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Transistor Current Source with Shifting Capacitor. Click to view full-size picture.

What is the idea behind this circuit?

Transistor Current Source with Shifting Capacitor

Idea: Keeping a constant current by following voltage (bootstrapping).

In order to keep a constant current, we may change the excitating voltage Vvar thus compensating the voltage drop VRL across the load. Click to view full-size picture.

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.

In thid clever circuit, the emitter follower T and the shifting capacitor C keep up steady voltage difference across the resistor R. As a result, a constant current passes through the resistor R and the load TL. Click to view full-size picture.

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.

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