Biasing in MOSFET Amplifiers

Biasing in MOSFET Amplifiers Biasing: Creating the circuit to establish the desired DC oltages and currents for the operation of the amplifier Four common ways:. Biasing by fixing GS. Biasing by fixing G and connecting a resistance in the Source 3. Biasing using a Drain-to-Gate Feedback Resistor 4. Biasing Using a Constant-Current Source MOSFETs

Biasing in MOSFET Amplifiers Biasing by fixing GS DD D k k ε n W L ox n μ n tox ( ) μ C n GS ox t GG RG G R D D M S R S - EE When the MOSFET deice is changed (een using the same supplier), this method can result in a large ariability in the alue of D. Deices and represent extremes among units of the same type. MOSFETs

Biasing in MOSFET Amplifiers Biasing by fixing G and connecting a resistance in the Source Degeneration Resistance MOSFETs 3

Biasing in MOSFET Amplifiers Biasing using a Drain-to-Gate Feedback Resistor Large Resistor MOSFETs 4

Biasing in MOSFET Amplifiers Biasing Using a Constant-Current Source Current Mirror Used in ntegrated Circuits Figure 4.33 (a) Biasing the MOSFET using a constant-current source. (b) mplementation of the constant-current source using a current mirror. MOSFETs 5

Current Mirror DC Analysis The width and length (the W/L aspect ratio) and the parameters of the two transistors can be different We can choose W/L freely n this circuit, consider W/L of both MOSFETs are the same and transistors are identical. The Gate-Source oltages are also the same, then REF k n k W L n W L ( ) GS ( ) GS t t REF W L L W REF W L - GS + + GS - MOSFETs 6 W L

Current Mirror DC Analysis Designing REF REF REF k n DD W L R GS + ( ) GS t SS t is often needed to find the alue of R in order to achiee a desired REF MOSFETs 7

Biasing of MOSFET Amplifier - ntro to MOS Field Effect Transistor (MOSFET) - NMOS FET 3- PMOS FET 4- DC Analysis of MOSFET Circuits 5- MOSFET Amplifier 6- MOSFET Small Signal Model 7- MOSFET ntegrated Circuits 8- CSA, CGA, CDA 9- CMOS nerter & MOS Digital Logic MOSFETs 8

MOSFET Design Space Modern integrated circuits use MOSFETs extensiely ery high densities of transistors up to 0 9 transistors/cm in some ULS memory arrays. Off-chip discrete resistors and capacitors are NOT commonly used On-chip resistors and capacitors generally small Multistage amplifiers are usually DC-coupled Transistors used whereer possible to implement current sources, resistors, capacitors, MOSFETs 9

Using MOSFETs to implement R s and C s Resistors: Actie Loads (large R s) Diode-connected loads (small R s) MOSFET Triode-Region (moderate R s) Capacitors Most obious is the gate-body capacitor Can be used to hae ariable-capacitors as well Current Mirrors MOSFETs 0

MOSFET Actie Loads MOSFETs used as an actie load for high resistances: MOSFET is held in saturation with the source and gate held at a constant DC oltage Drain connected to circuit r o is inersely proportional to D r o λ D gs 0 g m gs 0 R in r o R in? MOSFETs

Diode-Connected MOSFETs A Diode connected MOSFET can be used to achiee small resistances: The Drain is directly connected to Gate, and therefore it can only be operated in saturation (or cutoff) Source Absorption Theorem MOSFETs

MOSFET Current Mirrors Used extensiely in MOSFET C applications Often r o,is neglected. Since there is no gate current, the drain currents of M and M are identical n practice, REF due to finite r o. (Not included in EC) W REF kn X t ( + λx L X ( ) ) GS DS X - GS 0 0 + + GS The current will also depend on DS - MOSFETs 3

Current Mirror DC Analysis The width and length (the W/L aspect ratio) of MOSFETs can be designed almost freely Since the W/L of M and M need not be the same, the size ratios can affect current ratios REF k REF n k n W L W L W L ( ) GS ( ) GS L W t t W L W L - GS + + GS - MOSFETs 4

