拉扎维CMOS课件第四单元.pptx

Differential Amplifiers 1.Single-ended and differential way of working A single-ended signal is defined as one that is measured with respect to a fixed potential,usually the ground.A differential signal is defined as one that is measured between two nodes that have equal and opposite signal excursions around a fixed potential. The “center” potential in differential signaling is called the “common-mode” (CM) level. Advantages of differential (compared with single-ended) Transitions on line L2 corrupt the signal on line L1. If the clock line is placed midway between the two,the transitions disturb the differential phases by equal amounts, leaving the difference intact. if VDD varies by △V,then Vout changes by approximately the same amount So the differential signal can also increase the maximum voltage swing Advantages of differential (compared with single-ended) 2.Basic Diffrential Pair The first thing we think of is incorporating two identical single-ended signal paths to process the two phases. thus varying both the transconductance of the devices and the output CM level. 2.1 Qualitative Analysis----input and output characteristics Two important attributes. 2.1 Qualitative Analysis----commomn-mode behavior The role of tail current source is to suppress the effect of input CM level variations on the operation of M1 and M2 and the output level. According to M1 M2 M3 turn on and are saturated ,we recive The output voltage swings of a differential pair But because the op-amp usually need at least two stage amplifier to get actual magnification witch can be used, so reduce the demand for former swing . 2.2 Quantitative Analysis Squaring the two sides,we obtain The small-single differential voltage gain of the circuit in the equilibrium condition as The small-signal differential voltage gain The samll-signal behavior of differentia pairs 3.Common-Mode Respons We first assume the circuit is symmetric but the current source has a finite output inmpedance,RSS. Next we