Electronic Circuit Design Lab Experiments

Design a Common Emitter amplifier and determine its voltage gain and output resistance.

Sep 24 2023

Aim

Apparatus Required:

Breadboard ,Workstation, Function Generator, Trainer Kit, Patch cords, DC Power supply

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.

Circuit Diagram:

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 3.2 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. beyond which the gains, AL and AH are 1/√2 times the gain AM (maximum gain) at the middle frequencies. The two frequencies are called lower and higher cut off frequencies. The difference between them is called the bandwidth.

Procedure:

The circuit is set up as shown in figure 3.1. Input signal Vs is given to the circuit through a signal generator (sinusoidal signal is applied). Measure the magnitude (peak to peak) of the input by using CRO. Connect the CRO to the output side and the amplified output is observed. Increase the frequency in steps and observe the magnitude of VO. The frequency response is plotted in a semi log sheet.

Observations

Readings are to be taken till Vo decreases appreciably at high frequencies

Vin =.......................... (p-p)(mV)

Frequency f (Hz)	Vo(p-p) (mV)	Vo/Vin	Gain in db 20 log Vo/Vin

Result:The common emitter amplifier is designed, and its frequency response is plotted.

Voltage gain = Vo/Vin =

Lower cutoff frequency =

Upper cutoff frequency =

Bandwidth=

Viva Questions:

Q.-1. Explain in detail procedure for measuring β

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Q.-2. Using the values of β, determine the value of α.

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Q3- What is meant by bias stabilization? Why it is used?

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Q4- In the circuit, what should be the effect of reversing the polarity of VBB

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