To act transistor as an amplifier, the most important function is the transistor has to amplify the magnitude of a weak signal and so it serves as a good amplifier. The basic circuit of a transistor as an amplifier is shown in the figure below.
The weak ac signal is ΔVI, to be amplified is applied between the emitter-base junction of the transistor and the output is taken across the load resistor RL. The battery VEE forward biases the emitter-base junction and VCC reverse biases the collector-base junction of the transistor. This result in faithfull amplifification of transistor as an amplifier.
The process of increasing the strength of a weak signal without any change in the shape of the waveform is known as amplification.
- The input circuit has low resistance, whereas the output circuit has high resistance. The input resistance between the emitter and the base of the transistor will typically vary from 10 to 100 Ω, while the output resistance may vary from 100KΩ to 1MΩ.
- The collector current flowing through RL produces a large ac voltage drop ΔVO across it. Thus a weak signal is applied in the circuit appeared as large signal in the collector circuit.
Output voltage is given by ΔVO = ΔIC RL
= αΔIE RL (since ΔIC = αΔIE)
Voltage Amplification is defined as the ratio of output voltage to input voltage, and it is denoted by AV,
AV = ΔVO / ΔVI
= αΔIE RL / re’ αΔIE
We know that re’ =26/IE, where IE is the quiescent emitter current. Since the transistor transfer the entire input signal current from a low resistance base-emitter region to a high resistance base-collector region called “transfer resistor” or transistor in short.
TRANS FER+RES ISTOR ----> TRANSISTOR
Advantages of using transistor as an amplifier
- A common emitter amplifier is inverting and has low input impedance
- high output impedance
- high voltage gain
- high current gain.
- The common base circuit works best as a current buffer. It can take an input current at a low input impedance, and deliver nearly that same current to a higher impedance output.
- When placed between the the two stages it prevents the stage with the low input impedance from overloading the stage with the high output impedance.