27161549-Electronic-Circuits-I-lab-manual (1)

27161549-Electronic-Circuits-I-lab-manual (1) - ~ 1 ~...

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Unformatted text preview: ~ 1 ~ EC2208 - Electronic Circuits – I LAB Circuit Diagram CE Amplifier with Fixed Bias Pin Diagram Bottom view of BC107 E B C ~ 2 ~ EC2208 - Electronic Circuits – I LAB Ex. no: 1. COMMON EMITTER AMPLIFIER WITH FIXED BIAS Date: Aim To design and construct BJT Common Emitter Amplifier using fixed bias . To measure the gain and to plot the frequency response and to determine the Gain Bandwidth product (GBW). Apparatus Required S.No Equipments / Components Range / Details Qty 1. Power Supply (0 – 30) V 1 2. Resistor 5.1 K Ω , 3M Ω 1 3. Capacitor 1 μF 1 4. Transistor BC 107 1 5. AFO (0 – 1) MHz 1 6. CRO (0 – 20) MHz 1 Fixed Bias with Emitter Resistor The fixed bias circuit is modified by attaching an external resistor to the emitter. This resistor introduces negative feedback that stabilizes the Q-point. From Kirchhoff's voltage law, the voltage across the base resistor is V Rb = V CC- I e R e- V be ~ 3 ~ EC2208 - Electronic Circuits – I LAB Tabulation Model Graph Frequency (Hz) Vo (V) Gain = Vo / Vs Gain = 20log(Vo/Vs)dB ~ 4 ~ EC2208 - Electronic Circuits – I LAB From Ohm's law, the base current is I b = V Rb / R b . The way feedback controls the bias point is as follows. If V be is held constant and temperature increases, emitter current increases. However, a larger I e increases the emitter voltage V e = I e R e , which in turn reduces the voltage V Rb across the base resistor. A lower base- resistor voltage drop reduces the base current, which results in less collector current because I c = ß I B . Collector current and emitter current are related by I c = α I e with α ≈ 1, so increase in emitter current with temperature is opposed, and operating point is kept stable. Similarly, if the transistor is replaced by another, there may be a change in I C (corresponding to change in β-value, for example). By similar process as above, the change is negated and operating point kept stable. For the given circuit, I B = (V CC- V be )/(R B + ( β +1)R E ). Merits: The circuit has the tendency to stabilize operating point against changes in temperature and β- value. Demerits: In this circuit, to keep I C independent of β the following condition must be met: which is approximately the case if ( β + 1 )R E >> R B . • As β-value is fixed for a given transistor, this relation can be satisfied either by keeping R E very large, or making R B very low. • If R E is of large value, high V CC is necessary. This increases cost as well as precautions necessary while handling. • If R B is low, a separate low voltage supply should be used in the base circuit. Using two supplies of different voltages is impractical. • In addition to the above, R E causes ac feedback which reduces the voltage gain of the amplifier....
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This note was uploaded on 09/19/2011 for the course ECE 678 taught by Professor Asst during the Spring '11 term at Jönköping University.

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27161549-Electronic-Circuits-I-lab-manual (1) - ~ 1 ~...

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