# How do I solve the input resistance of this BJT and MOSFET in series?

This is a sedra-smith microelectronics excercise question for which I can not find the answer and I hoped for perhaps a solution here?

• The small signal parameter for a resistor is trivial. But what do you imagine are the small signal parameters for a BJT and FET? You might start by listing those so that we know what you think you are working with and what needs to go into the function. (There are at least two commonly used small signal models for the BJT (one of them applies to both BJT and FET) called the Y-model and the other called the linearized hybrid-$\pi$ model, for example. So which is it? (I don't have the book.) – jonk Oct 20 '19 at 1:23
• @jonk My main confusion is with the dotted lines, I do not not understand what it means. – dilinex Oct 20 '19 at 1:27
• It just means that the lines "go somewhere" and that you probably don't need to know where (because you understand how a collector behaves, for example) or else you are already told the circumstances for them elsewhere in the text and should know from reading the text. One of the two, most likely. – jonk Oct 20 '19 at 1:28
• @jonk Given the text gives no indication of where those lines go, it means it must go "somewhere". – dilinex Oct 20 '19 at 1:31
• @jonk I know Ix must be equal to the BJT base current however my problem is finding out what Vx is. – dilinex Oct 20 '19 at 1:33

I don't use the equations.

I figure it out from datasheet plots and excel as I did for the diode.

How to model a real diode 1N4148? Then I verified on Falstad Simulator with my parameters

Since Ri load line at BJT saturation or FET conduction has 50% factory tolerance , your choice of datasheets matters with overdrive levels. Then show the ON diode curves here. 