Class 5 – Aol, Bandwidth, and Slew Rate

1. Class 5 – Aol, Bandwidth, and Slew Rate Nov 18, 2011

2. Open Loop Gain Aol

3. Remember from Class 2 – Close Loop Gain

4. Aol for different Amplifiers

5. Aol on OPA369 Let’s examine what happens to closed loop gains for variations of Aol Note that Aol can be strongly effected by temperature

6. Closed Loop DC Gain Error(Error vs Gain Magnitude) Ideal Closed Loop Gain = 2 Avol = 120dB Ideal Closed Loop Gain = 101 Avol = 120dB

7. Closed Loop DC Gain Error(Error vs Gain Magnitude) Avol = 134dB, Gain = 101 Avol = 114dB, Gain = 101

8. Closed Loop DC Gain Error(Error vs Gain Magnitude) Avol = 134dB, Gain = 101 Avol = 90dB, Gain = 101 Over Temp!

9. AC Gain Error

10. Loop Gain= (Aol) – (Closed Loop Gain)

11. Aol Error vs. Frequency is a bandwidth limit

12. Bandwidth

13. Using the Gain Bandwidth Product

14. Dominant Pole

15. Gain Bandwidth Constant for 20dB/decade slope

16. Amplifiers where GBW is Different over Different Frequency ranges

17. Simulation – Noninverting Gain of 100

18. Bandwidth Limits Inverting Gain vs Noninverting gain

20. Slew Rate

21. Remember from Class 1Constant Current -> Voltage Changes Linearly

22. Slew Limit • For slow moving or small signals iout < iout(max) • For large rapid moving signals iout = iout(max) • The output is slew rate limited • This is the fastest rate the output can change • The input is no longer a virtual short • Large input differential voltages are possible • Iout is constant so vout increases linearly across capacitor

23. Slew Rate – Inside the Amplifier

24. Slew Rate for Different Amplifiers

25. Simulate Slew Rate OPA2188 Looking at the slope of the output signal. The rate of change is the slew rate.

26. Simulate Slew Rate OPA2188 The input no longer has the virtual short. The output changes cant keep up with the input.

27. Large Signal Step Response OPA827(Shows Slew Rate and Settling Time) SR = 10V / 0.4uS = 25V/uS (from graph) SR = 28V/uS (from data sheet table)

28. Slew Rate Over Temperature

29. Slew Boost Slows down as Vout approaches Vin.

30. Current to Miller Capacitance IccWith and With out Slew Boost

31. Settling Time

32. Settling Time Slew Rate Note: Settling Time includes Slew Rate time

33. Settling Time vs. Closed Loop Gain

34. Simulating Settling Time – OPA827

35. Simulating Settling Time Output Slew Output Settles Step input Settling time includes the slew and settling. We will zoom in on settling region. Step starts when step is applied at 1uS

36. Simulating Settling Time Use the post processor to set Limits Post processor Create limits for +/- 0.01% 9.999V and 10.001V

37. Simulating Settling Time 0.1% settling for a 10V signal 10V x 0.01% = 1mV |Output – 10V| < 1mV Data Sheet = 550nS Simulated = 400nS

38. Small Signal Step Response

39. Simulation vs. Data Sheet

40. Simulation Configuration Zoom in on rising Edge

41. Does a small signal Step put the device into slew limit? Rate of Change on Output Signal: ΔV/ Δt = 72.3mV / 6.17ns ΔV/ Δt = 11.7V/us From Data Sheet Slew Rate = 28V/uS This is what you would expect with a real device. A small signal step will NOT put the amplifier into slew limit.

42. The “Ringing” in small signal step response is related to stability Larger gain less overshoot gain=-1 is 2

43. Op-Amp + Cap on OutputSecond order system

44. Small Signal Step Rise Time (Relationship to Bandwidth)

45. Small Signal Step Rise Time (Relationship to Bandwidth)

46. Calculated vs. Simulated Small Signal Rise Time

47. Small Signal Step ResponseRise Time is independent of step size

48. In Slew Rate LimitRise Time Depends on Step Size

49. Max Output vs. Freq Full Power BW

50. Maximum Output vs Frequencyalso called Full Power Bandwidth For Vs = +/-15V 10Vpk Distortion! 7.5Vpk No Distortion!