I'm not sure how to succinctly phrase my question, so bear with me. I'm trying to make a very simple tail-pulse generator for a specific application. In the old days, this was done with a mercury-wetted relay. Imagine a SPDT relay. The common terminal would be attached to a capacitor, which was also in series with a resistor. The time constant of the tail is then just the RC time constant. When the relay is in the NC position, the capacitor is charged and a constant current is flowing through the resistor. Then the relay switches and the NO contact connects the output to the capacitor, the voltage on which decreases as it discharges through R.
So, that's the basic principle. In my case, this has to go somewhere where a mercury-wetted relay is not possible. I thought I could replace the relay with MOSFETs, but my simulation is giving me strange behavior. It works with a generic/ideal MOSFET, but as soon as I pick a real MOSFET, things go wonky. The trace on the left below is what I get in LTSpice with a generic MOSFET. The trace on the right is what I get with any randomly selected MOSFET. I've tried different Rdson values, etc.
Any advice? Where should I go next for troubleshooting? Maybe there is a better way to do this?
Thanks!
So, that's the basic principle. In my case, this has to go somewhere where a mercury-wetted relay is not possible. I thought I could replace the relay with MOSFETs, but my simulation is giving me strange behavior. It works with a generic/ideal MOSFET, but as soon as I pick a real MOSFET, things go wonky. The trace on the left below is what I get in LTSpice with a generic MOSFET. The trace on the right is what I get with any randomly selected MOSFET. I've tried different Rdson values, etc.
Any advice? Where should I go next for troubleshooting? Maybe there is a better way to do this?
Thanks!