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3 dB bandwidth

Discussion in 'Electronic Design' started by [email protected], Jun 21, 2005.

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  1. Pooh Bear

    Pooh Bear Guest

    I'm not sure but it isn't electronics as you and I know it !

    Some yrs ago I was the liaison guy between my client ( a UK audio manufacturer
    ) and Salford University who run a specialist electronic course in the
    electro-acoustic area. We ended up offering them a one-year placement for one
    of their students since they like to keep industry ties active. Indeed we were
    going to repeat the exercise to but then the company was placed in a situation
    where it had to cease trading.

    Whilst I was at Salford, I saw some of their labs and work assignments. One
    was a mic pre-amplifier. I had to point out to the lecturer who was their
    liaison guy with me that it was actually an instrumentation amplifer they
    were learning about !

    I had other occasions to criticise the student's level of relevant tuition (
    but the student himself was pretty ok I should add ). This lecturer sighed and
    admitted their electronics lecturer was more into heavy electrical so what did
    you expect ?

    I doubt that the skill of discrete design is taught at all these days.

  2. Pooh Bear

    Pooh Bear Guest

    It's just *attenuation* now is it ? Snip rest of trolling garbage.

    Holy Shit !

  3. Have a look at "Radio Designer's Handbook" by F. Langford-Smith (first
    published 1934). In my 4th Edition copy (1953) the whole of Chapter 9
    is devoted to the subject of decibels.

    It is quite clear that only the power ratio is directly described by
    decibels, if you wish to express the ratio of other electrical
    quantities in decibels you "must involve the resistance". (Page 807)

    I have no idea, I did my training in the 1960s.
  4. Pooh Bear wrote...
    Harvard teaches it in their very popular Physics 123 course, using
    the book Paul and I wrote. They also teach it in the summer, when
    most of the students are from elsewhere and come to Cambridge just
    to take the course.

    And many other schools use our book to teach how an engineer thinks
    about electronics circuit design.
  5. Mike Monett

    Mike Monett Guest

    Tony Williams wrote:

    Thanks, Tony.

    As you point out, the calculation using voltage is identical to the
    calculation using power:
    We can examine the -3dB point in a RC low pass filter. This gives the
    half-power point as well as a phase angle of 45 degrees, so it is a
    useful and meaningful parameter.

    Although the source impedance changes as the frequency is varied, we
    assume the scope or voltmeter has negligible loading on the signal.

    The voltage ratio is then 1/sqrt(2), which gives a power ratio of

    (1/sqrt(2))^2 = 0.5


    10 * log10((1/sqrt(2))^2) = -3.0103 dB


    20 * log10(1/(sqrt(2))) = -3.0103 dB

    As you point out
    Since power is voltage squared, the two equations are identical and
    give identical results.

    So the legitimate unit for voltage calculations is the dB.

    Mike Monett
  6. Mike Monett

    Mike Monett Guest


    Please be a little more tolerant. As you know, gain can also be
    understood as a form of attenuation. In other words, gain = - attenuation

    Tony Williams is one of the oldest members of sed, and he rarely gets
    involved in stupid arguments such as this. If you look through the
    archives, you will find his contributions are often quite brilliant and
    certainly far above the general level we now experience in this

    Tony is not a troll, and it is ungenerous to refer to his post in such a

    Mike Monett
  7. Jim Thompson

    Jim Thompson Guest

    But MIT doesn't. MIT teaches basic skills, not compendia of circuits
    to store away for later use.

    ...Jim Thompson
  8. Guy Macon

    Guy Macon Guest



    <more applause>
  9. Pooh Bear

    Pooh Bear Guest

    Don't you consider it somewhat foolish to consider one to the exclusion of
    the other though ? *Both* are required IME to do a decent job. No point
    re-inventing the wheel constantly. Indeed many or even most practical
    circuits have been around for decades.

    The really nice bit is tweaking a popular configuration to enhance

  10. Jim Thompson

    Jim Thompson Guest

    My point is, if you don't know fundamentals, what good is it showing

    To wit, look at all the folderol that continues here on these NG's
    about blinking LED's. Not once have I observed a calculation, just
    things like, "I took off two turns (on the inductor) and it started
    working". No one has a clue WHY. That's NOT design... that's

    ...Jim Thompson
  11. Mike Monett

    Mike Monett Guest

    I guess I'd have to turn the argument around and say the dB is
    dimensionless. If you want the result in power, use 10*log. If you want
    the result in volts or amps, use 20*log.

