When using a Transistor as a Switch

[Jerry James]

I was trying to solve one of the questions posted in this forum and while solving I started to have some doubt.
If a transistor is designed to work as a switch I can control the voltage and current of the output when the transistor is in ON mode i.e voltage is Vcc(supply voltage) and current is Vcc/RC (supply voltage/collector resistor). But I cannot control the current in the OFF mode and the voltage is fixed at 0V. Am I correct in my explanation? Also will there be a situation where in OFF mode I need to supply some minimum current? How should I solve this kind of problem?

[Jerry]

     Your first problem is that when the transistor is ON your output voltage would be near zero, not Vcc.  In this state your transistor would be operating in the "saturation mode".  When you say you can control the voltage and current when it is ON, you want the voltage near zero (voltage control), you want the current to be whatever is required to put the transistor into saturation (current control).

     Now in the OFF mode the transistor would be operating in the "cutoff mode".  At cutoff the current would be zero (current control) and the voltage would be Vcc (voltage control).

     As you can see we only have two states and we don't have much choice as to what the current and voltage can be.