Transistor as an Amplifier – Circuit Diagram, and Its Working

A transistor is a three terminal semiconductor device, and the terminals are E(Emitter), B (Base) & C (Collector). The transistor can work in three different regions like active region, cutoff region & saturation region. Transistors are turned off while working in the cut-off region and turned on while working in the saturation region. Transistors work as an amplifier while they work in the active region. The main function of a transistor as an amplifier is to enhance the input signal without changing much. Here this article discusses how a transistor works as an amplifier.

Transistor as an Amplifier

Amplifier circuit can be defined as, a circuit which is used to amplify a signal. The input of the amplifier is a voltage otherwise current, where the output will be an amplifier input signal. An amplifier circuit which uses a transistor otherwise transistors is known as a transistor amplifier. The applications of transistor amplifier circuits mainly involve in audio, radio, optical fiber communication, etc.

Thetransistor configurationsare 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. Because in CB configuration, the gain is <1, and in CC configuration, the gain is almost equivalent to 1.

The parameters of a good transistor mainly include different parameters namely high gain, high slew rate, high bandwidth, high linearity, high efficiency, high i/p impedance, and high stability etc.

Transistor as an Amplifier Circuit

A transistor can be used as an amplifier by enhancing the weak signal’s strength. 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.

Common Emitter Amplifier Circuit Diagram

In most of the electronic circuits, we use commonly NPN transistor configuration which is known as NPN transistor amplifier circuit. Let us consider a voltage divider biasing circuit which is commonly known as a single stage transistor amplifier circuit.

Basically, the biasing arrangement can be built with two transistors like a potential divider network across the voltage supply. It provides the bias voltage to the transistor with their middle point. This type of bias is mainly utilized in the bipolar transistor amplifier circuit design.

In this kind of biased, the transistor will reduce the current amplification effect factor ‘β’ by holding the base bias on a constant steady voltage stage & permits precise stability. The Vb (base voltage) can be measured with the potential divider network.

In the above circuit, the entire resistance will be equal to the amount of two resistors like R1 & R2. The produced voltage level at the two resistors junction will hold the constant base voltage at a supply voltage.

The following formula is the simple voltage divider rule, and it is used to measure the reference voltage.

Vb = (Vcc.R2)/(R1 + R2)

The similar supply voltage also decides the utmost collector current, as the transistor is activated that is in saturation mode.

Common Emitter Voltage Gain

Common emitter voltage gain is equivalent to the modification within the input voltage ratio to the modification within the amplifier o/p voltage. Consider Vin and Vout as Δ VB.& Δ VL

In conditions of resistances, the gain of the voltage will be equivalent to the signal resistance ratio within the collector toward the signal resistance within the emitter is given as

Voltage Gain = Vout/Vin= Δ VL/Δ VB = – RL/RE

By using the above equation, we can simply determine common emitter circuit voltage gain. We know that bipolar transistors include minute internal resistance built into their emitter section that is ‘Re’. Whenever the inside emitter resistance will be connected in series by the outside resistance, the customized voltage gain equation is given below.

Voltage gain = – RL/(RE + Re)

The whole resistance in the emitter circuit at low-frequency will be equivalent to the amount of the inner resistance & the external resistance that is RE + Re.

For this circuit, the voltage gain at high frequencies as well as low frequencies includes the following.

The voltage gain at high frequency is = – RL / RE

The voltage gain at low frequency is = – RL/(RE + Re)

By using the above formulas, voltage gain can be calculated for the amplifier circuit.

Thus, this is all about transistor as an amplifier. From the above information, finally, we can conclude that a transistor can perform like an amplifier only when it is biased properly. There are several parameters for a good transistor which includes high gain, high bandwidth, high slew rate, high linearity, high i/p impedance, high efficiency, and high stability etc. Here is a question for you, what is 3055 transistor amplifier?