The purpose of this circuit is to control the current through the load device (shown in this circuit as a diode)
so that it is proportional to the input voltage but independent of the voltage
drop across the load and of the power supply voltage (within limits).
The slider adjusts the input voltage to control the current through the load device.
The feedback loop insures that the voltage across the series resistance Rs
is almost exactly equal to the input voltage.
The voltage across the series resistance Rs is related to the
current through it by Ohm's Law. The current through Rs is almost exactly equal to the current through the load, because they are connected in series and because the
operational amplifier's input draws a negligible current.
The operational amplifier's output only needs to supply the transistor's base current,
which is much less that the current through by the load device because of the current gain (beta) of
To see the current flow throught the transistor, click on the "Show current flow" button.
You can change the voltage drop across the load device, the series resistance
Rs, and the power
supply voltage (Vcc). Just click on the number with the mouse pointer and
edit like any text field. You can also inspect and modify the specifications
of the transistor - click on the "Show specs" button on the left; this display a
table of several specifications (such as the transistor's current gain, beta) that
you can modify.
Updated and corrected May 5, 2001. This page is maintained by Prof. T. C. O'Haver , Department of Chemistry and
Biochemistry, The University of Maryland at College Park.
Comments, suggestions and questions should be directed to
Prof. O'Haver at firstname.lastname@example.org.
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