Submitted by- RAMSHANKAR KUMAR S7,ECE, DOE,CUSAT Division of Electronics Engineering, SOE,CUSAT1

INTRODUCTION Many low voltage design techniques have been developed to meet the needs of present era of low power portable electronic equipment, which drove the analog designers to look for innovative design techniques like self cascode MOSFETs With decreasing device dimensions, the intrinsic gain of the single MOSFET amplifier decreases. Common method to achieve higher gain is – Cascode Structure Cascode device shields the bottom transistor from variations in the output, thus achieving the benefits of high output impedance. It reduces the ratio errors due to difference in output and input voltages. Division of Electronics Engineering, SOE,CUSAT2

1. TECHNIQUE FOR IMPROVING ANALOG CIRCUIT PERFORMANCE. 2. IT IS A CONTRACTION OF THE PHRASE “CASCADE TO CATHODE”. 3. FIRST USED BY F.V.HUNT AND R.W.HICKMAN. Division of Electronics Engineering, SOE,CUSAT3

 IT IS A SINGLE STAGE AMPLIFIER WITH GAIN ENHANCEMENT.  WHILE A CASCADE IS TWO STAGE AMPLIFIER.  IT HAS SINGLE NODE IS AT HIGHER IMPEDENCE,WHERE GAIN IS REALIZED.  IN THIS GBW IS DETERMINED BY INPUT TRANSCONDUCTANCE AND LOAD CAPACITANCE. Division of Electronics Engineering, SOE,CUSAT4

Cascode and cascade amplifier Cascode amplifier is a one stage amplifier with gain enhancement at low frequencies. It has one node at higher impedance. Cascade amplifier is a 2 stage amplifier with 2 nodes at higher impedance which can cause stability problems. It is eliminated by Miller capacitance across second stage. Introduction of capacitance increases the power consumption So, cascode amplifier gives increased performance. Division of Electronics Engineering, SOE,CUSAT5

The cascode connection is an effective method to suppress the channel- length modulation and thus enhance the working. Here the cascode device “shields” the bottom transistor from variations in the output, which results in high output impedance. It also reduces the ratio errors due to difference in output and input voltages. The idea behind cascode structure is to convert the input voltage to a current and apply the result to a common source stage. This has been employed in various LV topologies Division of Electronics Engineering, SOE,CUSAT6

Single Transistor stage In this a single transisotr is biased by a DC current source IB. Small signal input current is superimposed on the drain current. Putting the input current source with another transistor will give 2 transistor cascode. Division of Electronics Engineering, SOE,CUSAT7

Voltage gain = product of gains both transistors In single transistor, output resistance is rds1. In a transistor with another r ds in its source it is r ds (g m r ds ) 2 So larger the output impedance larger the gain Best method to improve gain- putting a cascode on to of a single transistor amplifier. Other gain methods- Gain boosting Bootstrapping Current cancellation Starving schemes Division of Electronics Engineering, SOE,CUSAT8

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Composite Cascode Connection Self cascode is a 2 transistor structure which can be treated as a single composite transistor. Composite structure has high effective channel length and lower effective output conductance. Division of Electronics Engineering, SOE,CUSAT15

Lower transistor M1 is equivalent to a resistor. The composite structure has both the gates of M1 and M2 driven by the input signal and share a single a single bias source V GG. Division of Electronics Engineering, SOE,CUSAT16 ConditonM1M2Feature M2 and M1 have similar w/L ratio Triode region Active region Composite cascode works like common source stage, but with higher voltage gain W/L ratio of M2 > M1, m>>1 (optimum operation ) Effective gm of the composite structure is g m =g m2 /m

For the composite transistor to be in saturation M1-Linear,M2-Saturation In this case the derived relations are, In cascode structure, M1 operates in linear region, while M2 operates in saturation or linear region. So voltage between source and drain of M1 is small. Division of Electronics Engineering, SOE,CUSAT17

