[John O'Flaherty]

On Sun, 21 Nov 2010 23:14:27 -0800 (PST), PStamler
wrote:

On Nov 21, 10:41*pm, John O'Flaherty wrote:

There is a sense in which calling a voltage gain of 10 a gain of 20 dB
does refer to power. In a circuit in which nothing is changed but that
gain (including output loading and input signal level), if that gain
is reduced to 0 dB, the output power level will be reduced by a factor
of 100.

Huh-uh. We had this discussion years ago.

Given a non-inverting opamp with high input impedance and negligible
output impedance, a 1k resistor from the + input to ground, and a 100k
resistor from the output to ground (in other words a 100k load).
Assume the feedback resistance network is high enough that it draws
negligible current.

1V into 1k, at the input, means 1mA, so the power is 1mW. 10V into
100k at the output means 0.1mA, so the power is 1mW again. There's 0dB
power gain, but there's voltage gain of 10x, which is coded +20dB in
the voltage-gain realm of decibel calculation. This deviates from the
"real" standard of what decibels are, by divorcing the voltage and
power gains, but using the same unit for them, dB. The usage, however,
is nearly universal, so speaking as a descriptivist who believes that
dictionaries should reflect how people actually use the language,
perhaps the official definition needs revision to take into account
the dual usage of the term. I've suggested "dBG" as an indicator that
voltage gain is being discussed rather than power gain, and perhaps
that's the way to go.

My point, though, was that it's not a comparison of input to output
power, but of the difference in output power that holds between an
amplifier with 20 dB gain vs. one with 19 dB or 0 dB, all else being
equal. If it ever gets to, say, a loudspeaker, there will be that
difference. I incline to descriptivism too, but it just seems to me
that this actual usage of "dB" does refer to power, in the sense I
meant.
--
John