PHY301 Final Term by Rana Abubakar Khan

Rana Abubakar Khan

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PHY301 exam held today:

What is resistance level of PhotoDiode in darkness? (2 marks) Two figures were given. It was required to tell, which input signal is clipper, and for what input signal the output signal is Clamper. (2 marks) Define Diode. (2 marks) In which bias a diode conducts current? (2 marks) For the given circuit, convert left most voltage into current using Source transformation. Draw diagram. (3 marks) For a given circuit, value of I1 & I2 using mesh analysis was required. (3 marks) For the given circuit, how can we calculate only RTH using Thevenin's Theorem? (3 marks) Consider a diode with n=2 biased at 1mA. Find the change in current as a result of changing the voltage by –20mV using small signal model. (3 marks) Consider a diode with n=2 biased at 1mA. Find the change in current as a result of changing the voltage by –20mV using the exponential model. (5 marks) Identify meshes in given circuit. Write its SuperMesh equation. (5 marks) For the given circuit, find Vo using Thevenin's Theorem. (5 marks) What are modes of operation of BJT? (5 marks) PHY301 (26-8-2016)

1. Circuit was given. Which values are wrongly labelled in circuit.

Q2. What is Dopping? Why it is necessary?

3. Describe action of transformer.

Q4. In NPN transistor, what section is made very thin compared with other two section?

Q5. When BJT is said to be in saturation mode?

6. Write detailed steps of Thevenin’s theorem.

Rest questions were from circuits and asked to solve.

One question about to define the polarity of reverse biase Write KVL equation. Circuit was given Circuit was given ask to write the KVL equation for a mesh Figure was given ask to identify the figure either as half wave rectifier or full wave rectifier Thevenin’s theorem. Ask to find Rth. Circuit was given Norton’s theorem and ask to find the Inor. Circuit was given Write the function of a transistor Figure was given ask to trace the polarities of AC and also points the polarity of load resistance (did not remember clearly) Ask to write three differences between n-p-n and p-n-p amplifier Ask to find the voltage by superposition principle considering only current source. Circuit was given Ask to write difference between the construction of simple diodes and tunnel diodes One question from transformer. Ask to find voltage in the circuit on March 8, 2014 at 10:21pm

Describe the function of transistor.

In electronics, a transistor is a semiconductor device commonly used to amplify or switch electronic signals. A transistor is made of a solid piece of a semiconductor material, with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current flowing through another pair of terminals. Because the controlled power can be much larger than the controlling power, the transistor provides amplification of a signal. The transistor is the fundamental building block of modern electronic devices, and is used in radio, telephone, computer and other electronic systems. Some transistors are packaged individually but most are found in integrated circuits.

In a reverse biased PN-junction, which current carriers cause leakage current

In reverse biased leakage current is primarily limited by the supply of thermally generated minority carriers that are more or less independent of the reverse bias voltage, but tends to be very sensitive to temperature.

The leakage current is composed both of holes from the n-region seeing the potential and drifting to the pregion and electrons generated in the p-region drifting to the n-region, where both components generate a net negative current. If the doping of both sides of the junction are equal than the half of the current is from holes and half from electrons. As the doping becomes unequal the leakage becomes dominated by the minority carriers generated in the lighter doped region

Or

the minority carriers in the device is responsible for leakage current. holes in n-type material and electrons in p-type cause the flow of leakage in the pn junction device

Describe Source Transformation method for simplifying circuit.

A source transformation is a process of representing a circuit from the point of view of the load, or of the next circuit. The concept of source transformation suggests that any power source can be represented as a voltage source or a current source. If the impedance presented to the load or next circuit can be calculated, analysis of the circuit is simplified. Source transformation is applied to the design and testing of various types of circuits – from relatively simple direct current (DC) circuits, for steady-state power computations, to more complex circuits. For high frequencies of alternating current (AC), such as radio frequencies, source transformation aids in designing impedance matching circuits for maximum power transfer.

Difference between half wave and full wave rectifier.