Op-amp technicalities Flashcards

1
Q

What are the limits of op-amps? (5 limits, 1 outcome)

A

> Output voltage cannot exceed the supply voltages

> Input voltage cannot exceed the supply voltages

> Input voltage × Gain (G) cannot exceed supply voltages

> Differential input limit is the maximum difference between the V- and V+ inputs.

> The common-mode input voltage (the average between the two input voltages) cannot be too high.

> If limits are exceeded, the op-amp may start to draw significant current.

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2
Q

What makes a system unstable?

A

When the output is not entirely controlled

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3
Q

How does feedback impact a systems stability? What is the purpose of feedback?

A

> Feedback does not necessarily improve stability as an open-loop system is stable, but it is not very useful.

> Feedback allows us to control a system whilst maintaining stability.

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4
Q

When does instability occur?

A

> When the phase of the feedback sifts by π radians (180°).

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5
Q

What happens to the equations for the inverting and non-inverting amplifiers when instability occurs?

A

The denominators f both equations become 1 - AB.

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6
Q

What are the 3 different cases of positive feedback signals?

A

> Small positive feedback: -1 < AB < 0

> Unstable system: AB = -1

> Large positive feedback: AB < -1

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7
Q

What happens when there is small positive feedback?

A
  • 1 + AB < 1 - G > A
  • The system is not strictly unstable but it is approaching instability.
  • The gain is increasing rapidly.
  • G ⇒ ∞
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8
Q

What happens when there is positive feedback and the system is unstable?

A
  • 1 + AB = 0
  • The equation becomes undefined (Does not apply anymore)
  • Output is either at saturation at one of the supply voltages or has a self-sustaining oscillation
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9
Q

What happens when there is large positive feedback?

A

> It would appear that the system would become stable again with a negative output, but this does not occur.

> Gain equations are no longer valid in the region beyond where G ⇒ ∞

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10
Q

How does instability occur?

A

> Due to the design of an op-amp, there are several low pass filter stages in series

> Each low-pass stage produces an amplitude roll-off of 20dB/decade and a phase shift of -45° to 90° (at higher frequencies).

> Because they are in series, each phase shift acts on the preceding one.

> Eventually the phase shifts enough to cause positive feedback

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11
Q

What is the solution that op-amps use to prevent instability?

A

> We use a low-pass filter which reduces at a relatively low frequency such that it dominates the frequency response.

> We force the open-loop gain to reduce to unity well before the phase shifts can accumulate to a critical -180°

> This is called dominant-pole frequency compensation

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12
Q

What are the properties of the low pass dominant-pole frequency filter?

A

Roll off:

> 20dB/decade

> 6dB/ octave

> If we halve the signal frequency, then we will have twice as much open-loop gain Graph aspects:

> fT = The point where the gain is unity

> GBP: Gain bandwidth product GBP = fT

> GBW: Gain bandwidth GBW = fT Gain constraint:

> The closed-loop gain (G) cannot exceed the open-loop gain (A) and so the closed-loop is bounded by the open-loop. If the gain that we want is greater than the open-loop gain then we have a gain error.

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13
Q

What is phase margin?

A

> This is the difference between the total phase shift at fT and the critical -180° phase shift.

> This is a measure of how close the op-amp is to instability at the transition frequency

> We need to ensure that we do not introduce any additional phase shifts externally to the op-amp

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14
Q

Explain the different types of op-amp compensations

A

Fully compensated op-amp:

> The open-loop gain is rolled off to unity at a frequency where the phase shift is well below -180°

> This is done internally of the op-amp

Uncompensated op-amp:

> There is no in-built dominant-pole compensation

> The user is expected to add their own externally

Undercompensated op-amp:

> When the phase shift is at -180° the gain has not quite been rolled off to unity

> Intended to operate with a closed loop gain of 10 or more for high frequency circuits

> They are often called fast op-amps

> They are not stable at all

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15
Q

How do you calculate the gain error?

A
  1. Calculate the resistor configuration that will obtain the closed-loop gain that you wish to have
  2. Calculate the value of B for that configuration
  3. Decide what frequency you wish the op-amp to operate at
  4. Use the response graph to calculate the open-loop gain (A) at that frequency
  5. Insert the value of A and B into the gain equation for that type of op-amp and see what the actual gain becomes
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16
Q

How can inverting and non-inverting op-amps produce a gain that is less than unity/

A

> By using gain error to their advantage

> this method is not recommended

17
Q

What can be said about the useful bandwidths of inverting and non-inverting amplifiers?

