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Question-1. What is a transistor?
Answer-1: A junction transistor is simply a sandwich of one type of semiconductor material between two layers of the other type. A transistor is a three terminal current sensing device. It can be looked upon as two pn junction placed back to back. The three terminals are named as emitter base & collector.
Question-2. What are the types of transistor?
Answer-2: The transistor may be NPN or PNP type. An NPN bipolar transistor is so called because the outer layers are N-type semiconductors, while the base is a P-type. N stands for negative charge carriers or electrons, and P for positive charge carriers or holes.
Question-3. What are the different configurations of transistor?
Answer-3: A transistor may be connected in three configurations namely: 1) Common Emitter (CE) 2) Common Base (CB) 3) Common collector (CC)
Question-4. What is the meaning of term ?Common??
Answer-4: The term common is used to denote the element that is common to both input and output circuits. Because the common element is often grounded, these configurations are frequently referred to as grounded emitter, grounded base, and grounded collector.
Question-5. What are the characteristics o f each configuration that make it suitable for specific applications?
Answer-5: Each configuration has particular characteristics that make it suitable for specific applications. An easy way to identify a specific transistor configuration is to follow three simple steps: 1) Identify the element (emitter, base, or collector) to which the input signal is applied. 2) Identify the element (emitter, base, or collector) from which the output signal is taken. 3) The remaining element is the common element, and gives the configuration its name.
Question-6. What is Common Base configuration?
Answer-6: The common-base configuration (CB), base is common between input and output circuits. In emitter-base terminal input signal is applied and in collector-base terminal output is taken from it.
Question-7. What are the characteristics of CB?
Answer-7: since it has a low input resistance (30 ohms-160 ohms) and a high output resistance (250 kilohms-550 kilohms). However, two factors limit its usefulness in some circuit applications: (1) its low input resistance and (2) its current gain of less than 1. Since the CB configuration will give voltage amplification, and use in some microphone amplifiers. The input and output signals in the common-base circuit are in phase.
Question-8. What is current gain for CB?
Answer-8: The current gain in the common-base circuit is the ratio of change in collector current(output current ) to the change in emitter current(input current) at constant VCB . The term ALPHA (a) is used for gain.
Question-9. What is input characteristic of CB?
Answer-9: Input Characteristics, a graph of the base current IE versus VBE with constant VCB , which is the voltage between the base and the collector, looks like that of an ordinary diode. IE increases rapidly with small increase in VBE. The IE is almost independent of VCB.
Question-10. Output Characteristics of CB configuration?
Answer-10: Output characteristics are found by considering the collector loop. A graph of the collector current IC versus the collector-base voltage VCB with constant IE. The IC varies with VCB only at very low voltages (< 1 v). Beyond this value IC became constant and dependent upon only IE.
Question-11. What are the characteristics of CE configuration?
Answer-11: CE configuration has following characteristics: i) High voltage gain; ii) High current gain; iii) Medium input impendence; and iV) Output impendence equal too the load resistance.
Question-12. What is CE configuration?
Answer-12: In common emitter or CE circuit, the base terminal of the transistor serves as the input, the collector is the output, and the emitter is common to both, hence its name. It is used for amplification. A small signal introduced into the base produces a larger signal at the output.
Question-13. What is transistor gain for CE configuration?
Answer-13: The transistor gain, or ?, is the ratio of the change of collector current IC to the change of base current IB. The gain tells how much the input signal is amplified. It is a constant that depends on the transistor type.
Question-14. Input Characteristics of CE configuration?
Answer-14: Input Characteristics, a graph of the base current IB versus VBE with constant VCE , which is the voltage between the base and the emitter, looks like that of an ordinary diode. The current is zero until VBE reaches 0.7 volts, where it then increases very suddenly.
Question-15. Output Characteristics of CE configuration?
Answer-15: Output characteristics are found by considering the collector loop. A graph of the collector current IC versus the collector-emitter voltage VCE with constant IB. As VCE increases, IC will remain zero and then suddenly shoot up when the voltage reaches a certain value, much the same way as IB. Unlike IB, IC will remain constant as VCE increases.
Question-16. What are the characteristics of CE?
Answer-16: The common emitter also has a somewhat low input resistance (500 ohms-1500 ohms), because the input is applied to the forward-biased junction, and a moderately high output resistance (30 kilohms-50 kilohms or more), because the output is taken off the reverse-biased junction.
Question-17. What is gain?
Answer-17: The term GAIN is used to describe the amplification capabilities of the amplifier. It is basically a ratio of output versus input. Each transistor configuration gives a different value of gain even though the same transistor is used.
Question-18. Applications of CE configuration?
Answer-18: In Low frequency voltage amplifier, in Radio, as a low-noise amplifiers, as a switch etc.
Question-19. What is the advantage of CE configuration?
Answer-19: The common-emitter configuration (CE) is the arrangement most frequently used in practical amplifier circuits, since it provides good voltage, current, and power gain.
Question-20. What is the property of CE transistor?
Answer-20: The input signal to the common emitter goes positive when the output goes negative; the two signals (input and output) are 180 degrees out of phase. The common-emitter circuit is the only configuration that provides a phase reversal.
Question-21. What is Common Collector configuration (CC)?
Answer-21: In the common-collector circuit, the input signal is applied to the base, the output is taken from the emitter, and the collector is the element common to both input and output. The common collector is equivalent to the electron-tube cathode follower.
Question-22. What are the characteristics of CC configuration?
Answer-22: CC configuration has high input and low output resistance. The input resistance for the common collector ranges from 2 kilohms to 500 kilohms, and the output resistance varies from 50 ohms to 1500 ohms. Since a small change in base current controls a large change in emitter current, it is still possible to obtain high current gain in the common collector. The common collector is also referred to as an emitter-follower because the output developed on the emitter follows the input signal applied to the base.
Question-23. What is current gain?
Answer-23: It is based on the emitter-to-base current ratio called GAMMA (?), because the output is taken off the emitter. The common-collector current gain is denoted by gamma (?).
Question-24. Difference among CB, CE, and CC in terms of gain?
Answer-24: The current gain of CC is higher than that in the common emitter, but it has a lower power gain than either the common base or common emitter. Like the common base, the output signal from the common collector is in phase with the input signal.
Question-25. Relation among a, ?, and ??
Answer-25: A given transistor may be connected in any of three basic configurations; there is a definite relationship, as pointed out earlier, between alpha (a), beta (?), and gamma (?). These relationships are: a = ? / (? + 1), ? = a / (1 - a), and ? = ? + 1
Question-26. Application of CC configuration?
Answer-26: The common-collector configuration (CC) is used mostly for impedance matching. It is also used as a current driver, because of its substantial current gain. It is particularly useful in switching circuitry, since it has the ability to pass signals in either direction (bilateral operation).
Question-27. What is power rating of transistor?
Answer-27: The maximum power that a transistor can handle without destruction is known as power rating of the transistor. While connecting transistor in a circuit, it should be ensure that its power rating is not exceeded otherwise the transistor may be destroyed.
Question-28. Can you explain transistor testing?
Answer-28: An ohmmeter can be used to check the transistor, forward biased base-emitter junction should have low resistance (100 ohm-1kohm) and reverse biased collector-base junction should have higher resistance (100kohm).
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