Hi Andy,

At 01:19 AM 3/10/00 -0000, andy howard wrote:
>When calculating power ratios the formula is 10*log(x1/x2) where x1 and
>x2 are power measurements in e.g. watts. For voltage (or current) ratios
>you use 20*log(x1/x2) where x1 and x2 are voltages (or currents).
>
>If you double the voltage across a given load you will quadruple the
>power dissipated since power is proportional to the square of the
>voltage (p=v2/r).  Thus 6dB is a doubling of voltage but 3dB is doubling
>of power.
>
>Thus is the mighty Ohm's Law preserved.
>

Yes, but what about circuits where Ohm's law doesn't apply (constant
current or voltage devices) or circuits where you are comparing voltage
levels on two different impedances(voltage amplifier with high input
impedance, low output impedance, for example)? I never could understand why
people were willing to accept the generalization that we just multiply by
two when talking about voltage or current ratios. It would make more sense
to me to just consider it to be a log scale and specify what kind of ratio
you were talking about (power, voltage, etc.) I used to get REALLY confused
when trying to read op-amp datasheets. They used to say 140dB voltage gain,
which I thought meant 10^14. Things later made much more sense when I found
out about the 20*log definition, but I have since felt that it is an
inherently confusing way of doing things.

Sean


|
| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
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