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Maybe it means 0db equals some notional figure such as 1 watt into 8 ohms.
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Quote, But newer amplifiers consist with decibel figures!!! I really confused.It will go from -25dB to +18dB.What does it means?įigure like that can be a bit meaningless because they are not quoted with "respect to anlything". In db that ratio is 10Log(Pout/Pin) which is 26.4db (the same figure as the voltage gain) We can also say that the 100mv input would produce a power of 0.00125 watts into 8 ohms if it were applied across the load (same watts equals v squared over R) Using the same figures we can say that the output of 2100 mv RMS produces a power of 0.55 watts RMS (W= V squared/R). Now imagine its running into an 8 ohm load. At high frequencies the gain falls off again due to limitations of the IC. If we shorted those caps out (don't ) the amplifier would maintain its gain down to DC. The capacitor C2 also rolls the gain "of the amplifier itself" down to 1 or unity at DC. Why midband ? Because the input coupling capacitor rolls the gain off to zero at DC. For that amplifier it is 26.4db at AC in the "midband". So if your amplifier really did give 100 mv output for 2100 mv input (an attenuator) then the figure of -26.4db would indicate that. Also you'll notice if you swap Vin and Vout around in the formula that the answer is the same except with a minus sign in front of it. The formula is Gain = 20Log(VOut/Vin) So using either of the above two sets of numbers we get 26.4 db using either set of numbers. If we talked of 300 mv peak input we would get 6300 mv peak output and so on. That means that 100 millivolts RMS input would deliver 2100 mv RMS output.
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Numerically the gain is (R4 + R3)/R3 which is 21. R3 and R4 determine the voltage gain at AC.