Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I Biasing in MOSFET Amplifiers • Biasing: Creating the circuit to establish the desired DC voltages and currents for the operation of the amplifier • Four common ways: 1. Biasing by fixing VGS 2. Biasing by fixing VG and connecting a resistance in the Source 3. Biasing using a Drain-to-Gate Feedback Resistor 4. Biasing Using a Constant-Current Source MOSFETs 1 Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I Biasing in MOSFET Amplifiers • I D = 12 k n′ k n′ = μ n VDD Biasing by fixing VGS ε ox t ox W (VGS − Vt )2 L = μ n Cox VGG VG RD VD RG M1 VS RS -VEE When the MOSFET device is changed (even using the same supplier), this method can result in a large variability in the value of ID. Devices 1 and 2 represent extremes among units of the same type. MOSFETs 2 Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I Biasing in MOSFET Amplifiers • Biasing by fixing VG and connecting a resistance in the Source Degeneration Resistance MOSFETs 3 Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I Biasing in MOSFET Amplifiers • Biasing using a Drain-to-Gate Feedback Resistor Large Resistor MOSFETs 4 Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I Biasing in MOSFET Amplifiers • Biasing Using a Constant-Current Source Current Mirror Used in Integrated Circuits Figure 4.33 (a) Biasing the MOSFET using a constant-current source I. (b) Implementation of the constant-current source I using a current mirror. MOSFETs 5 Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I 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 • In this circuit, consider W/L of both MOSFETs are the same and transistors are identical. The Gate-Source voltages W1 are also the same, then L1 I REF = k n′ I = 12 k n′ 1 2 I W1 (VGS − Vt )2 L1 W1 (VGS − Vt )2 L1 I I REF W1 L1 = ⋅ =1 L1 W1 - VGS + + W1 L1 VGS - I = I REF MOSFETs 6 Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I Current Mirror DC Analysis • Designing IREF I REF VDD − VGS + VSS = R I REF = 1 ′ k 2 n W1 (VGS − Vt )2 L1 • It is often needed to find the value of R in order to achieve a desired IREF MOSFETs 7 Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I Biasing of MOSFET Amplifier • • • • • • • • • 1- Intro to MOS Field Effect Transistor (MOSFET) 2- NMOS FET 3- PMOS FET 4- DC Analysis of MOSFET Circuits 5- MOSFET Amplifier 6- MOSFET Small Signal Model 7- MOSFET Integrated Circuits 8- CSA, CGA, CDA 9- CMOS Inverter & MOS Digital Logic MOSFETs 8 Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I MOSFET Design Space • Modern integrated circuits use MOSFETs extensively – Very high densities of transistors – up to 109 transistors/cm2 in some ULSI 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 wherever possible to implement current sources, resistors, capacitors, MOSFETs 9 Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I Using MOSFETs to implement R’s and C’s • Resistors: Active Loads (large R’s) Diode-connected loads (small R’s) MOSFET Triode-Region (moderate R’s) • Capacitors Most obvious is the gate-body capacitor Can be used to have variable-capacitors as well • Current Mirrors MOSFETs 10 Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I MOSFET Active Loads • MOSFETs used as an active load for high resistances: – MOSFET is held in saturation with the source and gate held at a constant DC voltage 1 r = – Drain connected to circuit o λ ⋅ ID – ro is inversely proportional to ID vgs = 0 gmvgs = 0 Rin=ro Rin= ? MOSFETs 11 Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I Diode-Connected MOSFETs • A Diode connected MOSFET can be used to achieve small resistances: – The Drain is directly connected to Gate, and therefore it can only be operated in saturation (or cutoff) Source Absorption Theorem MOSFETs 12 Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I MOSFET Current Mirrors • Used extensively in MOSFET IC applications • Often ro,is neglected. Since there is no gate current, the drain currents of M1 and M2 are identical • In practice, IREF ≠ I due to finite ro. (Not included in EC1) 1 W1 2 I REF = k n′ (V X − Vt ) (1 + λV X ) 2 L1 V X = VGS 1 = VDS 1 VX 0 - VGS + I 0 + VGS - The current I will also depend on VDS2 MOSFETs 13 Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I 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 M1 and M2 need not be the same, the size ratios can affect current ratios I REF = 12 k n′ W1 (VGS − Vt )2 L1 W2 (VGS − Vt )2 I = k n′ L2 1 2 I I REF = W2 L1 ⋅ L2 W1 I W1 L1 W2 L2 - VGS + + VGS - MOSFETs 14 Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I Current Scaling (Steering) W L W L W L W L • Ratio of aspect ratios can be selected to achieve nearly any scale factor I/IREF Note: All gates are connected MOSFETs 15 Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I Current Mirroring – Pushing and Pulling MOSFETs 16 Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I Small Signal • Transistor M1 is diode connected and acts like a resistor to s.