A Diode Limiter Circuit 

** Circuit Description **

Vi 1 0 DC 0V
R 1 2 1k
D1 2 0 D1N4148
D2 0 2 D1N4148
* diode model statement
.model D1N4148 D (Is=0.1pA Rs=16 CJO=2p Tt=12n Bv=100 Ibv=0.1p)

** Analysis Requests **
* sweep the input voltage level from -5 V to +5 V in 100 mV increments
.DC Vi -5V 5V 100mV
** Output Requests **
.PLOT DC V(2) 
.probe
.end