PSPICE 计算机仿真

Simulation Program with Integrated Circuit Emphasis

CH5

OPERATIONAL AMPLIFIER运算放大器

5.1 Modeling OP Amps with Resistors and Dependent Sources

How to use PSpice to analyze a circuit containing an operational amplifier?

Fig.53

Example7 The parameters for the operational

amplifier in the circuit shown in Fig.53 are Ri=200kΩ, A=10E4, and Ro=5kΩ. 1)Use PSpice to find Vi and Vo when

Vg=1V(dc). 2)Compare the PSpice solution with

the analytic solution for Vi and Vo.

预备知识 An equivalent circuit for an

operational amplifier (Fig. 52)

Fig. 52 ex7_pre_ddb

Solution Analog lib: R, E(VCVS) Source lib: VDC

Fig. 54 ex7_sch

Fig. 55 output file

1)We used simple bias point analysis, therefore, V(in) = -0.0028V( 书上 :0.0027V--?) V(out) = -10.03V( 书上 :-9.9697V--?)

Ideal operational amplifer If we assume that the operational

amplifer in the circuit shown in Fig. 53 is ideal, we can altera the PSpice op amp schematic representation by making Ri and A very large and Ro=0.

To illustrate, we modify the schematic in Fig. 54, with R2=200 MΩ, A=10E8, and R4 removed from the schematic entirely. (Fig. 54a)

Fig. 54a

The resulting PSpice output file after bias point analysis is shown in Fig. 56. The values of vi and vo from this

simulation are Vi=v(in)=-10E-9V ( 书上 :100E-9V--?) Vo=v(out)=-10V

Fig. 56 output file

2)We obtain the analytic solution for Vi and Vo by solving the following simultaneous node-voltage equations:

0102001

1

oiii vvvv

045

10

10

4

oioio vvvvv

You should verify that the solutions for Vi and Vo are Vi=2.742mV Vo=-9.970V

5.2 Using OP Amps Library Models

Example8 Repeat the analysis of the circuit

shown in Fig.53, but this time use the uA741 model from the PSpice model library.

Solution uA741 在 EVAL 库中 ;

需要在 Place Part 对话框，点 Add Library… 按钮，添加该元件库。

Fig. 57a Add EVAL Library

Fig. 57b EVAL Path

Fig. 57 ex8_sch

Fig. 58 output file

5.3 Modifying OP Amp Models Place the model of the uA741 from

the Eval Library in your circuit schematic.

Highlight the op amp component, select Edit/PSpice Model from the Capture menu. (Fig. 59a)

Fig. 59a

Fig. 59 Model Editor

You define a subcircuit by inserting a .SUBCKT control statement .SUBCKT SUBNAME N1 N2 N3

You terminate the subcircuit description by inserting an .ENDS control statement .ENDS <SUBNAME>

Now we create a new subcircuit based on the ideal op amp, consisting of a 1MΩ input resistor, a voltage-controlled voltage source with a gain of 10E6, and a 200 Ω output resistor.

Fig. 60 Model Editor for IdealOpAmp

Highlight the IdealOpAmp component;

Select Edit/Part from the Capture menu;

The result should look like the part shown in Fig. 61.

Fig. 61 Edit model

Highlight the new IdealOpAmp component;

Select Edit/Properties from the Capture menu.

The modified template is shown in Fig. 62.

Fig. 62 Edit Property

example9 Repeat the analysis of the circuit

shown in Fig.53, but this time use the IdealOpAmp model we have just created.

Solution

Fig. 63 ex9_sch

Fig. 64 output file

From Fig. 64, we can see that the output voltage is -10 V.