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CIRCUIT ANALYSIS USING SPICE AND PSPICE

[CHAP. 15

15.6 THE´VENIN EQUIVALENT .TF Statement The .TF command provides the transfer function from an input variable to an output variable and produces the resistances seen by the two sources. It can thus generate the The´venin equivalent of a resistive circuit. The syntax is :TF

EXAMPLE 15.10 15-10.

houtput variablei

hinput variablei

Use the command .TF to find the The´venin equivalent of the circuit seen at terminal AB in Fig.

Fig. 15-10 The node numbers and element names are shown on Fig. 15-10.

The source file is

Transfer Function in Fig. 15-10 Vs 1 0 DC 12 E1 4 0 2 0 10 R1 1 2 1k R2 2 0 2k R3 2 3 1k R4 3 4 200 .TF V(3) Vs .END The output file contains the following results: NODE (1)

VOLTAGE 12.0000

NODE (2)

VOLTAGE 2:0000

NODE (3)

VOLTAGE 17:0000

NODE (4)

VOLTAGE 20:000

VOLTAGE SOURCE CURRENTS NAME CURRENT Vs 1:400E  02 TOTAL POWER DISSIPATION

1:68E  01 WATTS

SMALL-SIGNAL CHARACTERISTICS V(3)/Vs ¼ 1:417E þ 00 INPUT RESISTANCE AT Vs ¼ 8:571E þ 02 OUTPUT RESISTANCE AT V(3) ¼ 6:944E þ 01 Therefore, VTh ¼ 1:417ð12Þ ¼ 17 V and RTh ¼ 69:44 .

15.7

OP AMP CIRCUITS

Operational amplifiers may be modeled by high input impedance and high gain voltage-controlled voltage sources. The model may then be used within a net list repeatedly. EXAMPLE 15.11 Find the transfer function V3 =Vs in the ideal op amp circuit of Fig. 15-11(a). The op amp is replaced by a voltage-dependent voltage source with a gain of 106 [see Fig. 15-11(b)]. The source file is

Mahmood_Nahvi_eBook_Schaum_s_Outlines_Theory_An