A

The useful bandwidth of the inverting configuration is only half that of the non-inverting configuration.

18
Q

What happens when you require a unity gain that is close to fT?

A

> You need to include the imaginary aspect of phase shifting

Non-inverting: G = -j / (1 + (-j × 1)) = -j / (1 - j)) G ≈ 1/√2

Inverting: G = -j0.5 / (1 - 0.5j) ≈ 0.45

> If the open-loop gain is large (you are not near fT) then this does not matter

19
Q

Describe what can be said about choosing an op-amp based off fT?

A

> It is poor practice to design an op-amp circuit for operation close the fT

> We also do not want to choose an op-amp with an excessively high fT

> 20dB gain can be ensured for: freq < 0.1fT

20
Q

What is the issue with capacitive loads on op-amp circuits?

A

> Any load impedance which presents a capacitance between the output and ground introduces further phase shifting to the feedback path

21
Q

What is the definition of slew rate?

A

The limit for the rate of change of output voltage with time.

22
Q

Why will there be a limit to the slew rate?

A

Slew rate is limited by the finite internal current flow. An op-amp with a higher slew rate will draw more current from the power supply.

23
Q

How is the maximum slew rate requred calculated? What are its units?

A

Max slew rate = Aω

Differentiating the input signal: V = Asin(ωt) ⇒ dV/dt = Aωcos(ωt)

Units: V/s

24
Q

What is the problem if the slew rate is too large?

A

An op-amp with a very high slew rate may exhibit overshoot due to the combination of a fast output response and small but non-zero feedback delay.

25
Q

What is input bias current? When does it occur?

A

> Even though the impedance between the two inputs of an op-amp is very high, it is not infinite and so a very small current will flow between the inputs.

> Consequence 1: If there is a series capacitor on the input then it will accumulate a charge which will eventually saturate the output voltage.

> Consequence 2: A voltage error is added to input signal because there is a voltage across the impedance between the two inputs.

26
Q

How is input bias current prevented?

A

> Solution 1: Add a bias resistor between the input and ground. The problem with this is that this causes a high-pass filter on the input so you should pick the values of Rbias and capacitor accordingly.

> Solution 2: Ensure that the bias resistor is less than 1MΩ so we dont load the high resistance between the op-amp inputs

27
Q

What is the input offset voltage?

A

> A small internal offset voltage is added to the external differential input.

> This is due to the nature of how op-amps work

28
Q

What are the 2 consequences and two solutions to input offset voltage?

A

> Consequence 1: If the difference between the two inputs is very small ≈ 0 then this bias voltage is enough to cause the output to saturate.

> Solution 1: When using negative feedback this stops the input offset from affecting the output.

> Consequence 2: When using negative feedback, the output voltage will be offset from ground when the input is at reference voltage (e.g. 0V).

> Solution: There are offset pins that allow the designer to compensate for the offset voltage by including a potentiometer between the compensation pins and the negative supply voltage. But it is advisable to ignore this technique wherever possible because thermal drift will make the compensation ineffective.

29
Q

Define “Power-supply-rejection-ratio”

A

The ability of an op-amp to reject fluctuations on power supply rails.

30
Q

What is the problem with fluctuations in the power supply rails?

A

> It can cause instability

> Small high-frequency signals can be added to the op-amp output

31
Q

How can variations in supply voltage be removed externally?

A

> Using bypass capacitors between each supply and ground (or reference voltage) to lower the resonant frequency added by the supply.

> The type of capacitor is more important than its value

  • Ceramic or polyester capacitors are preferred.
  • 100nF is normal

> Inductance in power supply wires often makes it worse so keep wires short

32
Q

What are the equations for PSRR? What are the units?

A

PSRR = ∆Vdiff / ∆VPSU

PSRR = -20Log(∆Vdiff / ∆VPSU)

Units: μV/V or dB

33
Q

What is thermal noise performance?

A

> Main source of noise in op-amps

> Noise power is proportional to both temperature and bandwidth.

> Function of resistance (External resistors will add thermal noise)

> Units V/√Hz

34
Q

What is thermal drift?

A

> The characteristics of the op-amp will change depending on the temperature of the device.

35
Q
A