Transistor Constant Current Sources !

Transistor Constant Current Sources !
Transistor Constant Current Sources !
Transistor Constant Current Sources !

 A current source is a device that outputs a specific amount of current, generating as much or as little voltage across its terminals to ensure that exact amount of current through it. Current sources are notoriously difficult to find in nature (unlike voltage sources, which by contrast attempt to maintain a constant voltage, outputting as much or as little current in the fulfillment of that task), but can be simulated with a small collection of electronic components. As we are about to see, transistors themselves tend to mimic the constant-current behavior of a current source in their ability to regulate current at a fixed value. 

If we assume that the ratio of currents in the transistor is constant (i.e. IC / IB ), an excellent current source can be built using the simple configuration shown in the following figure because no matter what the value of the load's resistor is, the collector's or load's current will remain the same. The base current is fixed and independent of the load at the collector, the current at the collector will ideally be the same: a perfect current source. However, as the real transfer characteristic of the transistor tell us that the current gain does change from one point to another, and in spite of the current being fixed, the current gain will change and thus the current at the collector and the load.

A voltage divider configuration causes a low level of sensitivity of the current gain hfe and the current source will be closer to the ideal one and the overall stability of the source will improve because an increase in the collector current IC will also increase the current in the emitter IE, and thus the voltage VRE, but if VB is fixed, only VBE can fall, which will cause IB and then IC to fall, tending to stabilize the system. The dependence on temperature can also be decreased by increasing de value of RE in order to make the negative feedback stronger, or by adding a diode at the branch containing R2 in order to compensate the temperature dependence throught the diode's voltage.

Transistor Constant Current Sources !

 Left: Sensitive configuration for a transistor constant current source; Right: Less sensitive configuration for a transistor constant current source

Short introduction to FET Transistors:

Although field-effect transistors will be introduced in the next Uni-Train Course, it is important that you know that while the bipolar junction transistors (BJT) we have been using up to now are current-controlled devices (the base current controls the amount of collector current), the FET is a voltage-controlled device. The voltage at the gate terminal controls the amount of current through the device. Besides, the FET has a very high input resistance, which makes it superior for certain applications. Its three terminals are called Gate (G), Source (S), and Drain (D). The FET operates as a voltage-controlled constant current device. It is always operated with the gate-source junction reverse-biased. The channel width can be controlled by varying the gate voltage, whereby the amount of drain current ID can also be controlled.

Transistor Constant Current Sources !

Transistor Constant Current Sources !

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