Frequency Analysis of Amplifier

Discussion in 'General Electronics Chat' started by baseball07, Jun 1, 2010.

  1. baseball07

    Thread Starter Active Member

    Apr 24, 2007
    Hi everybody. I have a question regarding a frequency analysis of am amplifier. Normally, when an AC analysis is done, frequency is plotted on the x-axis and dB is plotted on the y-axis. However I just saw a plot where dB was replaced by mV. How does millivolts relate to dB when doing a frequency analysis? Thank you.
  2. Mike33

    AAC Fanatic!

    Feb 4, 2005
    I snagged this from another forum, it may help you out:

    The dB is a logarithmic unit used to describe a ratio. The ratio may be :
    3- VOLTAGE
    4- or intensity or several other things

    So this is just about a voltage ratio .. if you can describe the ratio between 2 power levels measured in "mW", then you can still also use the "dB" to describe the ratio between 2 voltage levels measured in "mV" ..
    Notice .. if you want to express power in "mW", then the formula will be like this :
    10 Log(P2/P1)

    if you express POWER in terms of the originating voltage, then the formula will be like this :
    10 log(P2/P1) = 10 log([V2/V1]^2) = 20 log(V2/V1)


    So you see, dB is not any absolute measurement, it's a ratio of measurements!
  3. someonesdad

    Senior Member

    Jul 7, 2009
    Mike's post is right on; lemme add a couple of minor points. A dB is ultimately just the ratio of two numbers. The Bel was defined as the base 10 logarithm of two numbers, but years ago engineers found it a bit big for convenience, so defined the decibel instead, or one tenth of a Bel.

    When you see someone write down dB, don't assume you know what it means, as there are a number of different conventions and some folks get a bit lazy and don't write down enough information -- and a reader might not know whether they are talking about a voltage ratio, power ratio, etc.

    Two commonly used dB measures are:

    dBm A power level referenced to 1 mW. If you want to translate it to a voltage, you must specify the load resistance. I've seen some rather childish arguments over this, as some common load resistances are 50, 75, and 600 ohms and people can argue over which one is the "right" one.

    dBV A voltage level referenced to 1 volt. If you use it, be sure to specify whether you're using 20 times the log of the ratio or 10 times.

    There are lots of others, so when you write something using dBs, do your readers a favor and define exactly what your ratios are.

    I have a voltmeter that has a handy feature -- I can enter a resistance and it will display voltage readings as dBm with respect to that resistance. It also can display 20 times the log of the reading with respect to a stored value. These features are handy, because dBs are handy things to work with.

    Senior Member

    May 26, 2009
    I'm curious to know the name of that forum?
  5. baseball07

    Thread Starter Active Member

    Apr 24, 2007
    Thanks everyone for the helpful information. I attached the circuit and corresponding frequency analysis I am talking about. In this configuration, the frequency is plotted against a numerical value in mV, which is not a ratio. What I am confused about is that other frequency analysis plots are plotting frequency vs dB. From the recents posts I would assume that dB in this case might be the Gain of the circuit (vout/vin), which is a ratio of two voltages. However, the attached analysis is plotted in mV, not a ratio. Can this be assumed to be a ratio (the gain of the amplifier)?

    Also, I would like help in piecing together how this Pre Amplifier works. I have started another thread regarding this. Any help would be appreciated.
  6. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    Provided you know the input voltage value correspond to the mV output values you can easily convert to a gain [dB]-vs-frequency plot. If you don't know the input conditions you are stuck. It would be unusual for the plot not to include relevant information as to the input signal conditions.

    At any frequency the Voltage Gain = 20log(Vout/Vin) dB.

    It would be easier to do this repeated calculation over the required frequency range if you had a spreadsheet table of values for Vout vs frequency, rather than having to read off the output values from the graph at various frequencies and plug them into the formula.