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UNIT -V

UNIT -V. IC MOSFET Amplifiers. Outline. IC Amplifiers IC Biasing –current steering circuit MOSFET current sources Amplifiers with active load CMOS common source and source follower CMOS Differential amplifier. Introduction. Integrated-circuit fabrication technology imposes constraints

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UNIT -V

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  1. UNIT -V IC MOSFET Amplifiers

  2. Outline • IC Amplifiers • IC Biasing –current steering circuit • MOSFET current sources • Amplifiers with active load • CMOS common source and source follower • CMOS Differential amplifier

  3. Introduction • Integrated-circuit fabrication technology imposes constraints • large capacitors are not available • very small capacitors are easy to fabricate • One objective is to realize as many functions as possible using MOS transistors only. • Reduction of device size is of great concern.

  4. Why current source needed for IC biasing? • Resistors take too much space on the chip. Source degeneration with resistor is not implemented in ICs. • The goal of a good bias is to ensure drain current (ID) and Drain source voltageVDS would not change (e.g., due to temperature variation). One can force ID to be constant using a current source.

  5. IC Biasing • Biasing in IC design is based on the use of constant- current sources. • A constant dc current (called a reference I) is generated at one location and is then replicated at various other locations for biasing the various stages through a process known as current steering.

  6. Biasing Mechanism for ICs • MOSFET Circuits • The basic MOSFET current source • MOS current-steering circuits • The bias currents of the various stages track each other in case of changes in power-supply voltage or in temperature

  7. Current source • Two types of basic gain cells exist: • Common-source (CS) • Common-emitter (CE) • Both are loaded with constant-current source. • This is done because of difficulties associated with fabrication of exact resistances. • It also facilitates increased gain. • These circuits are referred to as current-source loaded / active loaded.

  8. current source - contd… • NMOS current source sinks current to ground • PMOS current source sources current from positive supply

  9. Basic current mirror • Simple current-source loads reduce the gain realized in the basic gain cell because of their finite resistance (usually comparable to the value of ro of the amplifying transistor).

  10. nMOSFET current source Two matching MOSFET transistors connected back-to-back, such that both have the same Gate-to-Source voltage

  11. nMOSFET current source - contd… • Need: Using the transistors geometries (W/L)1and (W/L)2as design parameters create a DC current Io, as long as transistor Q2 is in Saturation Mode

  12. PMOS Current source

  13. PMOS CURRENT SOURCE - contd…

  14. Basic current mirror - contd…

  15. Basic current mirror - contd… • Assume Q2 to be operating in saturation mode • To ensure that Q2 is saturated, the circuit to which the drain of Q2 is to be connected must establish a drain voltage Vo that satisfies the relationship

  16. Basic current mirror - contd… • Identical devices Q1 and Q2. The drain current of Q2, IO, will equal the current in Q1, IREF, at the value of V0 that causes the two devices to have the same VDS, that is, at V0 = VGS. • As V0 is increased above this value, I0 will increase according to the incremental output resistance ro2 of Q2.

  17. Basic current mirror - contd… VA is proportional to the transistor channel length; thus, to obtain high output-resistance values, current sources are usually designed using transistors with relatively long channels.

  18. Output characteristics

  19. Current mirror circuits - contd… • Two performance parameters need to be improved: • The accuracy of the current transfer ratio of the mirror. • The output resistance of the current source.

  20. MOS Cascode current source • Q1 is CS configuration and Q2 is CG configuration. • Current source biasing.

  21. MOS Cascode current source - contd…

  22. Small signal of cascode current mirror

  23. Performance of MOS Cascode • Open-circuit voltage gain • The cascoding increases the magnitude of the open-circuit voltage gain from Ao to Ao2 • Output resistance

  24. Performance of MOS Cascode • If ro2 is infinite, then Rin2 reduces to 1/gm2. • If ro2 cannot be neglected, as is always the case in IC amplifiers, the input resistance depends on the value of RL in an interesting fashion. • The load resistance (RL) is divided by the factor (gm2ro2).

  25. Cascode MOS current mirror MOS cascode current mirror requires large load to increase the gain

  26. Cascode MOS current mirror - contd… • Q1 and Q3 are always in saturation. • Q2 and Q4 both have to be in saturation for current mirror to work. • Condition for saturation • VDS2 > VGS – Vt • VDS4 > VGS – Vt

  27. PMOS CASCODE CURRENT MIRROR Working of PMOS cascode is same as NMOS cascode.

  28. Cascoding • To raise the output resistance of the CS or CE transistor, we stack a CG or CB transistor on top. This is cascoding. • The CG or CB transistor in the cascode passes the current gm1vi provided by the CS or CE transistor.

