tsb_nph
Full Member level 2
Re: how to run transient simulation with time varying resist
Hi,
I hope you understand the difference between a grounded and floating resistor.
If you have a two-terminal resistor, then each of the nodes can be connected to two arbid nodes.
You have a grounded resistor from node 2 to ground and not a two-terminal resistor, that was why i was specifically asking what are the two nodes of the resistor.
The original question was how you model a time varying resistor. That resistor can be connected between vdd and output node of a resistively loaded diff pair, between output and input nodes of an opamp etc.. Hope you see the difference.
When the resistor is tied between input and output nodes of an opamp, the
current through the resistor is defined by the input and output voltages of the opamp, but whenever you vccs, you can't independently define the voltage across two nodes of your model. This is when impedances connected to the two resistor nodes (as i mentioned in my second reply) are important.
If you want a generic two-port resistor model, your model will not work.
Or maybe, can you show me some modification where you model a true two-port time-varying resistor terminal model that can be connected across any 2 nodes in a circuit.
Bharath
v_c said:I think you are getting confused about node R. That node is strictly for generating and arbitrary signal that is
supposed to give us the the value of R. Nothing else will be connected to that node.
Take a look at the attached file for a schematic of my system. What I gave was a particular application, but yes
you can use this as a subcircuit as follows.
.subckt var_res plus minus control
Gpm plus minus value={v(plus,minus)/v(control)}
Rcontrol control 0 10k
.ends var_res
so this resistor has the normal terminals plus and minus and has an extra terminal called control.
Even though control is a voltage, the subcircuit interprets this as the resistance value. So if you
put a 3V source at control, this will be a 3 ohm resistor. And this subcircuit is totally re-usable.
I think what may be confusing is the fact that the varying value of the resistance is being set
by a voltage -- there is nothing we can do about this, spice only understands voltage and current.
So just try out this subcircuit, first with a constant voltage at v(control) to see if this really works
as a resistor -- I think you will find that it works just fine. Then, you can try to put any arbitrary
voltage at the control node, whatever voltage you put there in volts will be the resistance (in ohms)
of the node from plus to minus.
Best regards,
v_c
Hi,
I hope you understand the difference between a grounded and floating resistor.
If you have a two-terminal resistor, then each of the nodes can be connected to two arbid nodes.
You have a grounded resistor from node 2 to ground and not a two-terminal resistor, that was why i was specifically asking what are the two nodes of the resistor.
The original question was how you model a time varying resistor. That resistor can be connected between vdd and output node of a resistively loaded diff pair, between output and input nodes of an opamp etc.. Hope you see the difference.
When the resistor is tied between input and output nodes of an opamp, the
current through the resistor is defined by the input and output voltages of the opamp, but whenever you vccs, you can't independently define the voltage across two nodes of your model. This is when impedances connected to the two resistor nodes (as i mentioned in my second reply) are important.
If you want a generic two-port resistor model, your model will not work.
Or maybe, can you show me some modification where you model a true two-port time-varying resistor terminal model that can be connected across any 2 nodes in a circuit.
Bharath