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Practice each question, then open the answer or worked solution.
In a CE transistor amplifier there is a current and voltage gain associated with the circuit. In other words, there is a power gain. Considering power a measure of energy, does the circuit violate conservation of energy?
The circuit won’t violate the conservation of energy as the power gain in the CE transistor is very high. Also, extra power is required for the amplification.
Short Answers23
(i) Name the type of a diode whose characteristics are shown in the figure (a) and in the figure (b)
(ii) What does the point P in the figure (a) represent?
(iii) What does the points P and Q in the figure (b) represent?
i) Figure a) represents the characteristics of Zener diode and figure b) represents the solar cell.
ii) Figure a) point P is the representation of Zener breakdown voltage.
iii) Figure b) point Q is the representation of zero voltage and negative current.
Three photodiodes D1, D2 and D3 are made of semiconductors having band gaps of 2.5eV, 2eV and 3eV, respectively. Which ones will be able to detect light of wavelength 6000 Ao?
Wavelength = λ = 6000 Ao = 6000 × 10-10m
Energy of the light photon = E = hc/ λ = 2.06 eV
If the resistance R1 is increased, how will the readings of the ammeter and voltmeter change?
IbR1 + Vbe = Vbb
Base current = Ib = Vbb – Vbe/R1
Ib is proportional to 1/R1
Therefore, when R1 is increased, Ib decreases.
Two car garages have a common gate which needs to open automatically when a car enters either of the garages or cars enter both. Devise a circuit that resembles this situation using diodes for this situation.
OR gate is used for explaining the situation:
| A | B | Y = A + B |
| 0 | 0 | 0 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 1 |
How would you set up a circuit to obtain NOT gate using a transistor?
a) It has only one input and only one output
b) Using the Boolean expression
c) Realization of NOT gate
d) Using truth table
Explain why elemental semiconductor cannot be used to make visible LEDs.
Elemental semiconductor cannot be used to make visible LEDs because the bandgap of elemental semiconductor is in the infrared region and not in the visible region.
The energy gap for Si is 1.1eV, wavelength is 1129 nm and the energy gap for Ge is 0.7eV, wavelength is 1725 nm.
Write the truth table for the circuit shown in the figure. Name the gate that the circuit resembles.
The given circuit is AND and the truth table is:
| A | B | X = A.B |
| 0 | 0 | 0 |
| 0 | 1 | 0 |
| 1 | 0 | 0 |
| 1 | 1 | 1 |
A Zener of power rating 1 W is to be used as a voltage regulator. If Zener has a breakdown of 5V and it has to regulate voltage which fluctuated between 3V and 7V, what should be the value of Rs for safe operation in the figure?
Power = 1W
Zener breakdown voltage = 5V
Minimum voltage = 3V
Maximum voltage = 7V
P = VI = 0.2A
Rs = 10 Ohms.
Long AnswersIf each diode in the figure has a forward bias resistance of 25Ω and infinite resistance in reverse bias, what will be the values of the current I1, I2, I3 and I4?
Forward bias resistance = 25 ohms
Reverse biased resistance = infinity
I3 = 0
Resistance in AB = 150 ohms
Resistance in EF = 150 ohms
AB is parallel to EF
Effective resistance, R’ = 75 ohm
Resistance R of the circuit = R’ = 100 ohms
Current = 0.05 A
I2 = 0.025 A
I4 = 0.025 A
Therefore,
I1 = 0.05 A
I2 = 0.025 A
I3 = 0
I4 = 0.025 A