  29. Cascoding-contd… • A MOS cascode amplifier operating with an ideal current source load achieves a gain of (gmro)2 = A02. • To realize the full advantage of cascoding, the load current-source must also be cascoded, in which case a gain as high as 1/2A02 can be obtained.

  30. Cascode amplifier with cascode current mirror

  31. Cascode amplifier with cascode current mirror - contd… • Advantage • Much better high-frequency response (high gain-bandwidth). • Simpler biasing.

  32. Double Cascoding • If still a higher output resistance and correspondingly higher gain are required, it is still possible to add another level of cascoding • Observe that Q3 is the second cascode transistor, and it raises the output resistance by (gm3ro3).

  33. Double Cascoding - contd…

  34. The Folded Cascode To avoid the problem of stacking a large number of transistors across a low-voltage power supply, one may use a PMOS transistor .

  35. Folded Cascode-contd… • Double cascoding is possible in the MOS case only. However, the large number of transistors in the stack between the power-supply rails results in the disadvantages of a severely limited output-signal swing. The folded-cascode configuration helps to resolve this issue.

  36. WILSON CURRENT Mirror • A Wilson current mirror is a three-terminal circuit. • It accepts an input current at the input terminal and provides a "mirrored" current source or sink output at the output terminal. • The mirrored current is a precise copy of the input current. • It may be used as a Wilson current source by applying a constant bias current to the input branch . • The circuit is named after George R. Wilson, an integrated circuit design engineer .

  37. MOS Wilson Current Source The drain current of Q2 to equal the input current and the output configuration assures that the output current equals the drain current of Q1. 

  38. MOS Wilson Current Source - contd… • An improved current source which has higher output resistance than the simple current mirror. • The output current is related to the reference current by the equation (assuming the transistors operating in saturation region):

  39. MOS Wilson Current Source - contd… • The reference current I1 may be approximated by

  40. Small signal-MOS Wilson Current source • The transistor pairs M1-M2 and M3-M4 are exactly matched and the input and output potentials are approximately equal, then in principle there is no static error. • The input and output currents are equal because there is no low frequency or DC current into the gate of a MOSFET.

  41. MOS Wilson Current Source • The threshold voltage of MOS devices is usually between 0.4 and 1.0 volts with no body effect depending on the manufacturing technology. • vgs  must exceed the threshold voltage by a few tenths of a volt to have satisfactory input-output current match, the total input to ground potential is comparable to 2.0 volts. • This difference is increased when the transistors share a common body terminal and the body effect in M4 raises its threshold voltage. 

  42. Limitation of Wilson current mirror • The principal limitation on the use of the Wilson current mirror in MOS circuits is the high minimum voltages between the ground connection. • The input and output nodes that are required for proper operation of all transistors in saturation. •  The voltage difference between the input node and ground is vgs1+vgs4 .

  43. MOS Widlar current source

  44. Summary • Biasing in integrated circuits utilizes current sources. As well, current sources are used as load devices. • The MOS current mirror has a current transfer ratio of (W/L)2/(W/L)1. For a bipolar mirror, the ratio is IS2/IS1.

  45. Mos steering circuits

  46. MOS steering circuits - contd… • Once a constant current is generated, it can be replicated to provide DC bias currents for the various amplifier stages in the IC. • Current mirrors can obviously be used to implement this current – steering function • The use of a negative DC supply –VSSdoes not change the fact that, by-structure, both transistors, in every mirror pair, have the same VGS voltage.

  47. Mos steering circuits - contd… • Here Q1 together with R determine the reference current IREF. Transistors Q1, Q2, and Q3 form a two-output current mirror,

  48. Mos steering circuits - contd… • To ensure operation in the saturation region, the voltages at the drains of Q2 and Q3 are • constrained as follows: • VD2,VD3≥ -VSS +VGS1-Vtn • VD2,VD3≥ -VSS +Vov1

  49. Summary • Accurate and stable reference current is generated and then replicated to provide bias current for the various amplifier stages on the chip. • The heart of the current-steering circuitry utilized to perform this function is the current mirror.

  50. CS Amplifier with active load • Current source acts as an active load. • Source lead is signal grounded. • Active load replaces the passive load.

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