W L Current Scaling (Steering) Ratio of aspect ratios can be selected to W L W L W L achiee nearly any scale factor / REF Note: All gates are connected MOSFETs 5

Current Mirroring Pushing and Pulling MOSFETs 6

Small Signal Transistor M is diode connected and acts like a resistor to s.-s. ground. r o MOSFETs 7

Outline of Chapter 5 - ntro to MOS Field Effect Transistor (MOSFET) - NMOS FET 3- PMOS FET 4- DC Analysis of MOSFET Circuits 5- MOSFET Amplifier 6- MOSFET Small Signal Model 7- MOSFET ntegrated Circuits 8- CSA, CGA, CDA 9- CMOS nerter & MOS Digital Logic MOSFETs 8

DC and AC - Body-Effect / CLM Three types of analysis: Neglect DC Body-Effect & DC CLM Use DC Body-Effect Three types of analysis: Neglect AC Body-Effect & AC CLM Use AC Body-Effect Use AC Body-Effect / Use CLM / Neglect DC CLM Use DC Body-Effect / Use DC CLM / Neglect AC CLM DC Analysis Use whateer DC alues for and in the small-signal analysis AC Analysis (small-signal) MOSFETs 9

Common Source Amplifier (CSA) Current source implemented with current mirror. Current mirror proides actie load at drain Source terminal grounded no DC or AC Body effect MOSFETs 0

CSA with Current Mirror r o CSA MOSFETs

CSA Small Signal Analysis out From MOSFET Current-Mirror: only r o appears in analysis g gs i ( r r ) o o m gs A out i g ( r r ) m o o MOSFETs

CSA nput/output Resistance nput Resistance R N Output resistance gs 0 g m gs 0 R r OUT r o o MOSFETs 3

CSA Calculations n practice, difficult to keep all transistors operating in saturation OUT is hard to control, and sensitie to: W/L, G, and CLM λ λ 0.0 k t 50μ A k 5μ A W L n R t p DD REF W 50, L 5, kω SS 40 g r o A G.596mA OUT m Hand:.567 3.855 5.9m 38.5kΩ 00 A A G SPCE:.58.57mA OUT.895 0.4 MOSFETs 4

Common Gate Amplifier (CGA) A pmos current mirror is used as REF including the output resistance. The gate terminal held at a DC oltage. (AC Ground) Since source terminal not at signal ground, the body effect is present. Typically used as second stage of a multi-stage amplifier circuit MOSFETs 5

CGA DC Analysis Current mirror is assumed to be ideal during the DC analysis, thus REF DC oltage at the source terminal ( S ) must be obtained from driing the current REF through the transistor. This assumes that the input oltage source is set to zero R is part of the source oltage Sole for O, with S and G known, and including CLM W ( ) k [ ( )] n G S t + λ O S L MOSFETs 6

CGA Replace with a current source including output resistance r o r o Choice of analysis: Neglect AC Body-Effect & CLM Use AC Body-Effect / Neglect CLM Use AC Body-Effect / Use CLM MOSFETs 7

CGA No Body Effect or CLM o g m gs r o i0 gs g + g m m R Non nerting A o r + g m o R MOSFETs 8

CGA R N & R OUT, No Body Effect or CLM R N g m R OUT 0 R N R OUT r o MOSFETs 9

CGA With Body Effect & no CLM X R N o o Sole at x first: bs ( g ) m x g mb x r o + gs ( g ) m gs g mb bs r o + x nclude R and sole for total oltage gain in term of R N A o A + x ( g + g ) o o x total m mb o i x i ( g m g mb ) r o r R N RN + R MOSFETs 30

CGA R N With Body Effect & no CLM i X i N X R N bs gs x i ) x ( g + m gmb x i ) in ( g + m gmb x Neglect nput oltage Source R N g m + g mb MOSFETs 3

CGA - With Body Effect & CLM i X x gs i X X Neglect nput oltage Source i x A o ( g + g ) m mb x i x + r o x r o o x ( r ) o ro + gm + gmb ro o MOSFETs 3

CGA R N With Body Effect & CLM i X X R N o i x r o Neglect nput oltage Source i x ( g + g ) R N m i x x mb r o + + g x r r m o o + + g x mb r o o MOSFETs 33