The applications of transistor amplifier circuits mainly involve in audio, radio, optical fiber communication, etc. The transistor configurations are classified into three types such as CB common base , CC common collector , and CE common emitter. But common emitter configuration is frequently used in the applications like an audio amplifier. With the help of the following transistor amplifier circuit, one can get an idea about how the transistor circuit works as an amplifier circuit.
In the below circuit, the input signal can be applied among the emitter-base junction and the output across the Rc load connected in the collector circuit. For accurate amplification, always remember that the input is connected in forward-biased whereas the output is connected in reverse-biased. For this reason, in addition to the signal, we apply DC voltage VEE in the input circuit as shown in the above circuit.
Generally, the input circuit includes low resistance as a result; a little change will occur in signal voltage at the input which leads to a significant change within the emitter current. Because of the transistor act, emitter current change will cause the same change within the collector circuit.
At present, the flow of collector current through an Rc generates a huge voltage across it. Therefore, the applied weak signal at the input circuit will come out in the amplified form at the collector circuit in the output. In this method, the transistor performs as an amplifier. The emitter current caused by the input signal contributes the collector current, which when flows through the load resistor R L , results in a large voltage drop across it.
Thus a small input voltage results in a large output voltage, which shows that the transistor works as an amplifier. Let there be a change of 0.
This emitter current will obviously produce a change in collector current, which would also be 1mA. Hence it is observed that a change of 0. As the common emitter mode of connection is mostly adopted, let us first understand a few important terms with reference to this mode of connection.
As the input circuit is forward biased, the input resistance will be low. The input resistance is the opposition offered by the base-emitter junction to the signal flow. The output resistance of a transistor amplifier is very high. The collector current changes very slightly with the change in collector-emitter voltage. The DC voltage V EE is applied to the input circuit along with the input signal to achieve the amplification.
The DC voltage V EE keeps the emitter-base junction under the forward biased condition regardless of the polarity of the input signal and is known as a bias voltage. When a weak signal is applied to the input, a small change in signal voltage causes a change in emitter current or we can say a change of 0. This change is almost the same in collector current because of the transmitter action. In the collector circuit, a load resistor R C of high value is connected.
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