transistors

Here we start with the basic element of micro electronics,obviously it will be Transistor


A Transistor can be thought of as a device that is active in only One Direction:  It can draw more or less current through its load resistor (sometimes referred to as a pull-up resistor). 

 It can either Source Current or it can Sink Current, it Cannot do Both.

Since the Transistor is a Current device, any signal Voltage must first be  Converted to a Current.   Voltage to Current Convertor  First, you must convert the input voltage to a current by  using a Voltage to Current Convertor--a resistor. Since the Transistor is a Current in/Current out device, any Current Output is  Converted to a Voltage Drop by the Current flowing thru a Load Resistor.   Current to Voltage Convertor Next, you convert the output current into a voltage by  using a Current to Voltage Convertor in the collector circuit--you  guessed it--a resistor.

Note voltage to current convertor in the base circuit.  A.K.A., current limiting resistor. .. Results of driving a transistor's base directly from a voltage  source, with no voltage to current convertor, i.e., a resistor.