<< prev story circuitfantasia > circuit stories > understanding circuits > capacitor shifted current source next story >> 

What is the idea behind this circuit?Transistor Current Source with Shifting Capacitor 

Idea: Keeping a constant current by following voltage (bootstrapping). The simple current source is imperfect as the load affects the current. The voltage drop VL across the load (suppose that it is a varying resistor R) is harmful as it enervates the excitation voltage V thus decreasing the current I. According to the basic idea above, we change continuously the voltage Vvar by following the voltage drop VL across the load, in order to compensate it. In this way, Vvar acts as a following voltage source keeping constant voltage difference Vvar  VL, which produces a constant current. 
Implementation: Using an emitter follower as a following voltage source. We may see this idea in the next BJT constant current source using following voltage compensation (bootstrapping). Here, the emitter follower ( transistor T) tracks the load voltage VTL (the potential of the point A). Then, the capacitor C lifts additionally the potential and applies it to the point B. As a result, the potential of the point B follows the potential of the point A. The voltage difference VR = VA  VB across the resistor R stays steady and a constant current passes through the resistor R and the load TL. 
<< prev story circuitfantasia > circuit stories > understanding circuits > capacitor shifted current source next story >> 