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# Question from a student about amps

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I have a weird feeling that these questions are unanswerable:

Compare linar to logarithmic
For this question I wrote that a logarithm is an exponent to which a standard number is raised to produce a given number. Then I explained the base of natural and common logs. Then to explain linear I described it as something that is uniform, or the same.

Why are amplifier gains commonly given in decibels? State 2 reasons
I explained that it was becuase of the dBm and dBk standards, and the fact that you can show loss easily.;

Coupling caps must have a reactance of less than 1/10th of the input impedance of the amplifier. Would this be for the uppermost, average, or lowest frequency? Why?
For this I wrote that it should be for all, shouldnt it be?

Why does the common emitter amp have a 180 phase shift?
I had zero clue why this happens.

My teacher claims that he told us this, but I have NO evidence of it in my notes. Any help here?

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Hi Amplifier,
Welcome to our forum.
1) The increase or decrease in logarithmic (to the base of 10) amplitude of a signal or voltage increases or decreases 10 times the actual amount for each doubling of distance along the graph. The amplitude is measured logarithmically to obtain a spread-out enormous range. If the graph was linear the low-level part of its amplitude would not be seen.
2) Video and audio amplifier gain is commonly given in dB because our vision and hearing respond logarithmically, for such an enormous range. An excellent signal to noise ratio is 120dB, which is 1 million times difference! Some opamps have a gain of 140dB or more, you add the zeros. From moonlight to sunlight is, well, you look it up (120dB?).
3) Coupling caps must have a reactance of less than 1/10th of the input impedance of the amplifier because they cause a voltage divider action with the input impedance at the calculated low frequency, and the resulting 9.1% loss is not noticed much.
4) The reactance of a coupling capacitor is very low at high frequencies so they are like a piece of wire (no loss). As the frequency decreases, their reactance increases which creates a voltage divider with the amplifier's input impedance reducing the signal (high loss).
If you like strong bass, use coupling capacitors with a high value for their low reactance at the low frequencies so they pass the bass without much loss. Even better, use DC coupling so you don't need any coupling capacitors, but don't drop the microphone! ;D

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It is the only two functions that you can compare. The question is why not take the difference instead of division. Division can make two numbers identical while a difference can't. 100-5=95,200-10=190. These two equations have the same result of 20, by division, but the difference is 95 and 190. You can see this becomes important when dealing with gain which is the exact same thing. Division. Vout/Vin versus 20 log Vout/Vin. And gain is the important aspect with electronic devices.

Now as far as the 180 degree transistor phase shift. This is because whenever you have an increase on the input, you have a decrease on the output. Follow the sinewave with this and you have 180 degree phase shift. What is more interesting is the 90 degrees phase shift associated with capacitors and inductors. I think this happens experimentally as I cannot determine how it works theoretically.

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Thanks Kevin,
I forgot to answer the transistor question.
Capacitors and inductors create a phase shift due to time delay.
I was taught ELI and ICE. The voltage leads the current in an inductor (ELI). The current leads the voltage in a capacitor (ICE). A coupling capacitor leads the current around a parallel resistor, a capacitor to ground lags in voltage when it follows a resistor.

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• 3 months later...

hello
we use dB because when we use dBs multiplication turns into addition as well as division turns into subtraction and this is very useful characteristic. and we can draw Bode plot which is a very helpful chart.

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We use Decibels because they measure a change in level logarithmically, like our hearing. Therefore the levels don't have many digits:
+120dB is a voltage (level) gain of 1,000,000.
-120dB is a loss to 0.000001.

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