Basic Principle work of transistor current source

Dear all hands

Transistor Current source although often neglected are as important and as useful as voltage source. Transistor current source often provide an excellent way to bias a transistors, an transistor current source are unequaled as “active load” for super gain amplifier stage and as emitter source for differential amplifier.

Integrator, saw tooth generator and ramp generator are need current source. Transistor current source also needed in outside world such of in electrophoresis or electrochemistry.

Basic principle work for current source is the circuit capable to hold current at desired value despite the load is change.

Below is transistor current source circuit with basic principle work :

Basic principle work of transistor current source circuit above is :

1. Applying VB to the base, with VB > 0,6 v
2. Ensures that emitter is always conducting :

So

VE =VB-0,6 V

IE = VE/RE = (VB-0,6 v)/RE

But , since IE =IC for very large hfe

IC = (VB-0,6)/RE

From latest expresion, load current in this current source circuit just only depend on RE value, an independent from load value

We can simulate transistor current source circuit above with proteus isis.