Chapter 4 Analog CMOS Subcircuits

1 Allen/Holberg : Chapter 4 : 1/13/011 Chapter 4 Analog cmos SubcircuitsFrom the viewpoint of Table , the previous two chapters have provided thebackground for understanding the technology and modeling of cmos devices andcomponents compatible with the cmos process. The next step toward our objective methodically developing the subject of cmos Analog - circuit design is to developsubcircuits. These simple circuits consist of one or more transistors; they are simple; andthey generally perform only one function. A subcircuit is typically combined with othersimple circuits to generate a more complex circuit function. Consequently, the circuits ofthis and the next Chapter can be considered as building operational amplifier, or op amp, to be covered in Chapters 6 and 7, is a goodexample of how simple circuits are combined to perform a complex function.

2 Figure presents a hierarchy showing how an operational amplifier a complex circuit mightbe related to various simple circuits. Working our way backward, we note that one of thestages of an op amp is the differential amplifier. The differential amplifier consists ofsimple circuits that might include a current sink, a current-mirror load, and a source-coupled pair. Another stage of the op amp is a second gain stage, which might consist ofan inverter and a current-sink load. If the op amp is to be able to drive a low-impedanceload, an output stage is necessary. The output stage might consist of a source followerand a current-sink load.

3 It is also necessary to provide a stabilized bias for each of theprevious stages. The biasing stage could consist of a current sink and current mirrors todistribute the bias currents to the other AmplifierBiasing CircuitsInput Differential AmplifierSecond Gain StageOutput StageCurrent SourceCurrent MirrorsCurrent SinkCurrent Mirror LoadInverterCurrent Sink LoadSource FollowerCurrent Sink LoadSource Coupled PairFigure Illustration of the hierarchy of Analog circuits for an operational subject of basic cmos Analog circuits has been divided into two chapters toavoid one lengthy Chapter and yet provide sufficient detail. Chapter 4 covers the simplersubcircuits, including: the MOS switch, active loads, current sinks/sources, currentmirrors and current amplifiers, and voltage and current references.

4 Chapter 5 willexamine more complex circuits like cmos amplifiers. That Chapter represents a naturalextension of the material presented in Chapter 4. Taken together, these two chapters arefundamental for the Analog cmos designer's understanding and capability, as mostdesigns will start at this level and progress upward to synthesize the more complexcircuits and systems of Table MOS SwitchAllen/Holberg : Chapter 4 : 1/13/012 The switch finds many applications in integrated- circuit design . In Analog circuits,the switch is used to implement such useful functions as the switched simulation of aresistor [1]. The switch is also useful for multiplexing, modulation, and a number of otherapplications.

5 The switch is used as a transmission gate in digital circuits and adds adimension of flexibility not found in standard logic circuits. The objective of this sectionis to study the characteristics of switches that are compatible with cmos begin with the characteristics of a voltage-controlled switch. Figure showsa model for such a device. The voltage vC control s the state of the switch ON or voltage-controlled switch is a three-terminal network with terminals A and Bcomprising the switch and terminal C providing the means of applying the controlvoltage vC. The most important characteristics of a switch are its ON resistance, rON, andits OFF resistance , rOFF.

6 Ideally rON is zero and rOFF is infinite. Reality is such that rONis never zero and rOFF is never infinite. Moreover, these values are never constant withrespect to terminal conditions. In general, switches can have some form of voltage offsetwhich is modeled by VOS of Fig. VOS represents the small voltage that may existbetween terminals A and B when the switch is in the ON state and the current is represents the leakage current that may flow in the OFF state of the switch. CurrentsIA and IB represent leakage currents from the switch terminals to ground (or some othersupply potential). The polarities of the offset sources and leakage currents are not knownand have been arbitrarily assigned the directions indicated in Fig.

7 The parasiticcapacitors are an important consideration in the application of Analog sampled-datacircuits. Capacitors CA, and CB, are the parasitic capacitors between the switch terminalsA and B and ground. Capacitor CAB is the parasitic capacitor between the switchterminals A and B. Capacitors CAC and CBC are parasitic capacitors that may existbetween the voltage-control terminal C and the switch terminals A and B. Capacitors CACand CBC contribute to the effect called charge feedthrough where a portion of thecontrol voltage appears at the switch terminals A and Model for a nonideal : Chapter 4 : 1/13/013 One advantage of MOS technology is that it provides a good switch.

8 Figure a MOS transistor that is to be used as a switch. Its performance can be determinedby comparing Fig. with the large-signal model for the MOS transistor. We see thateither terminal, A or B, can be the drain or the source of the MOS transistor dependingupon the terminal voltages ( , for an n-channel transistor, if terminal A is at a higherpotential than B, then terminal A is the drain and terminal B is the source). The ONresistance consists of the series combination of rD, rS, and whatever channel resistanceexists. Typically, by design , the contribution from rD and rS is small such that theprimary consideration is the channel resistance.

9 An expression for the channel resistancecan be found as follows. In the ON state of the switch, the voltage across the switchshould be small and vGS should be large. Therefore the MOS device is assumed to be inthe nonsaturation region. Equation (1) of Sec. , repeated below, is used to model = K'WL (VGS VT)VDS V 2DS2(1)Figure An n-channel transistor used as a VDS is less than VGS VT but greater than zero. (VGS becomes VGD if VDS isnegative.) The small-signal channel resistance given asrON = 1 ID/ VDS = LK'W(VGS VT VDS)(2)Figure illustrates drain current of an n-channel transistor as a function of thevoltage across the drain and source terminals, plotted for equal increasing steps of VGSfor W/L = 5/1.

10 This figure illustrates some very important principles about MOStransistor operation. Notice that the curves are not symmetrical about V1 = 0. This isbecause the transistor terminals (drain and source) switch roles as V1 crosses zero example, when V1 is positive, node B is the drain and node A is the source and VBS isfixed at volts and VGS is fixed as well (for a given VG). When V1 is negative, node Bis the source and node A is the drain and as V1 continues to decrease, VBS decreases andVGS increases resulting in an increase in : Chapter 4 : 1/13/014 Figure I-V characteristic of an n-channel transistor operating as a (mA) (volts) = 1 VVG = 2 VVG = 3 VVG = 4 VVG = 5 VABA plot of rON as a function of VGS is shown in Fig.