    Assuming the same impedance, the results are identical. They refer to the
    same thing. Here's a small table:

    dB Volts Watts
    -3dB 1/sqrt(2) 0.5
    -6dB 1/2 1/4
    -10dB 1/sqrt(10) 1/10
    -20dB 1/10 1/100
    -40dB 1/100 1/10000
    -60dB 1/1000 1/1000000

    You can also tie the dB to a specific unit. For example, dBm refers to
    milliwatts, and dBV to Volts. In these examples, the dimension is
    defined by the unit.

    So where's the problem?

    Mike Monett
  12. Jim Thompson

    Jim Thompson Guest

    None whatsoever, except for those who argue to hear their head
    resonate ;-)

    ...Jim Thompson
  13. Mike Monett

    Mike Monett Guest

    Thanks, Jim. It's amazing how these threads can go on forever and not
    solve anything. But when you entered the fray, hopefully that will put a
    stop to the nonsense. Only an idiot would be foolhardy enough to argue
    with you.

    Let's get back to design problems. I have a problem. I want to make a
    very high impedance buffer to drive a so-called 24-bit adc (actually only
    19 effective bits). The desired voltage range is whatever will be
    compatible with the adc. Perhaps 2.5V.

    The problem is I have not been able to find a suitable op amp with bias
    currents in the fA region, and common mode rejection good enough to use
    with a 19 bit adc.

    For example, the LMC6482 is spec'd at around 20fA, but has only 80dB or
    so common mode rejection. I believe this is only good enough for 12 bits.

    Do you have any suggestions on the perfect op amp to use?

    Mike Monett
  14. Fred Bloggs

    Fred Bloggs Guest

    Ahhh- good example of a very common misperception and confusion.

    In the case of using dB to characterize *amplifier* performance, there
    is an implied unit of normalization, and that is the amplifier
    performance at "midband". You are taking Log ratios of gains, and not
    voltages or watts. The dB is always computed as
    20*Log(Gain/Gain,midband) regardless of whether the gain is a voltage or
    power gain. To see this, note that a power gain, Gp, is
    Gp=Pout/Pin=(Vout^2/Ro)/(Vin^2/Rin), and therefore Pg/Pg,mb=
    (Vout/Vout,mb)^2=((Vout/Vin)/(Vout,mb/Vin))^2=(Gv/Gv,mb)^2, so that
    20*Log(Gp/Gp,mb)=2*20*Log(Gv/Gv,mb) or Gp,dBmb=2*Gv,dbmb. In words, the
    power gain loss relative to power gain at midband is 2x the voltage gain
    loss relative to voltage gain at midband in dB. Then a plot of amplifier
    voltage gain normalized to midband has its vertical dB scale multiplied
    by 2 to show power gain rolloff with frequency. When the Gv is down 3dB,
    the Gp is down 6dB, when Gv is down 6dB, the Gp is down 12dB, etc. This
    should show you that just because you are talking about power does not
    mean you mindlessly apply 10*Log(.).
  15. Jim Thompson

    Jim Thompson Guest

    So you have a differential signal? Does it have some bias return
    point, or totally floating and you have to establish common-mode?

    ...Jim Thompson
  16. Fred Bloggs

    Fred Bloggs Guest

    If the manu's are challenged to produce a "perfect buffer" internally,
    what hope do you have to do it externally. LTC has the "East Drive"
    technology, LTC2484:,C1,C1155,C1001,C1152,P11229
  17. Pooh Bear

    Pooh Bear Guest

    There is of course no problem.

    The dB is simply a *mathematical convenience* that allows more meaningfully
    understood interpretation of large ratios in particular.

  18. We have moved on from there. The book is dated, and simply not relevant
    to the modern world on this issue.
    Again, in short, quite nonsense today. dBs, today, are a general term.
    Its irrelevant how they may have been first used.

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
  19. You are mistaken. You are confusing the possible *first* use of the dB
    with what it actually means after the fact.

    The dB is a totally general term. It simply signifies that a log of a
    number was taken. Thats it. End of story.

    The db is not even a real unit, that is, it is not a dimension. The term
    under the log must be a unitless. You cant take a function of something
    with dimensions. e.g.


    note, qV/KT is dimensionless.

    db's are applied in many fields, the power use is no more than a
    historical incidental.

    You are trying to claim that, for example, if it is stated that an
    amplifier has 25dB gain more than another, that such a statment is
    wrong. You will be pretty much alone on that idea.

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
  20. Fred Abse

    Fred Abse Guest

    Nothing wrong with "Peak envelope power", if we're talking about SSB
    transmitters. Rather fundamental, in fact.
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