> I have read somewhere that 40 dB of gain is approximately
> 200 times gain. Does anyone know if this is true and what
> the formula is to have work out the relationship.

A "Bell" (named after Alexander Graham Bell) is a means of quantifying the
power ratio of two signals.  The scale is logarithmic, with an increase of 1
Bell denoting in increase in the power ratio by a factor of 10.  A factor of
10 is a rather large jump, so deciBells are almost universally used instead
of Bells.  A deciBell, abbreviated "dB", is a tenth of a Bell.  Therefore 10
dB denote a relative increase in power by a factor of 10, and 40dB therefore
denotes a relative increase in power by 10**4 = 10,000.  Note that this is
power, not voltage.  If an amplifier had a gain of 40dB and its input and
output impedence were the same, then it would have a voltage gain of
SQRT(10,000) = 100.

A few quick rules of thumb to remember.  A voltage gain of 10 at the same
impedence increases the power by 20dB, and similarly a voltage gain of 2
results in an increase of Log10(2**2) = Log10(4) = 6dB.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olin@embedinc.com, http://www.embedinc.com

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