Transistor acting as an amplifier

As probably you have often heard or know; transistors are used to switch and also amplify signals. This article describes with schematics transistor acting as amplifier of voltage or current and audio signals.

To use a transistor as signal amplifier, set it or bias it to operate in its active region. The low voltage and or current signal for amplification is applied to one pair of terminals of the transistor (input) for example; base (B) and emitter (E). The transistor then amplifies this signal at the other pair of terminals (output) for example; collector (C) and emitter (E). The output power depends on the supply and the transistor’s gain. You can read to understand more about transistor’s gain HERE.

Transistor Voltage or Current Amplifier

The circuits below show how transistors can be configured as voltage amplifiers using both NPN and PNP Bipolar Junction Transistors (BJTs). A small voltage (VInput­) is applied across the base (B) and emitter (E) of the transistor and it is amplified (VOutput) across the collector (C) and emitter (E). The configuration used is a Common Emitter (CE) voltage amplifier configuration. That is; the emitter (E) is the common terminal for both the input and output of the transistor.

npn transistor as voltage or current amplifier

PNP transistor as voltage or current amplifier

The value of R2 should be much greater than R1 at least by 10 to 60 times. Please read transistor active region if you are confuse.

Transistor Audio Signal Amplifier

However, if we want to amplify an audio signal, we couple the input and output signals by using a coupling capacitors. This is because; capacitors block direct current from passing through them but allow alternating current to pass freely. This cancels away any direct signal or noise from the input and output. The gain may be too high and may cause distortion in the audio quality. Connecting resistor like 22Ω to 50Ω in series from the emitter (E) to the 0V that is; for the NPN or to the positive supply for the PNP can control this.

NPN transistor acting as amplifier

PNP transistor acting as an audio amplifier

The configurations above are Common Emitter (CE) configurations. That is; the emitter (E) is the common terminal for both the input and output of the transistors. Transistor amplifiers have three (3) different configurations typologies for amplifications. These are; Common Emitter (CE); Common Collector (CC); and Common Base (CB).

The transistors’ biasing in this article is from base to rail voltage supply. However, there are different biasing methods which one can use. See also transistors biasing HERE.

I hope you like this article “transistor acting as amplifier”.

See Bipolar Junction Transistor (BJT) Cut Off (FULLY OFF) Region; Saturation (FULLY ON) Region; and Active Region.


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