Creating a Voltage Transfer Characteristic (VTC) for a Zener Diode Circuit

[foobag]

http://img847.imageshack.us/img847/1922/capturevq.jpg

I need help starting this.

So far I am getting Voh = +10V from the zener diode, and Vol = -10V, the upper and lower saturation limits based on the zener diodes

Then for threshold values I am getting an upper threshold Vth = -R1/R2 * Vol = 6.25 V

and a lower threshold as Vtl = - R1/R2* Voh = -6.25V

does this seem logical for the VTC for the first part?

Whats the point for R3 in this circuit other than protection of the zener diodes, does it/will it play effect on my calculations?

 

[vk6kro]

When you put zeners in series, one behaves like a zener and the other behaves like a diode, depending on the polarity. So, you get a Zener voltage plus 0.7 volts in each direction.

However, if you assume that this is a total of 10 volts, then your result looks OK.

The function of the Zeners would be to stabilize the Schmitt triggering point, which otherwise depends on the supply voltage.
The 2 K resistor gives a predictable current into the Zeners. It also limits this current.

It doesn't come into the gain calculation, though, because you get 10 volts across the Zeners, regardless of gain.

 

[foobag]

thanks a lot for your help! it makes more sense now

 

[foobag]

does anyone know what part I can use in PSPICE/ORCAD for use as a double anode zener diode?