CMOS Operational-Amplifier

Transcription

1 CMOS Operational-Amplifier 1

2 What will we learn in this course How to design a good OP Amp. Basic building blocks Biasing and Loading Swings and Bandwidth CH2(8) Operational Amplifier as A Black Box Copyright 2004 by Oxford University Press, Inc. 2

3 Figure 7.40 Two-stage CMOS op-amp configuration. 3

4 Copyright 2004 by Oxford University Press, Inc. Figure 6.1-8

5 Copyright 2004 by Oxford University Press, Inc. Figure 6.1-9

6 Copyright 2004 by Oxford University Press, Inc. Figure 6.1-1

7 Figure 7.41 Equivalent circuit of the op amp in Fig

8 Figure 7.42 Bias circuit for the CMOS op amp. 8

9 Figure P7.2 9

10 Figure P

11 Figure P

12 Figure P

13 Figure 9.1 The basic two-stage CMOS op-amp configuration. 13

14 Figure 9.2 Small-signal equivalent circuit for the op amp in Fig

15 Figure 9.3 An approximate high-frequency equivalent circuit of the two-stage op amp. This circuit applies for frequencies f f P1. 15

16 Figure 9.4 Typical frequency response of the two-stage op amp. 16

17 Figure 9.5 Small-signal equivalent circuit of the op amp in Fig. 9.1 with a resistance R included in series with C C. 17

18 Figure 9.6 A unity-gain follower with a large step input. Since the output voltage cannot change immediately, a large differential voltage appears between the op-amp input terminals. 18

19 Figure 9.7 Model of the two-stage CMOS op amp of Fig. 9.1 when a large differential voltage is applied. 19

20 Figure 9.8 Structure of the folded-cascode CMOS op amp. 20

21 Figure 9.9 A more complete circuit for the folded-cascode CMOS amplifier of Fig

22 Figure 9.10 Small-signal equivalent circuit of the folded-cascode CMOS amplifier. Note that this circuit is in effect an operational transconductance amplifier (OTA). 22

23 Figure 9.11 A folded-cascode op amp that employs two parallel complementary input stages to achieve rail-to-rail input common-mode operation. Note that the two + terminals are connected together and the two terminals are connected together. 23

24 Figure 9.12 (a) Cascode current mirror with the voltages at all nodes indicated. Note that the minimum voltage allowed at the output is V t + V OV. (b) A modification of the cascode mirror that results in the reduction of the minimum output voltage to V OV. This is the wide-swing current mirror. 24

25 Figure E9.8 25

26 Figure E

27 Figure 9.31 Cascading the small-signal equivalent circuits of the individual stages for the evaluation of the overall voltage gain. 27

28 Figure 9.32 Bode plot for the 741 gain, neglecting nondominant poles. 28

29 Figure 9.33 A simple model for the 741 based on modeling the second stage as an integrator. 29

30 Figure 9.34 A unity-gain follower with a large step input. Since the output voltage cannot change instantaneously, a large differential voltage appears between the op-amp input terminals. 30

31 Figure 9.35 Model for the 741 op amp when a large positive differential signal is applied. 31

32 Speed Limitation V in V A out 0 s V s 1 in2 1 1 Due to internal capacitances, the gain of op amps begins to roll off. CH2(8) Operational Amplifier as A Black Box Copyright 2004 by Oxford University Press, Inc. 32

33 Bandwidth and Gain Tradeoff Having a loop around the op amp (inverting, noninverting, etc) helps to increase its bandwidth. However, it also decreases the low frequency gain. CH2(8) Operational Amplifier as A Black Box Copyright 2004 by Oxford University Press, Inc. 33

34 Slew Rate of Op Amp In the linear region, when the input doubles, the output and the output slope also double. However, when the input is large, the op amp slews so the output slope is fixed by a constant current source charging a capacitor. This further limits the speed of the op amp. CH2(8) Operational Amplifier as A Black Box Copyright 2004 by Oxford University Press, Inc. 34

35 Comparison of Settling with and without Slew Rate As it can be seen, the settling speed is faster without slew rate (as determined by the closed-loop time constant). CH2(8) Operational Amplifier as A Black Box Copyright 2004 by Oxford University Press, Inc. 35

36 Slew Rate Limit on Sinusoidal Signals dv out dt R 1 V0 1 cos t R2 As long as the output slope is less than the slew rate, the op amp can avoid slewing. However, as operating frequency and/or amplitude is increased, the slew rate becomes insufficient and the output becomes distorted. CH2(8) Operational Amplifier as A Black Box Copyright 2004 by Oxford University Press, Inc. 36

37 Maximum Op Amp Swing V out V max V 2 min V sin t V 2 To determine the maximum frequency before op amp slews, first determine the maximum swing the op amp can have and divide the slew rate by it. CH2(8) Operational Amplifier as A Black Box Copyright 2004 by Oxford University Press, Inc. 37 max min FP SR V max V min 2

38 Nonzero Output Resistance v v out in R R R R In practical op amps, the output resistance is not zero. It can be seen from the closed loop gain that the nonzero output resistance increases the gain error. A 0 out 2 R R A out 1 0 R R 1 2 CH2(8) Operational Amplifier as A Black Box Copyright 2004 by Oxford University Press, Inc. 38

39 Copyright 2004 by Oxford University Press, Inc. Figure 6.2-3

40 Copyright 2004 by Oxford University Press, Inc. Figure 6.2-4

41 Copyright 2004 by Oxford University Press, Inc. Figure 6.2-5

42 Copyright 2004 by Oxford University Press, Inc. Figure 6.2-6

43 Copyright 2004 by Oxford University Press, Inc. Figure 6.2-7

44 Copyright 2004 by Oxford University Press, Inc. Figure 6.3-1

45 Copyright 2004 by Oxford University Press, Inc. Figure 6.3-2

46 Copyright 2004 by Oxford University Press, Inc. Figure 6.3-3