:MOS Amplifiers: Concepts and MOS Small-Signal-Model Sedra & Smith Sec. 5.4 & 5.6 akmal@ece.nitdgp.ac.in Analog Design 334.1012 NMOS Transfer Function (1) Transfer Function: Relation between output and input voltages. vi = Circuit Equations: o NMOS iv characteristics: iD = f (vGS , vDS ) o KVL: ECE334/ vo = vDS = VDD − RD iD NMOS Transfer Function (2) 1) For vGS < Vt , NMOS is in cutoff: iD = 0 & vDS = VDD − RD iD = VDD 2) Just to the right of point A: o VOV = vGS − Vt is small, so iD is small. o vDS = VDD − RD iD is close to VDD o Thus, vDS > VOV and NMOS is in saturation. ECE334 NMOS Transfer Function (2) 4) To the right of point B, vDS < VOV = vGS − Vt and NMOS enters triode. Point B is called the “Edge of Saturation” 3) As vGS increases: o VOV = vGS − Vt and iD become larger; o vDS = VDD − RD iD becomes smaller. o At point B, vDS = VOV ECE334 NMOS Transfer Function (2) 1) For vGS < Vt , NMOS is in cutoff: iD = 0 & vDS = VDD − RD iD = VDD 2) Just to the right of point A: o VOV = vGS − Vt is small, so iD is small. o vDS = VDD − RD iD is close to VDD o Thus, vDS > VOV and NMOS is in saturation. ECE334 4) To the right of point B, vDS < VOV = vGS − Vt and NMOS enters triode. Point B is called the “Edge of Saturation” 3) As vGS increases: o VOV = vGS − Vt and iD become larger; o vDS = VDD − RD iD becomes smaller. o At point B, vDS = VOV Graphical analysis of NMOS Transfer Function (1) vi = NMOS i-v Characterisitics : iD = f (vGS , vDS ) KVL : VDD = RD iD + vDS KVL equation is a plane in this space. Intersection of KVL plane with the iv characteristics surface is a line. NMOS operating point is on this line (depending on the value of vGS.) If we look from the bottom (iD axis out of the paper), we can see the transfer function. Graphical analysis of NMOS Transfer Function (6) Looking parallel to vGS axis Every point on the load line corresponds to a specific vGS value. As vGS increases, NMOS moves “up” the load line. Looking from the bottom Foundation of Transistor Amplifiers (1) A voltage amplifier requires vo/vi = const. (2 examples below) MOS transfer function is NOT linear vo/vi can be negative (minus sign represents a 180o phase shift) In saturation, however, transfer function looks linear (but shifted) Foundation of Transistor Amplifiers (2) In saturation, transfer function appear to be linear Approximate the transfer function with a tangent line at point Q (with a slope of − G): vDS − VDS = − G (vGS − VGS ) vds = − G vgs (linear relationship) for vds = vDS − VDS and vgs = vGS − VGS Foundation of Transistor Amplifiers (3) Let us consider the response if NMOS remains in saturation at all times and vGS is a combination of a constant value (VGS) and a signal (vgs). vGS = VGS + v gs The response to a combination of vGS = VGS + vgs can be found from the transfer function Response to the signal appears to be linear Response (vo = vDS ) is also made of a constant part (VDS ) and a signal response part (vds). Constant part of the response, VDS , is ONLY related to VGS , the constant part of the input (Q point on the transfer function of previous slide). o i.e., if vgs = 0, then vds = 0 The shape of the time varying portion of the response (vds) is similar to vgs. o i.e., vds is proportional to the input signal, vgs Although the overall response is non-linear, the transfer function for the signal is linear! Constant: Bias Signal and response vds vGS = VGS + vgs vDS = VDS + vds iD = ID vgs + id Non-linear relationship among these parameters Approximately Linear relationship among these parameters Important Points and Definitions! Signal: We want the response of the circuit to this input. Bias: State of the system when there is no signal. o Bias is constant in time (may vary extremely slowly compared to signal) o Purpose of the bias is to ensure that MOS is in saturation at all times. Response of the circuit (and its elements) to the signal is different than its response to the Bias (or to Bias + signal): o Signal iv characteristics of elements are different, i.e. relationships among vgs , vds , id is different from relationships among vGS , vDS , iD . o Signal transfer function of the circuit is different from the transfer function for total input (Bias + signal). Issues in developing a MOS amplifier: 1. Find the iv characteristics of the elements for the signal (which can be different than their characteristics equation for bias). o This will lead to different circuit configurations for bias versus signal 2. Compute circuit response to the signal o Focus on fundamental MOS amplifier configurations 3. How to establish a Bias point (bias is the state of the system when there is no signal). o Stable and robust bias point should be resilient to variations in µnCox (W/L),Vt , … due to temperature and/or manufacturing variability. o Bias point details impact small signal response (e.g., gain of the amplifier). Signal Circuit 1) We will find signal iv characteristics of various elements. 