1. Compensation Methods Electronic Engineering

2. The following presentation is a part of the level 5 module -- Electronic Engineering. This resources is a part of the 2009/2010 Engineering (foundation degree, BEng and HN) courses from University of Wales Newport (course codes H101, H691, H620, HH37 and 001H). This resource is a part of the core modules for the full time 1 st year undergraduate programme.Engineering (foundation degree, BEng and HN) The BEng & Foundation Degrees and HNC/D in Engineering are designed to meet the needs of employers by placing the emphasis on the theoretical, practical and vocational aspects of engineering within the workplace and beyond. Engineering is becoming more high profile, and therefore more in demand as a skill set, in today’s high-tech world. This course has been designed to provide you with knowledge, skills and practical experience encountered in everyday engineering environments. Contents Instructions Gain Curve Compensation Methods Dominant Pole Compensation Frequency Compensation Lead Lag Compensation. Summary Credits In addition to the resource below, there are supporting documents which should be used in combination with this resource. Please see: Clayton G, 2000, Operational Amplifiers 4th Ed, Newnes James M, 2004, Higher Electronics, Newnes Compensation Methods

3. For our amplifier we are going to apply negative feedback to reduce the gain to 55dB. Comment on the amplifiers stability. Compensation Methods

4. Gain Curve f C1 f C2 Phase Curve -40dB/dec

5. For our amplifier we are going to apply negative feedback to reduce the gain to 55dB. Comment on the amplifiers stability. Unstable as it crosses the Gain Curve on the -40dB/dec line. The amplifier is therefore unsuitable for this application. Can we do anything about it? Compensation Methods

6. There are three methods which we will examine. 1. Dominant Pole compensation 2. Frequency compensation 3. Lead Lag compensation. Compensation Methods

7. Dominant Pole Compensation In this method the Manufacturer introduces an artificial break frequency (pole) which causes the gain to drop to 0dB before the first natural one occurs. f C1 f C2 Unstable Stable New f C Original Curve New Curve gain frequency Desired gain

8. Try this out on our plot and answer the following questions: At what frequency does the dominant pole need to be placed? What is the Bandwidth of the compensated amplifier? Compensation Methods

9. Gain Curve f C1 f C2 Phase Curve x 9.5dB – 5.1kHz 29.5dB – 510Hz 89.5dB – 0.51Hz 15dB – 2.8kHz 35dB – 280Hz 55dB – 28Hz

10. Try this out on our plot and answer the following questions: At what frequency does the dominant pole need to be placed? What is the Bandwidth of the compensated amplifier? 0.51 Hz 28 Hz Compensation Methods

11. Notes This method is stable for all applications. The user does not need to carry out the compensation exercise. Bandwidths are limited in size using this method. Compensation Methods

12. Frequency Compensation This method is similar to the first in that a dominant pole is introduced. This time its position is selected by the user and is positioned so that the gain drops to the desired gain at the point the first natural break frequency occurs. f C1 f C2 Unstable Stable New f C Original Curve New Curve gain frequency Desired gain Compensation Methods

13. A table supplied by the amplifier manufacturer allows the user to convert the new pole frequency measured from the plot into a capacitor value which is connected between two pins on the amplifier. Compensation Methods

14. Try this out on our plot and answer the following questions: At what frequency does the dominant pole need to be placed? What is the Bandwidth of the compensated amplifier? Compensation Methods

15. Gain Curve f C1 f C2 Phase Curve x 69.5dB – 3kHz 89.5dB – 300Hz f C1 – 15kHz

16. Try this out on our plot and answer the following questions: At what frequency does the dominant pole need to be placed? What is the Bandwidth of the compensated amplifier? 300 Hz 15 kHz (f C1 ) Compensation Methods

17. Notes The user must compensate each amplifier according to its use. The Bandwidth, using this method is always at the first break frequency. Once compensated if the gain is reduced the amplifier will become unstable. Compensation Methods

18. Lead Lag Compensation. Let us examine the problem we have with our amplifier. f C1 f C2 Unstable Original Curve gain frequency Desired gain The amplifier would be stable if either f C1 occurred at a lower frequency or f C2 occurred at a higher frequency Compensation Methods

19. We cannot move the two break frequencies as they are inherent parameters of the amplifier. BUT We can make f C1 “look” as though it started at a lower frequency using the circuit below. R1 R2 C V IN V OUT Compensation Methods

20. Series combination of R2 and C Now we can generate an equation for V OUT in terms of V IN Compensation Methods

21. Quantative Analysis At low frequencies ω parts << 1 so the gain = 1 phase will be 0° At high frequencies ω parts >> 1 so the gain = R2/(R1 + R2) phase will be 0° In between gain must roll off in some way. e.g. R1 = 10k , R2 = 1k , C = 10nF Plot over the range 100 Hz to 100 kHz

23. FrequencyGainGain dBPhase 1000.99764-0.0205249-1.99651 2000.990659-0.0815145-3.97231 4000.964149-0.3171164-7.78572 7000.901053-0.9049962-12.9442 10000.82426-1.6787185-17.2528 20000.590744-4.572012-26.0858 40000.350726-9.1006474-31.1146 70000.221129-13.107079-30.3226 100000.169115-15.436358-27.5696 200000.115878-18.719977-19.0967 400000.097777-20.195268-10.903 700000.093209-20.610813-6.45841 1000000.092044-20.720125-4.5634

24. Compensation Methods

25. Referring to our equation for the network, we will have two break frequencies, one for the top line and one for the bottom. Break frequency where roll off begins f 1 is given by: Break frequency where roll off stops f 2 is given by: Gain after roll-off is given by Compensation Methods

26. x x f1f1 f2f2 -20dB/dec Compensation Methods

27. Note The phase is not a concern as it returns to a low value by the time the gain curve stops reducing. Let us return to our amplifier. Compensation Methods

28. f C1 f C2 Unstable Original Curve gain frequency Desired gain Introduce a lead lag network which starts before f C1 and stops at f C1 f1f1 f2f2 Stable f C1 now looks as though it starts at f 1 Compensation Methods

29. Try this out on our plot and answer the following questions: What component values do we need – assume C has a value of 100pF What is the Bandwidth of the compensated amplifier? Compensation Methods

30. Firstly f 2 must equal f C1 Secondly what drop in gain is required Compensation Methods

31. Gain Curve f C1 f C2 Phase Curve Required drop in gain = -10dB

32. Firstly f 2 must equal f C1 Secondly what drop in gain is required Finally, what is f 1 Compensation Methods

33. Gain Curve f C1 f C2 Phase Curve x x f1f1 f2f2 Bandwidth f C2

34. Try this out on our plot and answer the following questions: What component values do we need – assume C has a value of 100pF What is the Bandwidth of the compensated amplifier? 229k  106k  100pF V IN V OUT f C2 = 300kHz Compensation Methods

35. Notes The user must compensate each amplifier according to its use. The Bandwidth, using this method is always at the second break frequency. Once compensated if the gain is reduced the amplifier will become unstable. Compensation Methods

36. Summary No compensation – Unstable Dominant Pole Compensation – B.W. = 28Hz Frequency Compensation – B.W. = 15kHz Lead Lag Compensation - B.W. = 300kHz Compensation Methods