. Comparator
2. Non inverting or Inverting Amplifier
3. Summing Amplifier
4. differential Amplifier
5. Inverting integrator or Differentiator.

Question 7

What are the charateristics of an Ideal Op Amp?

Answer 7

V0 = A(V2 - V1).
A is the open loop gain which for an ideal op amp is equal to infinity.
It has infinite input impedance so no current will flow into its input terminals.
Zero Output Impedance
Infinite Bandwidth. Gain is the same for all signals and independant of frequency.

Question 8

What are the characteristics of a Real Op Amp?

Answer 8

They are nearly ideal, except the gain of the Op amp is closer to 200 000 than infinity.

Question 9

What is the Virtual Earth Principle?

Answer 9

This principle makes it easier to analyse Op Amps. We say that it is a constant reference voltage that is equal to zero. Generally it is connected to the positive pin in the Op Amp diagram and so if this is 0 then we say that the negative pin is also zero.

Question 10

What is a comparator?

Answer 10

It will compare two voltage levels( sometimes this will be a reference voltage) to see which one is bigger.

Question 11

What is a non inverting Op Amp?

Answer 11

An Ideal op amp. The phase of the input and the output are the same.

Question 12

What is an Inverting Op Amp?

Answer 12

The output Voltage will be half a cycle out of phase with the input.

Question 13

What is a differential Op Amp?

Answer 13

Used for finding the differences between voltages. Multiply each voltage by a constant that will be determined by the resistors in the circuit.

Question 14

What is a summing Op Amp?

Answer 14

An Op Amp that will combine two or more voltages into a single voltage.

Question 15

How do we tell the difference between an integrating Op

Amp and a differentiating Op Amp?

Answer 15

The integrating one will have a capacitor in the feed back loop. it will produce an output voltage that is proportional to the input voltage of the integral of Vin.
A differentiating Op Amp will have a capacitor before the feedback loop. It will produce an output that is the first derivative of the input voltage. It is directly proportional to the rate of change of the input voltage over time.

Question 16

What are the categories of Non Ideal Op Amps?

Answer 16

1. Imperfections in the linear range of operation
2. Non linear limitations
3. DC imperfections

Question 17

What are the limitations of Non Ideal Op Amps?

Answer 17

A real op amp wont have infinite gain or input impedance. it wont have zero output impedance.
the open loop gain will start to decrease around 100hz, it will continue to decrease as the frequency increases.

Question 18

What is Clipping in relation to an Op Amp?

Answer 18

It is the idea that the Output voltage cannot exceed the max value of the supply voltage.

Question 19

What is Slew Rate?

Answer 19

The max rate at which an Op Amp output voltage can change. Usually around 105 - 108 V/s. It determines the max frequency for which full power can be delivered.

Question 20

What is the Offset Voltage?

Answer 20

It allows us to negate the idea that if the inputs of the Op Amp are joined together and grounded, then this means that the output voltage should be zero. In reality it isn’t so it behaves as if there is an internal voltage in the circuit.

Question 21

How do we null the offset Voltage?

Answer 21

Connect a potentiometer.

Question 22

What is the Offset Current?

Answer 22

To make the Op Amp work, we must provide the inputs of the Op Amp with some current regardless of the fact that they have infinite input impedance. We therefore model the current into the Op Amp through a Bias Current (the same for both inputs) and an offset current that is difference between the input currents. Offset current is generally lower than the Bias.