Transistor as amplifier

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In addition to being used as switches, transistors can also be used as amplifiers. Using a transistor this way, a low power signal is effectively translated into a higher power signal. The following example exhibits this behavior.

In this example, the photo-resistor is used as a variable resistor. This photo-resistor resistance is used in a classic Voltage Divider circuit to feed an analog signal into the base of the transistor. The transistor then opens the flow of current through the LED in proportion to how much current is flowing into the transistor base. The potentiometer is included in order to allow you to tune the sensitivity of the voltage divider circuit for different light conditions.

Note: This example uses the Arduino 5V pin as the power supply and LEDs as the "load" for simplicity. The power coming out of the Arduino 5V pin (max 40mA) is generally too small to drive much more than a few LEDs or a very small motor. For larger loads, such as bigger motors or relays, or stronger lights or sound, a stronger power supply, such as from batteries or a power adaptor, would be used instead.

The circuit

Transistor as amplifier circuit

This type of circuit is called using a transistor in "common emitter mode", since the base->emitter circuit and collector->emitter circuits both connect current to ground at the same emitter.


  1. How does turning the potentiometer to maximum resistance affect the sensitivity of the circuit to the photo-resistor's signal, as compared to turning the potentiometer all the way to minimum resistance?

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