I would like to add a "negative" resistance onto the lumped element in CST, but I don't know how to do that. Could anybody help me? Thank you very much for your kindness!
What the hell is CST (see https://en.wikipedia.org/wiki/CST)? But you can use a tunnel diode to get a negative resistance. Another name for it is Esaki-diode.
What is the meaning of the single letters CST, the name e.g. Central Standard Time. You should also update the list of the different CSTs in Wikipedia. What's about the tunnel diode, success?