Amplifier

upto my knowledge i think op amps amplifies the voltage. and some speacial op amps are available to amplify sound signals. then why do they use dB for all op amps.

Just a note: there is a slight difference in how power ratios are expressed versus amplitude (be it voltage, current, etc) ratios when both are expressed in decibels.

When referring to measurements of power or intensity, the ratio of a value P (measured power or intensity) to another value P₀ (reference power or intensity), represented here by P_dB, is

P_dB = 10 log₁₀(P/P₀) dB

When referring to measurements of amplitude, it is usual to consider the ratio of the squares of A (measured amplitude) and A₀ (reference amplitude). This is because in most applications power is proportional to the square of the amplitude, and it is desirable for the two decibel formulations to give the same result in such cases, hence,

A_dB = 20 log₁₀(A/A₀) = 10 log₁₀((A/A₀)²) = 10 log₁₀(A²/A₀²) dB .

An elaboration of an elaboration very often gets into the realm of just picking nits. There is one little nit in your excellent explanation that has bugged me.

The reason that power is nominally proportional to the square of the amplitude of other units is because one usually assumes that the load is linear.

ΔV=ΔI*a → ΔP=(ΔV^2)*a or ΔP=(ΔI^2)/a

On a very, very rare occasion (exam question) the load will not be in linear region in a change in power. In this case one must do whichever non-linear math technique to identify the actual power change.

i got it. Is it used only to represent values of ratios. and how to convert the value to decibels.

"Instead of publishing easy to confuse large numbers like 3,456,000 10,234,000 the values of 131 db and 260 db are used."

Then why doesn't Zimbabwe express the face value of their largest bank note in decibels

where (this morning's) reference value is the ratio

?

I think Zimbabwe needs to look at their monetary system a little closer, it might be time tor a new currency.

The Zimbabwe franc will be equal to 100 billion Zimbabwe dollars, and transactions will be allowed in both Francs and ZD for a period of one year.

After that, the banks will convert all ZD accounts into Franc accounts. The banks will still accept deposits in ZD, but will convert them into Francs to deposit into the account, and will only allow withdrawals in Francs.

One year after that, there will be a government-sponsored program where armored cars (with police escort) will tour the countryside, exchanging peoples ZD for Francs at no cost to the people (well, no fee for the exchange, the cost of the program will probably come out of taxes, which means a small, (hopefully) temporary increase in taxes to fund the program.

After the armored car moneychanger program ends, It will be illegal for merchants and companies to give change in ZD, although they can still accept ZD as payment. This should pull out the last of the ZD from circulation and allow the country to work with a simpler money system.

Note that all I did was move the decimal point 11 places to the left.

Then why doesn't Zimbabwe express the face value of their largest bank note in decibels

And 265 db$ doesn't make the national debt seem so bad!

Many of life's senses and reactions are more conveniently expressed on a logarithmic scale such as dB. Just note that the reference level must be specified, eg dBA, dBu, dBm, dBfs etc for the absolute value to be apparent..

As another reason, it is very convenient to use the dB principle for drawing the frequency dependence of the gain. This is the basis for the well-known BODE diagram.

Using a log scale on the vertical as well as on the horizontal frequency axis allows a very simple procdure for scetching the BODE diagram: It consists of piecewise linear parts only - with a slope which depends on the degree of the frequency dependence only: -n*20dB/dec. (n= degree). Change of the slope occurs at pole and/or zero frequencies only.

I forgot to add that it is not correct to speak in this context of "units". Decibels are no units. For example, you can add x (in dB) and y (given in dBm).

The mark "dB" (or dBm or...) is an indication only that the corresponding quantity id given in log form - nothing else.

The first use for [deci]bels was not for sound level but for attenuation / amplification data.

The T. U. (Transmission Unit) was invented by Bell Labs to deal with signals over telephone lines. Instead of multiplying gains times losses etc., if you take the logarithms your problem can be described in a budget form, i. e., plus or minus only.

The T. U. was later renamed a "bel" (B).