Passive, active, and digital filters (3ed., CRC, 2009) - tiera.ru

17-22 Passive, Active, and Digital Filtersduring f 1 and hold it during f 2 while the second op-amp do the same functions during the complementaryclock phases. In general, having the differential outputs available makes the system more flexible andgives several advantages while designing high-performance analog signal processors.Fully differential amplifiers are widely used technique to reduce the effect of charge injection,clock feedthrough, and better rejection to common-mode signals such as substrate and powersupply noise and an improved dynamic range when compared to single-ended circuits. As long as thenoise present at the output of the amplifier is present in both outputs of the amplifier with thesame amplitude and same phase, it will be rejected by the differential nature of the following stage.In addition, an additional loop that operates on the common-mode signals presents low outputimpedance for these signals, it fix the operating point of the differential outputs at the desired leveland further minimizes the common-mode output fluctuations up to the unity-gain frequency of the loop.Their disadvantage comes from the fact that they require a common-mode feedback (CMFB) circuit.The fully differential version of the folded cascode amplifier is depicted in Figure 17.18a. It requires thesame amount of power as the single-ended version shown in Figure 17.11. Since the current mirrorpresent in the single-ended amplifier for the conversion of the fully differential current of the differentialpair into single-ended output is eliminated, the fully differential amplifier has a single internal polelocated at the source terminal of M 3 . Since several parasitic poles and phantom zeros are eliminated,the fully differential amplifier has inherently better phase margin than its single-ended counterpart. Thecircuit is fully symmetric and any noise injected through M 0 is evenly split by the differential pair and willappear at the two outputs as a pure common-mode signal. The common-mode noise present at thetransistors M 1 is further attenuated by the small sensitivity of the differential pair to common-modeV B3M 2V DDM 2V B2M 3V B2M 4M 3V out–V in+ M 1 M 1V B1M 4M 5 V CMFB V B M 0V in–V CMFBV B1M 5V out+(a)V SSv 0+v 0–V DDV O-DCφ 2 φ 1φ 1 φ 2VO-DCC S C C C SV refφ 2 φ 1φV 1 φ 2 CMFBV refC CV SSV refM 4M 5I BIASV B1(b)FIGURE 17.18(a) Fully differential folded cascode amplifier and (b) switched-capacitor based CMFB circuit.