Differential Amplifier: Asymmetric Saturation Current 

** Circuit Description **
* power supply
Vdd 6 0 DC +15V
Vee 7 0 DC -15V
* differential-mode signal level
Vd 101 0 DC 0V AC 1V
Rd 101 0 1
EVp 1 100 101 0 +0.5
EVm 2 100 101 0 -0.5
* common-mode signal level
Vcm 100 0 DC 0V
* differential pair
Q1 4 1 3 npn_transistor1
Q2 5 2 3 npn_transistor2
* load resistors
Rc1 6 4 10k
Rc2 6 5 10k
* bias source
I 3 7 DC 1mA
R 3 0 200k
* transistor model statements
.model npn_transistor1 npn ( Is=13.65fA Bf=100 VAf=100V )
.model npn_transistor2 npn ( Is=14.35fA Bf=100 VAf=100V )
** Analysis Requests **
.DC Vd -10mV +10mV 100uV
.OP
.TF V(5,4) Vcm
** Output Requests **
.PLOT DC V(5,4)
.probe
.end