22BIT222-3

. AIM : BJT as an Amplifier Apparatus Required: Resistors, capacitors, transistor, LED, voltmeters, ammeters, battery, connecting wires, ac source Theory: Amplifiers are classified as small signal amplifiers and large signal amplifiers depending on the shift in operating point, from the quiescent condition caused by the input signal. If the shift is small, amplifiers are referred to as small signal amplifiers and if the shift is large, they are known as large signal amplifiers. In small signal amplifiers, voltage swing and current swing are small. Large signal amplifiers have large voltage swing and current swing and the signal power handled by such amplifiers remain large. Voltage amplifiers come under small signal amplifiers. Power amplifiers are one in which the output power of the signal is increased. They are called large signal amplifiers. Figure shows the circuit diagram of a common emitter amplifier. Voltage amplifiers come under small signal amplifiers. Power amplifiers are one in which the output power of the signal is increased. They are called large signal amplifiers. FREQUENCY RESPONSE The gain of an ideal amplifier should remain the same for any frequency of the input signal. Therefore, the frequency response curve (gain in db plotted against frequency) becomes a straight line parallel to the frequency axis. In actual practice, the coupling capacitors and the emitter bypass capacitor reduce the gain at lower frequencies. The capacitance internal to the transistor and stray capacitance due to the wiring reduce the gain at higher frequencies. Fig 2 shows the typical frequency response characteristics of CE amplifier. The curve is flat only for middle range of frequencies. There is one low frequency fL and one high frequency fH . Current gain = 1.14 Voltage gain = 4.02 Power gain = 1.114 ∗ 4.02 = 4.48 Ideally there is a phase shift of 180° between current and voltage but we do not observe it due to zeit or time delay