davew wrote:
On Nov 22, 7:14*am, PStamler wrote:
[...]
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.
But, if I were to double the input voltage I would see a 6dB increase
in power output would I not? In context, the use of dB for voltage
gain is entirely correct and acceptable.
It can be used that way as long as the two voltages being compared are
across the same impedance and both relate to the power in the same way.
This was the case in your example.
You can compare voltages at two different places as long as the
impedances at those places are the same. So an amplifier working with
600-ohm input and output terminations could have its power gain
expressed in dB by just measuring the voltage gain.
What you must not do (which is all-too frequently done) is to use the
same calculation to compare voltages in different impedances. For that,
you must use the voltages to calculate the power at each point and then
compare the powers to get a result in dB. If you want to use a
logarithmic scale to compare voltages, call it something different (dBu,
dBV) because it is not "dB".
The convenient shortcut dB formula relating voltages is often taught
with insufficient emphasis being placed on the conditions for which it
is valid. After a while, people begin to believe that the voltage
formula is the true representation of dB until a situation arises where
it generates nonsense results - then they get confused and sometimes
adamant. At least one well-known textbook has got it wrong.
--
~ Adrian Tuddenham ~
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www.poppyrecords.co.uk