V-I characteristics at constant area Division of Electronics Engineering, SOE,CUSAT18

V-I characteristics of β Division of Electronics Engineering, SOE,CUSAT19

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ALSO KNOWM AS O/P IMPEDENCE CASCODE AMPLIER. USED TO FURTHER INCREASE THE O/P IMPEDENCE WITHOUT USING MORE CASCODE DEVICES. SOURCE OF M2 AND DRAIN OF M1 IS AT SAME VOLTAGE. Rout =A1Gm2Rds1Rds2 Division of Electronics Engineering, SOE,CUSAT21

IT USES NEGATIVE FEEDBACK AMPLIFIER TO FORCE THE SOURCE OF M2(THE DRAIN OF M1) BE AT THE SAME VOLTAGE. OUTPUT RESISTANCE OF GAIN BOOSTED CASCODE AMPLIFIER IS FURTHER ENHANCEHED BY A1 COMPARE TO THAT OF CONVENTIONAL CASCODE AMPLIFIER. IT IMPROVE THE GAIN BUT FURTHER REDUCES THE O/P SWING. Division of Electronics Engineering, SOE,CUSAT22

Self cascode Technique for LV topologies Widely used gain boosting scheme in analog design Here, it is power efficient to increase the output resistance by lowering the bias current. To increase gain of the stage Transconductance of the stage can be improved Output resistance seen by the first or second stage has to be increased. The transconductance(g m ) and the output resistance (r ds )increases with the bias current as follows Division of Electronics Engineering, SOE,CUSAT23

Inferences Operating voltage of regular cascode is much higher than SC and SC is suitable for Low voltage design SC offers higher output impedance than the regular cascode structure. But the output voltage requirements are similar to the simple transistor. Division of Electronics Engineering, SOE,CUSAT24

Similar results can be obtained for conventional cm and cascode conventional CM shown in the following figures. Division of Electronics Engineering, SOE,CUSAT25

V-I characteristics of β in case of CM Division of Electronics Engineering, SOE,CUSAT26

Self cascode CM structures M1&M2, M3&M4, M7&M8 are pairs of cascode composite structures. They have replaced m1,m2,m3 in figure of VI characteristics for Beta. Division of Electronics Engineering, SOE,CUSAT27

It is seen that there are two distinct advantages of using cascode structures. Firstly, it provides higher output impedance similar to a regular cascode structure while output voltage requirements are similar to that of a single transistor It reduces the effect of miller capacitance on the input of the amplifier. Division of Electronics Engineering, SOE,CUSAT28

Limited input common mode range Small output swing Relatively high power supply requirements Division of Electronics Engineering, SOE,CUSAT29

Cascode amplifier is a suitable alternative for high gain Comparing with other SC structure, regulated and conventional structures indicate that self cascode approach is a good alternative for low voltage applications Division of Electronics Engineering, SOE,CUSAT30

S.S. Rajput, “Low voltage current mode circuit structures and their applications.” Ph.D. Thesis, Indian Institute of Technology, Delhi, S.S. Rajput and S.S. Jamuar, “Current mirrors: Essential units for analog circuit structures.” IETE J. of Education, vol. 4.3, no. 4, S.S. Rajput and S.S. Jamuar, “A high performance current mirror for low voltage designs.” In Proc. APCCAS-2000/IEEE, Tianjin, Dec. 2000, pp. 170–173, China. E. Sackinger and W. Guggenbuhl, “A high swing, high impedance MOS ascode current mirror.” IEEE J. Solid State Circuits, vol. 25, pp. 289–298, E. Sanchez-Sinencio and A.G. Andreou, ed., Low Voltage/Low Power Integrated Circuits and Systems. IEEE Press, A. Zeki and H. Kuntman, “Accurate and high output impedance current mirrors suitable for CMOS current output stages.” Electron. Lett., vol. 33, pp. 1042–1043, Division of Electronics Engineering, SOE,CUSAT31