Voltage to Current Convertor

When driving the input (base) of a transistor gain stage: you must convert the input voltage to a current by using a voltage to current convertor --a resistor.

Current to Voltage Convertor

When deriving a voltage from the collector of a transistor amplifier stage: you convert the output (collector) current into a voltage by using a Current to Voltage convertor in the collector circuit--you guessed it--a resistor.