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.
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=VoutAt low frequencies, M3 carries ISS/2,VGS3 is constant for a bias currentof ISS/2, therefore, noise from VDDcouples 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 notcontributemuch noise)
32 Noise in a Telescopic Op-Amp Observation:1. Low impedance pathto output via M3.2. Divde Vout, M1 by Av2(Flicker noise)Account for M1 and M2
33 Rule of ThumbMentally 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 contributemuch noise