-s. ground. ro2 MOSFETs 17 Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I Outline of Chapter 5 • • • • • • • • • 1- Intro to MOS Field Effect Transistor (MOSFET) 2- NMOS FET 3- PMOS FET 4- DC Analysis of MOSFET Circuits 5- MOSFET Amplifier 6- MOSFET Small Signal Model 7- MOSFET Integrated Circuits 8- CSA, CGA, CDA 9- CMOS Inverter & MOS Digital Logic MOSFETs 18 Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I DC and AC - Body-Effect / CLM Three types of analysis: Neglect DC Body-Effect & DC CLM Use DC Body-Effect / Neglect DC CLM Use DC Body-Effect / Use DC CLM Three types of analysis: Neglect AC Body-Effect & AC CLM Use AC Body-Effect / Neglect AC CLM DC Analysis Use whatever DC values for V and I in the small-signal analysis AC Analysis (small-signal) Use AC Body-Effect / Use CLM MOSFETs 19 Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I Common Source Amplifier (CSA) • Current source I implemented with current mirror. • Current mirror provides active load at drain • Source terminal grounded – no DC or AC Body effect MOSFETs 20 Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I CSA with Current Mirror ro2 CSA MOSFETs 21 Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I CSA Small Signal Analysis • From MOSFET Current-Mirror: only ro2 appears in analysis v gs1 = vi vout = − g m1 (ro1 ro 2 )v gs1 vout AV = = − g m1 (ro1 ro 2 ) vi MOSFETs 22 Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I CSA Input/Output Resistance • Input Resistance RIN ⇒ ∞ • Output resistance vgs1 = 0 gm1vgs1 = 0 ROUT = ro1 ro 2 MOSFETs 23 Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I CSA Calculations • In practice, difficult to keep all transistors operating in saturation VOUT is hard to control, and sensitive to: W/L, VG, and CLM λ1 = λ2 = 0.01V −1 Vt1 = Vt 2 = 1V k ′p = 50μ AV Hand: SPICE: VG = 2.567V VG = 2.58V I = 2.596mA k n′ = 125μ AV VOUT = 3.855V I = 2.57mA VOUT = 2.895V W2 W = 50, 1 = 40 L2 L1 g m1 = 5.192m AV AV = −102.4 V V VDD = 5V ,VSS = −2V RREF = 1kΩ ro = 38.52kΩ AV = −100 V V MOSFETs 24 Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I Common Gate Amplifier (CGA) • A pMOS current mirror is used as IREF including the output resistance. • The gate terminal held at a DC voltage. (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 25 Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I CGA – DC Analysis • Current mirror is assumed to be ideal during the DC analysis, thus IREF=I • DC voltage at the source terminal (VS) must be obtained from driving the current IREF through the transistor. • This assumes that the input voltage source VI is set to zero • RI is part of the source voltage • Solve for VO, with VS and VG known, and including CLM 2 1 ′W I = 2 k n (VG − VS − Vt ) [1 + λ ⋅ (VO − VS )] L MOSFETs 26 Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I CGA • Replace with a current source including output resistance ro2 ro2 Choice of analysis: Neglect AC Body-Effect & CLM Use AC Body-Effect / Neglect CLM Use AC Body-Effect / Use CLM MOSFETs 27 Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I CGA – No Body Effect or CLM vo = − g m v gs ⋅ ro 2 i=0 v gs = − Non Inverting AV = 1 g m1 1 + RI g m1 vI vo ro 2 = 1 vI + RI gm MOSFETs 28 Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I CGA – RIN & ROUT, No Body Effect or CLM RIN 1 = g m1 ROUT vI = 0 RIN ROUT = ro 2 MOSFETs 29 Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I CGA – With Body Effect & no CLM Solve at vx first: vX RIN vbs1 = v gs1 = −v x vo = −(g m1v gs1 + g mb1vbs1 )ro 2 vo = −( g m1v x + g mb1v x )ro 2 vo AV = = ( g m1 + g mb1 )ro 2 vx Include RI and vo vo v x R IN solve for total AV − total = = ⋅ = ( g m + g mb )ro 2 ⋅ voltage gain in vi v x vi R IN + R I term of RIN MOSFETs 30 Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I CGA – RIN With Body Effect & no CLM iX vX iIN RIN vbs1 = v gs1 = −v x ix = ( g m1 + g mb1 )v x iin = ( g m1 + g mb1 )v x Neglect Input Voltage Source RIN 1 = g m1 + g mb1 MOSFETs 31 Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I CGA - With Body Effect & CLM iX iX vX Neglect Input Voltage Source v x = −v gs vo = ix ⋅ ro 2 v x − vo i x = ( g m1 + g mb1 )v x + ro1 ⎛1 ⎞ vo AV = = (ro1 ro 2 ) ⋅ ⎜⎜ + g m1 + g mb1 ⎟⎟ vx ⎝ ro1 ⎠ MOSFETs 32 Department of Electrical and Computer Engineering ECSE-330B Electronic Circuits I CGA – RIN With Body Effect & CLM iX vX vo = ix ⋅ ro 2 RIN v x − vo ix = ( g m1 + g mb1 )v x + ro1 1+ Neglect Input Voltage Source RIN = ro 2 ro1 vx = 1 ix + g m1 + g mb1 ro1 MOSFETs 33