Hi I just have a quick question. I was wondering if someone could tell me the difference between internal resistance and input resistance? Thanks

 

the internal resistance is within the device (input device) but input resistance is output of device , maybe is input resistant of circuit

 

The input resistance is the resistance offered at the input of the transistor (normally the transistor is at the input stage of an amplifier). You can see the concept of input resistance, or input impedance applied to the inputs of amplifiers (op-amps audio amps and such), However, I never saw it applied to an individual transistor.

Internal resistance should be the resistance measured across the emitter and colector of a transistor.

 

To expand the answer with an example -- If you have common emitter amplifier using a BJT then the input resistance is the resistance "looking" into the base. It is roughly equal to thermal voltage divided by the base current or Beta times the thermal voltage divided by collector current. Say your collector current is 1mA and your beta is 100 then your input resistance would be ~2.5K Ohms.