Op-amp technicalities Flashcards
What are the limits of op-amps? (5 limits, 1 outcome)
> 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.
What makes a system unstable?
When the output is not entirely controlled
How does feedback impact a systems stability? What is the purpose of feedback?
> 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.
When does instability occur?
> When the phase of the feedback sifts by π radians (180°).
What happens to the equations for the inverting and non-inverting amplifiers when instability occurs?
The denominators f both equations become 1 - AB.
What are the 3 different cases of positive feedback signals?
> Small positive feedback: -1 < AB < 0
> Unstable system: AB = -1
> Large positive feedback: AB < -1
What happens when there is small positive feedback?
- 1 + AB < 1 - G > A
- The system is not strictly unstable but it is approaching instability.
- The gain is increasing rapidly.
- G ⇒ ∞
What happens when there is positive feedback and the system is unstable?
- 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
What happens when there is large positive feedback?
> 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 Gain equations are no longer valid in the region beyond where G ⇒ ∞
How does instability occur?
> 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
What is the solution that op-amps use to prevent instability?
> 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
What are the properties of the low pass dominant-pole frequency filter?
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.
What is phase margin?
> 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
Explain the different types of op-amp compensations
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
How do you calculate the gain error?
- Calculate the resistor configuration that will obtain the closed-loop gain that you wish to have
- Calculate the value of B for that configuration
- Decide what frequency you wish the op-amp to operate at
- Use the response graph to calculate the open-loop gain (A) at that frequency
- Insert the value of A and B into the gain equation for that type of op-amp and see what the actual gain becomes