The slider adjusts the input voltage to control the voltage across the load resistance. The feedback loop insures that the voltage across the load resistance is almost exactly equal to the input voltage. The voltage across the load resistance determines the current through it by Ohm's Law. This is also the transistor's collector current. The operational amplifier only needs to supply the transistor's base current, which is much less that the collector current because of the current gain (beta) of the transistor.
To see the current flow throught the transistor, click on the "Show current flow" button.
You can change the load resistance and the power supply voltage (Vcc). Just click on the number with the mouse pointer and edite 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.
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