oh, k , got them bkwards! , Please feel free to use any voltage source, or any gain level for the transistors, or modify the resistors as you see fit, im just trying to figure out a general method for my switching apps, i just dont get why(or when) you would use resistors at all, since all the power u need to switch should just come off an input line,no?
The tutorial is WRONG.
Beta or hFE is used only in a linear amplifying transistor that has plenty of collector to emitter voltage. A saturated transistor has a very low collector to emitter voltage so beta or Hfe is not used to calculate base current. Instead, the datasheets for most little transistors show a base current that is 1/10th the collector current.
The concern about using the input line to switch the transistor is that a bjt has no limit on the amount of current it will allow to go into the base connection...even if it causes smoke. If the input signal is capable of supplying too much current, you must limit the current with a resistor.
Usually the transistor configuration is selected based on what the application is. That is, what is required of of the application in what does it have to do. After you select the configuration, then you select the operating point.
In other words, if you would tell us your end application that would help a lot.
well.. im using a op amp light chasing circuit and i need to connect the op amp to a 555 timer(astable config) as a VCO, connected to a 4bit counter to drive a h bridge for a stepper motor,
In the simulation the op amp goes haywire so when i wire it in reality i know what to do when : a)the op amp underpowers the 555 timer
B)to balance the op amp and the timer vco input
the main circuits im trying to integrate are as follows: