# Physics Class 12 NCERT Solutions: Chapter 14 Semiconductor Electronics Materials Devices and Simple Circuits Part 2

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Q: 9. For a CE-Transistor amplifier, the audio signal voltage across the collected resistance of is . Suppose the current amplification factor of the transistor is , find the input signal voltage and base current, if the base resistance is .

Answer:

Collector resistance,

Audio signal voltage across the collector resistance,

Current amplification factor of the transistor,

Base resistance,

Input signal voltage

Base current

We have the amplification relation as:

Voltage amplification

Therefore, the input signal voltage of the amplifier is .

Base resistance is given by the relation:

Therefore, the base current of the amplifier is .

Q: 10. Two amplifiers are connected one after the other in series (cascaded). The first amplifier has a voltage gain of and the second has a voltage gain of. If the input signal is volt, calculate the output ac signal.

Answer:

Voltage gain of first amplifier,

Voltage gain of second amplifier,

Input signal voltage,

Output AC signal voltage

The total voltage gain of a two-stage cascaded amplifier is given by the product of Voltage gains of both the stage, i.e.,

We have the relation:

Therefore, the output AC signal of the given amplifier is

Q: 11. A p-n photodiode is fabricated from a semiconductor with band gap of . Can it detect a wavelength of ?

Answer:

Energy band gap of the given photodiode,

Wavelength,

The energy of a signal is given by the relation:

Where,

But

The energy of a signal of wavelength is , which is less than the energy band gap of a photodiode. Hence, the photodiode cannot detect the signal.