How does a transistor amplify a signal? A low-amplitude signal is applied to the transistor's base, and this signal is amplified at the collector. How does this happen? The transistor's collector-emitter junction is a resistor with a variable resistance. This resistance is set by a resistor connected to the transistor's base, or bias.
Connect an ohmmeter between the transistor's collector and emitter, and apply bias current to the base using a 500 kOhm variable resistor and a 6 kOhm resistor in series with it. The transistor is an NPN transistor, and the resistor is connected to the + power supply. By changing the bias resistor, you will see how the transistor's collector-emitter junction resistance changes,In my case, from 40 to 378 Ohm.
I have assembled an equivalent transistor, which consists of a 1.6 k resistor and a 1 k variable resistor. I connected a voltmeter to a variable resistor and applied a voltage of 7 V to an equivalent transistor. By changing the resistance of the variable resistor, I change the voltage. This is called a voltage divider. The voltage divider consists of a collector-emitter junction of a transistor and a resistor connected to the collector of the transistor.
My voltage divider produces from 0 to 2.7V
This also happens with a real transistor amplifier. The signal at the transistor's base controls a voltage divider consisting of the transistor's junction and a resistor in the collector. The signal changes the junction's resistance and, consequently, the voltage drop the collector-emitter.
Connect a 10 μF capacitor to the voltage divider and connect it to an oscilloscope. By smoothly changing the resistance of the variable resistor up and down, you will see a sine wave on the oscilloscope screen. This is how the signal is amplified.
Why does the amplifier invert the signal? When the positive half-wave of the signal is applied to the base of an NPN transistor, this causes the transistor to be fully closed, and therefore the collector-emitter junction will have minimal resistance and minimal voltage drop. The amplifier's output signal will be negative. Conversely, when the negative half-wave is applied to the input, this causes the transistor to be fully open, and its junction resistance will be higher, resulting in a greater voltage drop across the junction, and the output half-wave will be positive.
I sharply increase the resistance of the variable resistor and the signal will be positive and vice versa
Where does the amplifier get its power? It gets its power from the power supply.
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