1.2.3. The transistor as an amplifier experiment

In order to conduct an experiment, it is necessary to determine the polarity of the transistor that will be used, which is shown in the following figure and should be consulted in the datasheet.

Figure 1‑24 NPN BC547 transistor orientation
(Source: referenced)

Components:

• breadboard
• 5V power supply
• 3mm red LED
• BC547 NPN transistor
• 220Ω resistor
• 1KΩ potentiometer

Figure 1‑25 The transistor as an amplifier experiment

Figure 1‑26 The transistor as an amplifier realization

Explanation: 220Ω resistor serves to limit the current through the LED when the transistor is in saturation and is effectively a closed switch. Transistor is connected in an emitter-follower configuration because the voltage at the emitter follows the voltage at the base – rising the base voltage will raise the emitter voltage, or in this case – across the LED. It is worth mentioning that the emitter voltage will be ~0.7V lower than the base voltage. The potentiometer acts as a voltage divider whose output voltage is biasing the base of the transistor. Let’s try to find out what happens when the potentiometer is in the middle position.  According to the formula, the output voltage on the base of the transistor will be:

Then, the voltage on the emitter, or across the diode is:

This voltage is sufficient for the LED to be biased and it starts glowing. If the position on the potentiometer is before the middle point, the emitter voltage is too low to bias the LED. And if the position on the potentiometer is after the middle point, the LED glows brighter. A transistor amplifier circuit can easily be converted into a circuit that uses a transistor as a switch.

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References:

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