Download presentation

Presentation is loading. Please wait.

1
**Operational Amplifier (2)**

Chapter 9

2
**Topics Two-Stage Op-Amps Gain-Boosting Input Rage Limitation Slew Rate**

Power Supply Rejection Noise

3
**Simple Implementation of a Two-Stage Op-Amp**

4
**Two-Stage Op-Amp Employing Cascoding**

(small voltage swings) High gain stage

5
**Two-Stage Op-Amp With Single-Ended Output**

6
Gain-Boosting Idea behind gain boosting: increase the output impedance without adding more cascode devices.

7
**Increasing the Output Impedance by Feedback**

Io Io is sensed by ro1, convert into voltage, subtracted Vb. Current-Voltage Feedback. Loop gain Increased by A1

8
**Output Resistance of a Source Degenerated Transistor**

9
**Gain Boosting Using Feedback Analysis**

10
Implementation (Small signal gain) Vout, min=VOD2+VGS3

11
**Differential Implementation**

Minimum voltage at the drain of M3:

12
**Folded Cascode Gain Boosting**

(Minimum Vx)

13
**Various Implementations of Gain Boosting**

14
**Input Range Limitations**

(Vin is input limited, as opposed to output limited)

15
**Extension of input CM Range**

As Vin, cm →VDD, the PMOS input pair turns off. As Vin, cm →0, the NMOS input pair turns off.

16
Slew Rate “Linear settling” is only applicable to sufficiently small inputs. With a large input step, the output displays a linear ramp with a constant slope. The slope of the ramp is called the slew rate. While the small signal bandwidth of a circuit suggests a fast time-domain response, the large signal speed may be limited by the slew rate simply because the current available to charge the dominant capacitor is limited.

17
**Response of a linear circuit to an input step**

The slope of the step response is proportional to the final value of the output; if we apply a larger input step, the output rises more rapidly.

18
**Response of a linear circuit to an input step**

19
**Linear Op-amp to Step Response**

20
**Linear Settling in Time Domain**

21
**Slewing in an Op-Amp Circuit**

22
**Slewing During Low to High Transition**

23
**Slewing During High to Low Transition**

24
Slewing Slewing is a nonlinear phenomenon. If the input doubles, the output level does not double at all points because the ramp exhibits a slope independent of the input!

25
**Slewing in telescopic Op-Amp**

26
**Slewing in a Folded Cascode Op-Amp**

27
**Power Supply Rejection**

Op-Amps are supplied from noisy lines, and must “reject” the noise adequately. Power Supply Rejection Ratio (PSRR) is defined as the gain from input to the output divided by the gain from the supply to the output.

28
**Example (1) If M3 and M4 carry the same amount current, then**

VGS3=VGS4=VDS3=VDS4. Therefore VX=Vout At low frequencies, M3 carries ISS/2, VGS3 is constant for a bias current of ISS/2, therefore, noise from VDD couples directly to VX. Since VX=Vout, the VDD noise is coupled to Vout, with a gain of unity. The PSRR at low frequencies:

29
**Example (2) Calculate the Low Frequency PSRR of the feedback circuit**

(KCL) (KVL)

30
**Example (3) (PSRR) β=C1/(C1+C2), Vout/Vin=1/ β=1+C2/C1**

(Low frequencies analysis, C1 and C2 do not draw any current) (PSRR) β=C1/(C1+C2), Vout/Vin=1/ β=1+C2/C1

31
**Noise in a Telescopic Op-Amp**

(Do not contribute much noise)

32
**Noise in a Telescopic Op-Amp**

Observation: 1. Low impedance path to output via M3. 2. Divde Vout, M1 by Av2 (Flicker noise) Account for M1 and M2

33
Rule of Thumb Mentally change the gate voltage of each transistor by a small amount and predict the effect at the output.

34
**Noise in a Folded Cascode Circuit**

Do not contribute much noise

35
Noise Analysis

36
Equivalent CS Stages

37
Noise due to M7

38
**Noise-Voltage Swing Trade-Off**

If the VOD of M9 and M10 is Reduced to increase output swing, the noise of M9 will increase.

39
**Noise in a Two-Stage Op-Amp**

Noise of stage 2 not so significant

40
**Summary (Telescopic) (Folded cascode, Only thermal noise is included)**

(Two Stage Op-Amp)

Similar presentations

© 2021 SlidePlayer.com Inc.

All rights reserved.

To make this website work, we log user data and share it with processors. To use this website, you must agree to our Privacy Policy, including cookie policy.

Ads by Google