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

# 27161549-Electronic-Circuits-I-lab-manual (1) - Circuit...

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~ 1 ~ EC2208 - Electronic Circuits – I LAB Circuit Diagram CE Amplifier with Fixed Bias Pin Diagram Bottom view of BC107 E B C

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~ 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 , 3M 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

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~ 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. Usage: The feedback also increases the input impedance of the amplifier when seen from the base, which can be advantageous. Due to the above disadvantages, this type of biasing circuit is used only with careful consideration of the trade-offs involved.
~ 5 ~ EC2208 - Electronic Circuits – I LAB Design Choose β = 250, V CC = 12V, I C = 1 mA By applying KVL to output side, V CC – I C R C – V CE = 0 V CC = I C R C

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

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