Computer Aided Design Course 5 Transient Analyses

These time-based analyses evaluate circuit performance in response to independent input source signals varying with time. The transient, or time-domain, response is the most used analysis in PSpice simulation. Using this type of analysis means that the operation of your circuit is being simulated as time progresses and various input variables change their values, or as oscillations are developed depending on component values. On the other hand, one must bear in mind that the transient analysis tends to cause trouble due to the inevitable compromises which have to be made between speed and accuracy.

In order to set up and run Transient Analysis, for example, you would go through the following steps: Specifying Time-dependent stimulusTime-dependent stimulus Setting up Time responseTime response Setting up Fourier ComponentsFourier Components Selecting OptionsOptions

1.Time-dependent stimulus The generators of time-dependent input signals for your circuit can be classified in two categories according to the way of setting their transient behaviour parameters: by using Standard Symbol names at Schematics stage where parameter adjustment is possible by introducing the desired input waveform as a function or a graphic plot with the help of Stimulus Editor

2. Time response The Transient response analysis allows the response of the circuit to be calculated from TIME = 0 to a specified time. Minimum circuit design requirements Circuit should contain one of the following: an independent source with a transient specification (if it is an input excitation) - see previous stepprevious step an initial condition on a reactive elementinitial condition a controlled source that is a function of time During a transient analysis, any or all of the independent sources may have time-varying values.

Minimum program setup requirements A transient analysis specification includes:

*Detailed Bias Point: The transient analysis does its own calculation of a bias point to start with, using the same technique as described for DC analyses. This is necessary because the initial values of the sources may be different from their DC values. If you want to report the small-signal parameters for the transient bias point, you should use the Transient command and enable Detailed Bias Point. Otherwise, if all you want is the result of the transient run itself, you should enable the Transient command only.

Detailed Bias Point.

**** INITIAL TRANSIENT SOLUTION TEMPERATURE = DEG C ************************************************************************* NODE VOLTAGE NODE VOLTAGE NODE VOLTAGE NODE VOLTAGE ( IN) ( OUT) ( VCC) (N00101) (N00563) (N00870) VOLTAGE SOURCE CURRENTS NAME CURRENT V_V E+00 V_V E-03 TOTAL POWER DISSIPATION 1.58E-02 WATTS **** 03/29/07 15:33:55 ******* PSpice (Jan 2004) ******* ID# ** Profile: "SCHEMATIC1-t1" [ C:\DOCUMENTS AND SETTINGS\OVI\MY DOCUMENTS\CARTE_CAD_V1\simulari\t1- PSpiceFiles\SCHEMATIC1\t1.sim ] **** OPERATING POINT INFORMATION TEMPERATURE = DEG C ************************************************************************* **** BIPOLAR JUNCTION TRANSISTORS NAME Q_Q1 MODEL Q2N2222 IB 7.69E-06 IC 1.21E-03 VBE 6.50E-01 VBC -4.06E+00 VCE 4.71E+00 BETADC 1.58E+02 GM 4.68E-02 RPI 3.72E+03 RX 1.00E+01 RO 6.43E+04 CBE 5.57E-11 CBC 3.87E-12 CJS 0.00E+00 BETAAC 1.74E+02 CBX/CBX2 0.00E+00 FT/FT2 1.25E+08

( - )/50 şi 2.

5. Fourier components A periodic signal can be represented by a Fourier series: where a0/2 is the dc component, ak and bk the coefficients of the series give the magnitude of the signal Ak of kth harmonic component and the frequency k .. Fourier series is:

where the amplitude Ak and the phase  k are given by:

To perform a Fourier analysis is only possible after a transient analysis has been completed. This is due to the very simple reason that the Fourier analysis actually calculates DC and Fourier components of the result of a transient analysis. By default, the 1st through 9th components are computed, however, more can be specified. So, first, you have to perform a transient analysis before doing a Fourier analysis. The sampling interval used during the Fourier transform is equal to the print step specified for the transient analysis. When selecting Fourier to run a harmonic decomposition analysis on a transient waveform, only a portion of the waveform is used. Using Probe, a Fast Fourier Transform (FFT) of the complete waveform can be calculated and its spectrum displayed.

A Fourier analysis setting up is performed in Transient dialog box. Specification includes:Transient dialog box

FOURIER COMPONENTS OF TRANSIENT RESPONSE V(OUT) DC COMPONENT = E+00 HARMONIC FREQUENCY FOURIER NORMALIZED PHASE NORMALIZED NO (HZ) COMPONENT COMPONENT (DEG) PHASE (DEG) E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E+03

The program compute the THD (Total Harmonic Distorsion): The results of the Fourier Analysis are only available in the output file (.out). They cannot be viewed in Probe.

7. Options The Options is used to set all the options, limits, and control parameters for the simulator. *These options are available for modification in PSpice, but it is recommended that the program defaults be used. **For these options zero means infinity.

Convergence problems: There are only a few remedies available for this one. Try relaxing RELTOL from to Try setting ITL4=40 in an OPTIONS statement. This setting will slow down the simulation, so it does not recommend using it for circuits that do not have a convergence problem in transient analysis. When using PSpice for high voltage and currents, it may be appropriate to relax VNTOL frm 1uV to 1mV and ABSTOL from 1pA to 1nA.

Example 1: A simple BJT Switching circuit

Example 2: Schmitt trigger comparator

Example 3: Astable circuit with OpAmp IC dat de PSF

IC=1V

Example 4: Hartley oscillator