2) In order to use circuit theory tools, we will use the signal iv characteristics of various elements to assign a circuit symbol. e.g., o We will see that the diode signal iv characteristics is linear so for signals, diode can be modeled as a “circuit theory” resistor. o In this manner, we will arrive at a signal circuit. Bias and Signal Circuits Bias & Signal Bias Signal only = (Bias + Signal) - Bias + ? MOS : VGS ,VDS , I D , MOS : vgs , vds , id , vR = VR + vr RD : RD : iR = I R + ir ..... MOS : vGS , vDS , iD , (vGS = VGS + vgs ,...) RD : ..... VR , I R ..... vr , ir Finding signal circuit elements -- Resistor Resistor Voltage Current iv Equation Bias + Signal: vR iR v R = R iR Bias: VR IR VR = R IR vr = vR − VR ir = iR − IR ?? Signal: vr = vR − VR = RiR − RI R = R (iR − I R ) vr = Rir A resistor remains as a resistor in the signal circuit. Finding signal circuit elements – IVS & ICS Independent voltage source Voltage Current iv Equation Bias + Signal: vIVS iIVS vIVS = VDD = const Bias: VIVS IIVS VIVS = VDD = const vivs = vIVS − VIVS iivs = iIVS − IIVS ?? Signal: vivs = vIVS − VIVS = VDD − VDD = 0 vivs = 0, iivs ≠ 0 An independent voltage source becomes a short circuit! Similarly: An independent current source becomes an open circuit! Exercise: Show that dependent sources remain as dependent sources Summary of signal circuit elements Resistors& capacitors: The Same o Capacitor act as open circuit in the bias circuit. Independent voltage source (e.g., VDD) : Effectively grounded Independent current source: Effectively open circuit o Careful about current mirrors as they are NOT “ideal” current sources (early effect and/or channel-length modulation was ignored!) Dependent sources: The Same Non-linear Elements: Different! o Diodes & transistors ? Formal derivation of small signal model Signal + Bias for element A (iA, vA) : iA = f (vA) Bias for element A (IA, VA) : IA = f (VA) Signal for element A (ia, va) : ia = g (va) i A = f (v A ) f ( 2 ) (VA ) 2 = f (VA ) + f (VA ) ⋅ (v A − VA ) + ⋅ (v A − VA ) + ... 2! f ( 2 ) (VA ) 2 (1) = f (VA ) + f (VA ) ⋅ va + ⋅ va + ... 2! ≈ f (VA ) + f (1) (VA ) ⋅ va (1) i A = ia + I A = I A + f (1) (VA ) ⋅ va ia = g (va ) = f (1) (VA ) ⋅ va (Taylor Series Expansion) Small signal means: f (1) f ( 2 ) (VA ) 2 ⋅ va (VA ) ⋅ va >> 2! f (1) (VA ) va << 2 ⋅ ( 2 ) f (VA ) Small signal model vs iv characteristics Small signal model is equivalent to approximating the non-liner iv characteristics curve by a line tangent to the iv curve at the bias point id = f (1) (VD ) × vd rd = 1 nVT ≈ f (1) (VD ) I D Derivation of MOS small signal model (1) MOS iv equations: iD = f (vGS, vDS) iG = 0 Signal + Bias for MOS (iD, vGS , vDS) : iD = f (vGS, vDS), iG = 0 Bias for MOS (ID, VGS , VDS) : ID = f (VGS, VDS), IG = 0 Signal for MOS (id, vgs , vds) : id = g (vgs , vds), (Taylor Series Expansion in 2 variables) I D + id = iD = f (vGS , vDS ) = f (VGS , VDS ) + ≈ ID ig = 0 + ∂f ∂vGS ∂f ∂vGS ⋅ (vGS − VGS ) + VGS ,VDS × v gs + VGS ,VDS ∂f ∂vDS id ≈ ∂f ∂vDS ⋅ (vDS − VDS ) + ... VGS ,VDS × vds VGS ,VDS ∂f ∂vGS × v gs + VGS ,VDS ∂f ∂vDS × vds VGS ,VDS Derivation of MOS small signal model (2) iD = 0.5µ nCox id = ∂f ∂vGS ∂f ∂vGS W (vGS − Vt ) 2 (1 + λvDS ) = f (vGS , vDS ) L ⋅ v gs + VGS ,VDS VGS ,VDS 0.5µ nCox =λ× 0.5µ nCox VGS ,VDS VGS ,VDS W (VGS − Vt ) 2 (1 + λVDS ) 2I L = D ≡ gm VOV (VGS − Vt ) = λ × 0.5µ nCox VGS ,VDS ⋅ vds W (vGS − Vt )(1 + λvDS ) L = 2 × 0.5µ nCox = 2× ∂f ∂vDS ∂f ∂vDS W (vGS − Vt ) 2 L VGS ,VDS W (VGS − Vt ) 2 (1 + λVDS ) 1 λI D L = ≈ λI D ≡ (1 + λVDS ) (1 + λVDS ) ro id = g m ⋅ v gs + vds ro ig = 0 MOS small signal “circuit” model vds ig = 0 and id = g m ⋅ v gs + ro Statement of KCL Two elements in parallel Input open circuit gm = 2⋅ ID VOV ro ≈ 1 λ ⋅ ID g m ro = 2 2V = A >> 1 λVOV VOV PMOS small signal model is identical to NMOS PMOS* NMOS = PMOS small-signal circuit model is identical to NMOS o We will use NMOS circuit model for both! o For both NMOS and PMOS, while iD ≥ 0 and ID ≥ 0, signal quantities: id, vgs, and vds , can be negative!

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