Revered members,
I list below some of the questions asked in a competitive exam to test the proficiency of electronics. I request the members to give the answers. I have produced the answers which i have written, below each question. I require your help in ascertaining if they are correct.
An oscillator differs from an amplifier , because it
a) has more gain b) requires no input signal c) requires no DC supply d) always has the same input
Ans: requires no input signal
Which of the following devices has the highest input impedance
a) JFET b) MOSFET C) Crystal Diode d) Ordinary transistor
Ans : Ordinary Transistor
Which of the following is not a characteristic of UJT
a) Saturation Current b) Negative Resistance c) Peak point Voltage d) Bilateral Conduction
Ans : Saturation Current
A zener diode is destroyed if it
a) is forward biased b) is reverse biased c) carries more than the rated current d) carries less than the rated current
Ans : carries more than the rated current
In a full wave rectification, if the input frequency is 50 Hz, then the output has a frequency of
a) 100 Hz b) 50 Hz c) 25 Hz d) 200 Hz
Ans : 50 Hz
The multivibrator which generates square wave of its own is the -------- multivibrator
a) monostable b) astable c) bistable d) tristable
Ans : astable
The output impedance of a transistor is
a) high b) low c) infinity d) zero
Ans : zero
Revered members, I have reproduced the questions verbatim.

 

Which of the following devices has the highest input impedance
a) JFET b) MOSFET C) Crystal Diode d) Ordinary transistor

Ans : Ordinary Transistor-> MOSFET I guess

In a full wave rectification, if the input frequency is 50 Hz, then the output has a frequency of
a) 100 Hz b) 50 Hz c) 25 Hz d) 200 Hz

Ans : 50 Hz -> 100Hz

The output impedance of a transistor is
a) high b) low c) infinity d) zero

Ans : zero-> low

 

Which of the following devices has the highest input impedance
a) JFET b) MOSFET C) Crystal Diode d) Ordinary transistor
Ans : Ordinary Transistor

If you look at the ON input current of these various devices you can determine which has the lowest current (and thus highest input impedance).

In a full wave rectification, if the input frequency is 50 Hz, then the output has a frequency of
a) 100 Hz b) 50 Hz c) 25 Hz d) 200 Hz
Ans : 50 Hz

Look at the full-wave rectified output of a sine-wave and you will see the obvious correct answer.

The output impedance of a transistor is
a) high b) low c) infinity d) zero
Ans : zero

An op amp output impedance may be near zero but not a transistor. Look at the characteristic curves of a transistor with the input constant and varying output voltage. You can determine the answer from that. Alternately look up a typical value for the Hybrid Parameter hoe of a transistor.

 

Look at the full-wave rectified output of a sine-wave and you will see the obvious correct answer.

Crutschow,
A typical AC wave has a frequency of 50Hz. Full wave rectifier output has both the negative and positive half cycles rectified. So the input 50 hertz is entirely rectified so, i presume the output has the frequency 50 hertz(100 percent of the input).

Correct me if i am wrong

 

If the input is 50 Hz, the output exhibits a 100 Hz ripple though it can be said to be DC since the voltage does not cross the zero line.

 

Thanks KJ6EAD. I have the following doubt.
Frequency is defined as number of vibrations or number of oscillations or number of cycles of oscillations per second. So, crudely, I assume that the diagram given in the left side(AC signal) has three peaks which has 50 Hz frequency and the DC signal(rectified output) also has three peaks. So how it is 100 Hz? I apologize if i sound silly or naive, but to understand the concepts, i need a reply.

 

Yes the frequency definition is as you stated. Key in on the "number of cycles per second". In the sine wave on the left of KJ6EAD's post one cycle is starting at 0 going to the positive peak crossing 0 going to the negative peak then back to 0 which is the stating point. It is here where the sine wave starts to repeat itself, 50 times per second.

In the wave to the right, one cycle is starting at 0 going to the positive peak then back to 0. Then, again it repeats itself. Since the bridge rectifier has done its job right the negative cycle of the sine wave has basically been flipped to the positive side. This means that there are two positive cycles for every cycle of the incoming sine. So the frequency has doubled, hence the 100Hz.

I hope this makes things a little clearer for you.

-Brian

 

Look at the full-wave rectified output of a sine-wave and you will see the obvious correct answer.

Crutschow,
A typical AC wave has a frequency of 50Hz. Full wave rectifier output has both the negative and positive half cycles rectified. So the input 50 hertz is entirely rectified so, i presume the output has the frequency 50 hertz(100 percent of the input).

Correct me if i am wrong

Your presumption is not correct. If you run the rectified output through a series capacitor to block the DC and look at only the AC portion, you will see a waveform that crosses the zero (average) point twice as often as the unrectified input. Now this waveform has some significant distortion as compared to a pure sinewave but the fundamental rectified frequency is twice the input frequency. If you run that rectified signal through a capacitor into a speaker you will hear mainly 100Hz, with some harmonics, not 50Hz.

 

Thanks KJ6EAD. I have the following doubt.
Frequency is defined as number of vibrations or number of oscillations or number of cycles of oscillations per second. So, crudely, I assume that the diagram given in the left side(AC signal) has three peaks which has 50 Hz frequency and the DC signal(rectified output) also has three peaks. So how it is 100 Hz? I apologize if i sound silly or naive, but to understand the concepts, i need a reply.

See attached, I've added something to that rectifier image. On the input, at 50 Hz, one cycle takes 20ms. At the output, since both positive and negative input peaks are now positive, the same 20ms now encompasses 2 cycles of the new waveform. Therefore, one cycle is only 10ms, which corresponds to 100Hz.