# SPICE design help

#### mcutitaru

Joined Sep 18, 2011
3
Hi,

I'm trying to make a PMOS spice design and the part where I am stuck is this: I have 2 pmos transistors and they need to go in serial in a certain way. The 1st transistor has the Source from Vcc, and Gate from Ground. Its Drain is connected to the 2nd transistor's Gate (to act as a control for the 2nd transistor). The 2nd transistor has its own connections to other parts of the circuit. This is what I have so far for this part of the circuit:
mpmos1 1 0 x x *1=Vcc, 0=GND
mpmos2 1 x 0 0 *(not complete)
Can anyone suggest a way to connect the two transistors? I tried adding a new node, but it doesn't solve the problem. Thank you!

#### SgtWookie

Joined Jul 17, 2007
22,227
If you connect PMOS1's gate to GND (node 0), then it will always be ON, so the the drain will always be at the potential of Vcc. This means that PMOS2's gate will also be at Vcc, so PMOS2 will always be OFF.

It sounds to me like you need a better plan.

As it is, your netlist should look more like:
Rich (BB code):
V1 Vcc   0 [voltage]
M1   1   0 Vcc Vcc PMOS
M2   ?   1 Vcc Vcc PMOS

#### mcutitaru

Joined Sep 18, 2011
3
Thank you for your reply! I know that using DC on the 1st PMOS will either have it ON or OFF, depending on the Gate input. However I plan on using AC and do a sinusoidal voltage swing from 0V to (say) 1V, so that the 1st PMOS would be ON to begin with but then slowly turn OFF as the voltage at the source is ramped up past the threshold. So I need to have the connections they way I have them. Any thoughts? (I hope I am explaining it well since my background is more in digital). Any help is appreciated! Thank you!

#### SgtWookie

Joined Jul 17, 2007
22,227
I'm not really sure what you want to do.

You need to explain it more completely.