﻿ Define magnifying power of a telescope. Write its expression. A small telescope has an objective lens of focal length 150 cm and an eye piece of focal length 5 cm. If this telescope is used to view a 100 m high tower 3 km away, find the height of the final image when it is formed 25 cm away from the eye piece. from Class 12 CBSE Previous Year Board Papers | Physics 2012 Solved Board Papers

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# CBSE Class 12 Physics Solved Question Paper 2012

31.

Draw a simple circuit of a CE transistor amplifier. Explain its working. Show that the voltage gain AV, of the amplifier is given by , where  is the current gain, RL is the load resistance and ri is the input resistance of the transistor. What is the significance of the negative sign in the expression for the voltage gain?

Circuit diagram of CE transistor amplifier,

When an ac input signal VI (to be amplified) is superimposed on the bias VBB, the output, which is measured between collector and ground, increases.

We first assume that Vi = 0.

Applying Kirchoff’s law to the output loop,

VCC = VCE + IC RL

Similarly, the input loop gives,

VBB = VBE + IB RB

When Vi is not zero, we have

VBE + Vi = VBE + IB RB

Change in IB causes changes in Ic

Hence,

As,

The change in VCE is the output voltage Vis,

The voltage gain of the amplifier is,

Negative sign in the expression shows the output voltage and input voltage have phase difference of .

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# 32.Define magnifying power of a telescope. Write its expression.A small telescope has an objective lens of focal length 150 cm and an eye piece of focal length 5 cm. If this telescope is used to view a 100 m high tower 3 km away, find the height of the final image when it is formed 25 cm away from the eye piece.

a) Magnifying power of telescope is the ratio of the angle subtended at the eye by the image to the angle subtended at the unaided eye by the object.

Mathematically, we can write

where, fo is the focal length of the objective, fe is the focal length of the eye-piece and D is the least distance of distinct vision.

b) Using, the lens equation for objective lens,

Hence, magnification due to the objective lens is given by,

Now, using lens formula for eye-piece, we get

Therefore, magnification due to eyepiece me =
Hence, total magnification,

So, size of final image =

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33.

Draw the circuit diagram of a full wave rectifier using p-n junction diode. Explain its working and show the output, input waveforms.

(b) Show the output waveforms (Y) for the following inputs A and B of

(i) OR gate

ii) NAND gate

a)

Working: The AC input voltage across secondary s1 and s2 changes polarity after each half cycle. Suppose during the first half cycle of input AC signal, the terminal s1 is positive relative to centre tap O and s2 is negative relative to O. Then diode D1 is forward biased and diode D2 is reverse biased. Therefore, diode D1 conducts while diode D2 does not. The direction of current (i1) due to diode D1 in load resistance RL is directed from A to B. In next half cycle, the terminal s1 is negative and s2 is positive relative to centre tap O. The diode D1 is reverse biased and diode D2 is forward biased. Therefore, diode D2 conducts while D1does not. The direction of current (i2 ) due to diode D2 in load resistance RL is still from A to B. Thus the current in load resistance RL is in the same direction for both half cycles of input AC voltage. Thus for input AC signal the output current is a continuous series of unidirectional pulses.

In a full wave rectifier, if input frequency is f hertz, then output frequency will be 2f hertz because for each cycle of input, two positive half cycles of output are obtained.

b) Output waveforms for the following inputs A and B of OR gate and NAND gate.

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