Add DC transfer

examples/diff_amp/BC547A.PRM

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+*
+.MODEL QBC547A NPN( 
++     IS=1.533E-14
++     NF=1.002
++     ISE=7.932E-16
++     NE=1.436
++     BF=178.7
++     IKF=0.1216
++     VAF=69.7
++     NR=1.004
++     ISC=8.305E-14
++     NC=1.207
++     BR=8.628
++     IKR=0.1121
++     VAR=44.7
++     RB=1
++     IRB=1E-06
++     RBM=1
++     RE=0.6395
++     RC=0.6508
++     XTB=0
++     EG=1.11
++     XTI=3
++     CJE=1.61E-11
++     VJE=0.4209
++     MJE=0.3071
++     TF=4.995E-10
++     XTF=139
++     VTF=3.523
++     ITF=0.7021
++     PTF=0
++     CJC=4.388E-12
++     VJC=0.2
++     MJC=0.2793
++     XCJC=0.6193
++     TR=1E-32
++     CJS=0
++     VJS=0.75
++     MJS=0.333
++     FC=0.7762 )
+*

examples/diff_amp/control.sp

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+* Note that inside control block the commands order is relevant
+
+.control
+* set variables
+set noaskquit
+set filetype=ascii ; rawspice.raw (see `write` command) in ascii format
+set nobreak ; output.log won't page break data
+set noacct
+
+options NOINIT NOMOD
+
+* DC transfer characteristic:
+*   - vary input voltage from -1V to 1V in steps of 0.1V
+dc v1 -1 1 0.1
+
+run ; run the actual simulation
+
+* raw file path can be specified with -r flag: ngspice -r data.raw
+* it defaults to ./rawspice.raw if a path is not specified with -r.
+write
+quit
+.endc

examples/diff_amp/main.cir

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+AoE 2_13 differential amplifier
+* this is 2_13.cir file
+* supply voltage resources
+vp 77 0 dc 15V
+vn 88 0 dc -15V
+* input voltages
+v1 1 0 dc 0.8V
+v2 2 0 dc 0
+* bipolar transistors
+q1 77 1 11 QBC547A
+q2 22 2 21 QBC547A
+*emitter resistors
+re1 11 20 1k
+re2 21 20 1k
+*collector resistor
+rc 77 22 75k
+*common emitter resistor
+r1 20 88 75k
+
+* model for a pnp transistors
+.include BC547A.PRM
+
+.end 
+