# Example a4 construct a spice netlist for the step

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Example A.4 Construct a SPICE netlist for the step response of the circuit depicted in Fig. A.6(a). out V in V (a) (b) out c1 20 nH 30 1 pF in vin 0 mid l1 r1 Figure A.6 Solution We begin with labeling the nodes and the elements [Fig. A.6(b)]. How do we choose the tran- sition time of the step? Ignoring the damping behavior of the circuit for now, we may consider ns as the time constant of the response and hence choose the transition time to be about 150 ps. The netlist is as follows: My RLC Circuit l1 in out 20n r1 out mid 30 c1 mid 0 1p vin in 0 pulse(0 1 0 150p 150p 1)

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BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 823 (1) Sec. A.2 Types of Analysis 823 .tran 25p 500p .end Note that the falling transition time is unimportant here. Example A.5 Suppose we wish to determine the frequency response of the RLC circuit illustrated in Fig. A.6(a). Revise the netlist accordingly. Solution We must often study both the transient and the ac response of circuits. For convenience, only one file should serve both purposes. Fortunately, SPICE allows us to “comment out” lines of the file by inserting a * at the beginning of each line. We therefore repeat the netlist from the above example, comment out the lines related to transient analysis, and add the lines necessary for ac analysis: My RLC Circuit l1 in out 20n r1 out mid 30 c1 mid 0 1p *vin in 0 pulse(0 1 0 150p 150p 1) *.tran 25p 500p *Added next two lines for ac analysis. vin in 0 ac 1 .ac dec 100 1meg 1g .end (The letter g at the end of the .ac line denotes .) As seen above, comment lines can also serve as reminders. A.2.3 DC Analysis In some cases, we wish to plot the output voltage (or current) of a circuit as a function of the input voltage (or current). Called “dc analysis,” this type of simulation requires that SPICE sweep the input across a range in sufficiently small steps. For example, we may write vin in 0 dc 1 Lower Upper Step End End Size .dc vin 0.5 2 1m The vin description specifies the type as dc with a nominal value of 1 V. The dc sweep command begins with .dc and specifies vin as the source that must be swept. The following two entries denote the lower and upper ends of the range, respectively, and the last entry indicates the step size. Example A.6 Construct a netlist to plot as a function of for the circuit shown in Fig. A.7(a). Assume This nominal value is arbitrary and unimportant in dc analysis.
BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 824 (1) 824 App. A Introduction to SPICE out V in V (a) (b) out 100 200 100 200 in vin 0 Figure A.7 an input range of V to V with 2-mV steps. Solution We label the nodes and the elements as illustrated in Fig. A.7(b). The netlist can be written as: Voltage Divider r1 in out 100 r2 out 0 200 vin in 0 dc 1 .dc vin -1 +1 2m .end Note that the values of r1 and r2 are not followed by a unit so that SPICE assumes they are expressed in ohms.

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