# The sourcedrain junction capacitances exhibit a

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The source/drain junction capacitances exhibit a voltage dependence that may not follow the square-root equation associated with “abrupt” junctions. SPICE allows an equation of the form (A.3)

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BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 832 (1) 832 App. A Introduction to SPICE where denotes the value for zero voltage across the junction, and typically falls in the range of 0.3 to 0.4. Thus for and , we specify (cjo, mj) and (cjswo, mjsw) A more complete MOS model may therefore appear as: .model mymod nmos (level=1, uo=360, tox=0.4n, vth=0.5, lambda=0.4, +cjo=3e-4, mj=0.35, cjswo=40n, mjswo=0.3) where the “level” denotes a certain complexity for the model. In practice, higher levels with many more parameters are used. Similarly, a PMOS model may be constructed as follows: .model mymod2 pmos (level=1, uo=150, tox=0.4n, vth=-0.55, +lambda=0.5, cjo=3.5e-4, mj=0.35, cjswo=35n, mjswo=0.3) A.4 Other Elements and Commands A.4.1 Dependent Sources In addition to the independent voltage and current sources studied above, we may need to incor- porate dependent sources in simulations. For example, as mentioned in Chapter 8, op amps can be viewed as voltage-dependent voltage sources. Similarly, a MOSFET acts as a voltage-dependent current source. Consider the arrangement shown in Fig. A.18, where the voltage source tied between nodes and is equal to three times the voltage difference between nodes and . For simplicity, V AB A B C D 3 Figure A.18 Voltage-dependent voltage source. we call the “input nodes,” the “output nodes,” and the factor of 3, the “gain.” Such a voltage-dependent voltage source is expressed as Output Input DC Gain Nodes Nodes Value e1 c d poly(1) a b 0 3 Note the element name begins with the letter “e” to signify a voltage-dependent voltage source. The next two entries are the output nodes, with the first representing the positive terminal. The entry poly(1) indicates a first-order polynomial relationship between and . Next, the controlling (input) nodes are specified, and the zero is entered to denote a zero additional dc voltage. Finally, the gain is specified. In a more general case, this expression can realize , where is the dc value (zero in the above example) and is the gain (3 in the above example). Example A.13 The circuit of Fig. A.19(a) employs an op amp with a gain of 500. Construct a SPICE netlist for the circuit.
BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 833 (1) References 833 in X in V out V X (a) (b) 1 k 10 k 1 k 10 k out r1 rf eopamp Figure A.19 Solution We first draw and label the circuit as shown in Fig. A.19(b). Thus, r1 in x 1k rf x out 5k eopamp out 0 poly(1) x 0 0 -500 For the voltage-dependent current source depicted in Fig. A.20, the description is as follows: V AB A B C D G m G m = (20 ) 1 Figure A.20 Voltage-dependent current source.
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