Hello, does anyone know why the output goes 5V and 4.9V when transistor is ON and off respectively? the output should be 5V and 0V
I am simulating something and i dont have my pc right now for ltspice simulation. Thanks.

 

hi 144,
This is what LTSpice shows, it appears Falstad is in error?
E

 

hi 144,
This is the plot with actual transistor types.
E

 

It comes down to the transistor model being used. With the output floating, the behavior of it is problematic for simple models since no current can flow. If the junctions are treated as having capacitance with no leakage, then a voltage can be maintained indefinitely across the junctions, with variations being due to charge injection as the base is moved up and down.

 

hi 144,
This is the plot with actual transistor types.
E
View attachment 359197

Thanks Eric for confirming, it seems I need my PC for simulation.

 

It comes down to the transistor model being used. With the output floating, the behavior of it is problematic for simple models since no current can flow. If the junctions are treated as having capacitance with no leakage, then a voltage can be maintained indefinitely across the junctions, with variations being due to charge injection as the base is moved up and down.

Hmm.. Thanks for the inputs. I don't think I can change the model in falstad, but will try later.

 

Hmm.. Thanks for the inputs. I don't think I can change the model in falstad, but will try later.

The bigger question is what are you doing where it matters?

If nothing is connected to the emitter, what does it matter what the voltage is there, since it isn't being applied to anything?

If it's being applied to something, then you can expect any simulator to tell you what the voltage is going to be unless you include it in your simulation -- simulators are lousy mind-readers.

 

The bigger question is what are you doing where it matters?

If nothing is connected to the emitter, what does it matter what the voltage is there, since it isn't being applied to anything?

If it's being applied to something, then you can expect any simulator to tell you what the voltage is going to be unless you include it in your simulation -- simulators are lousy mind-readers.

The output on the emitter serves as a pull up to an input signal. And yes I think you are correct on the model, since Ltspice shows different results based from Eric's simulation.

 

Put a load resistor, say 1 megohm, from the output to ground and see what Falstad then tells you.

 

Hi Alec, yup I already tried that. I was just confused on the falstad simulation since it shouldn't behave like that even if no load is connected.