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Question 1: Photodiodes and photovoltaics
You should refer to the attached documents:

?Datasheet A describes a C30617 photodiode (PD), designed for use in optical communications systems.
?Datasheet B provides material properties for a range of semiconductors.

Exam-question-4XCTG-1.1)

Using the information in the datasheets, explain in your own words:

(i)What type of photodiode is the C30617 model? How does its structure differ from a simple pn-junction device?
(ii)What are the advantages of this type of photodiode structure, and why it is particularly well-suited to optical communications?
(iii)Which semiconductor material is used in the C30617? Why is this a good choice for optical communications?

[6 marks]

Exam-question-4XCTG-1.2)

(i)Estimate the thickness of semiconductor needed to achieve the stated quantum efficiency of the C30617 at 1550-nm wavelength. See page 4 of Datasheet A for a plot of quantum efficiency.

You may assume that quantum efficiency equals the proportion of incident light absorbed in the device. Refer to the material properties given in Datasheet B.

(ii)Write down two factors that may reduce the external quantum efficiency in a photodiode.

[4 marks]

Exam-question-4XCTG-1.3)

Figure 1 shows a photodiode receiver circuit using an op-amp as a transimpedance amplifier.

Figure 1 Schematic diagram of a photodiode receiver

The output voltage of the circuit is given by ??out = ??f??d, where ??f is the feedback resistance, and ??d is the photodiode current.
Design a receiver, using the circuit above, for use in an optical communications link. Assume the following conditions:

?The communications signal is transmitted using a 1550-nm laser diode, with 1 mW output power.
?The signal is carried between two nearby cities over an optical fibre with a loss of 0.5 dB/km. (You may choose the distance between the cities).
?A C30617 photodiode (in an FC receptacle package) is used in the receiver circuit. See Datasheet A, Table 4 for typical responsivity.
?The output of the receiver should equal approximately 5 V when the laser signal is switched on and 0 V when it is switched off.

(i)Choose a suitable feedback resistor, showing all your working, and explaining your decisions clearly. You should use a standard (E12) resistor value.
(ii)Explain, briefly, which of the parameters in Datasheet A, Table 4 (excluding responsivity) are important for this photodiode application.

[5 marks]

Please turn to the next page

Question 2: Transistors
Exam-question-4XCTG-2.1)

Datasheet C describes a kind of field-effect transistor (2N5457). Explain how this kind of transistor works. Your answer should include descriptions of:

(i)The physical structure of the transistor, including a simple diagram.
(ii)The relationship between the drain current, and the drain– source voltage.
(iii)The effect of applying a negative gate–source voltage.

[6 marks]

Exam-question-4XCTG-2.2)

Figure 2 shows a schematic diagram for a common-source FET amplifier, in which a small input signal is applied to the gate.

Figure 2 Circuit schematic for a common-source amplifier

The small-signal voltage gain of the amplifier is given by ???? = ???m??D, assuming that the amplifier is operating under typical conditions.

Please turn to the next page

Discuss the limitations to the performance of the amplifier. Your answer should include:

(i)The factors that determine the maximum allowed input signal.
(ii)The effect on the shape, and the spectrum of the output signal if the input signal is too large.
(iii)The reasons why the gain of the amplifier cannot be increased infinitely, by simply increasing ??D.

[4 marks]

Exam-question-4XCTG-2.3)

Design a common-source FET amplifier, using a 2N5457 transistor and the scheme in Figure 2. Assume the following:

?The characteristics of the transistor are given in Figure 7 of
Datasheet C.
?The input signal typically has an amplitude of 0–10 mV
?The corresponding output signal should have an amplitude of approximately 0–750 mV.
?You may choose values of ??GG, and ??DD as you wish.
???D should be a standard (E12) resistor value.
Explain your design clearly, using appropriate calculations.

[5 marks]