Anatomy of a PSPICE output file - dc BJT example

This page shows the location of essential information in the PSPICE output file for a dc BJT analysis.

Contents

• Input File

Output File Components:

1. Input File Listing
2. Model Parameters
3. dc Node Voltages
4. Voltage Source Currents
5. Total Power Dissipation
6. Operating Point Information (.op statement)
7. Job Statistics

The input file(p1.cir):

practice problem 1
*The npn BJT named Q1 is defined below: 
Q1	coll	base	emit	nmod
.model	nmod	npn(is=1e-13A va=100V bf=100) 
vc	coll	0	dc	10V
ve	emit	0	dc	4.3V
vb	base	0	dc	5V
.op
.options nopage
.end

The output file (p1.out)

(1) PSPICE regurgitates the input file listing, annotated with the time and date:

**** 02/04/97 22:07:32 *********** Evaluation PSpice (Jan 1993) **************

 practice problem 1


 ****     CIRCUIT DESCRIPTION


******************************************************************************


*The npn BJT named Q1 is defined below:
Q1      coll    base    emit    nmod
.model  nmod    npn(is=1e-13A va=100V bf=100)
vc      coll    0       dc      10V
ve      emit    0       dc      4.3V
vb      base    0       dc      5V
.op
.options nopage
.end

**** BJT MODEL PARAMETERS
(2) Here are the values for the device models parameters used in the circuit. Some of these are obtained from the model statement(s) in the input file, others are calculated or default values. These parameters are described in most PSPICE reference books.

               nmod            
               NPN             
          IS  100.000000E-15 
          BF  100            
          NF    1            
         VAF  100            
          BR    1            
          NR    1   

**** SMALL SIGNAL BIAS SOLUTION TEMPERATURE = 27.000 DEG C

(3) This section lists the dc voltage at each node in the circuit. Also note that the default temperature is 27 C = 300.15 K. When performing hand calculations of junction currents using exponentials, e.g., IC = IS(exp(VBE/VT), in order for your results to agree with PSPICE it might be necessary to use a value for the thermal voltage calculated with high precision. For T = 27 C, VT = kT/q = 25.87 mV. The temperature can be set to any value by including a .TEMP statement in the input file. Also, the analysis can be repeated at a sequence of temperatures (see PSPICE reference).

 NODE   VOLTAGE     NODE   VOLTAGE     NODE   VOLTAGE     NODE   VOLTAGE


( base)    5.0000  ( coll)   10.0000  ( emit)    4.3000  

(4) The following section lists each voltage source by name, together with the dc current flowing through it. The convention is that positive current flows into the + node of the source, through the source, and leaves the - node. Thus, if the voltage source value is positive and the current is negative, then power is being delivered by the source.

    VOLTAGE SOURCE CURRENTS
    NAME         CURRENT

    vc          -5.958E-02
    ve           6.015E-02
    vb          -5.675E-04

(5) The next line gives the total power dissipation in the circuit.
TOTAL POWER DISSIPATION 3.40E-01 WATTS




**** OPERATING POINT INFORMATION TEMPERATURE = 27.000 DEG C
(6) This section provides detailed information about the solid state devices as requested by including the '.op' statement in the input file.

**** BIPOLAR JUNCTION TRANSISTORS

• The device name and its model name are listed first.
• The terminal curents IB and IC are listed next. IE is not listed, but it can easily be computed by Kirchhoff's current law (see note below).
• NOTE WELL: The PSPICE convention (for both npn and pnp BJT's and n- and p-channel FET's) is that ALL currents are defined to be ENTERING the device. This is DIFFERENT than the convention used in some texts (e.g., Jaeger), and must be taken into account to properly interpret the results. For example, an npn BJT in forward active would have IC, IB, and IE all positive using the Jaeger convention (for which IE=IB+IC), but IE would be negative using the PSPICE convention (for which IE+IB+IC=0). As another example, a pnp BJT in forward active would have all positive terminal currents using the Jaeger convention, but PSPICE will show IC and IB as negative, and IE positive (still IE+IB+IC=0).
• Next, PSPICE lists the terminal voltage differences VBE, VBC, and VCE. Examining VBE and VBC provides a quick way to check the operating region of the BJT.
• If the BJT is in forward active, BETADC is essentially the value of (beta-sub-F) as defined in Jaeger; it includes the effect of the Early voltage correction factor. If the BJT is in saturation, it is the value of the forced beta (beta-sub-forced), which is the ratio of IC to IB.
• GM, RPI, RX, and RO are the hybrid-pi small signal model parameters used in ac analysis.
• The various capacitances, BETAAC, and FT are useful for frequency-dependent analysis. In particular, FT is the unity-gain frequency of the common-emitter short circuit current gain.

NAME         Q1        
MODEL        nmod      
IB	5.67E-04	...This shows a dc current of 0.567 mA flowing into the base
IC	5.96E-02 	...This shows 59.6 mA flowing into the collector
VBE    	7.00E-01	...B-E junction is forward-biased
VBC   	 -5.00E+00	...C-B junction is reverse-biased (n-type coll. higher than p-type base)
VCE	5.70E+00	      ...These junction conditions indicate forward active operating region.
BETADC	1.05E+02 
GM	2.30E+00 
RPI	4.56E+01 
RX	0.00E+00 
RO	1.76E+03 
CBE	0.00E+00 
CBC	0.00E+00 
CBX	0.00E+00 
CJS	0.00E+00 
BETAAC	1.05E+02 
FT	3.67E+19 

JOB CONCLUDED
(7) The last section prints CPU statistics. More detail can be requested by specifying various parameters in the '.options' statement.

TOTAL JOB TIME .55
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