modified 10 years ago

Basic CMOS Inverter With Switched Input

01:34:30

Must tie gate\switch node to ground via high resistance pull-down resistor to get correct simulation results

published 10 years ago

mapplejacks

10 years ago

Put the resistor on the gate rather than the middle. As it is, when the switch is open, the gates are floating.

abalogh

10 years ago

Placing a resistor on the gate should not matter here as i'm putting a known voltage across the input. But anyway, it looks like the problem IS with the switch. Removing it and changing the voltage between 0&5V during simulation gives the expected output on the node.. I found the SPDT switch did'nt work correctly here either (with one of its outputs tied to ground).

mapplejacks

10 years ago

The spdt does work, but the high voltage isn't 5v because of your load. Ways around that: wider transistors or higher load resistance. The reason a resistor works at the gate for spst is because it gives connection to ground when switch is open: a pull down resistor. Otherwise it's just floating. One end goes to gates, the other to gnd. Not in series with the switch.

abalogh

10 years ago

Okay, i see your point, and it does work as such. I was using the switches with the assumption that the models had a built-in shunt resistence to ground

mapplejacks

10 years ago

Is this typical of real life switches?

abalogh

10 years ago

Fair enough; in real life for this setup you would need a pull-down resistor in order to ensure the input is tied to the correct logic level as you have pointed out. My confusion was that i percieved the switches in the app as stricly 'virtual' devices and not as actual ones.

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