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- 1. The Decibel - dB Power in Power Amplifier Power out The decibel (dB) is a logarithmic unit of measurement that expresses the magnitude of power or amplitude relative to a specified reference level and is commonly used in the measurement of sound or power when investigating electronic systems. A decibel is one tenth of a bel, a seldom-used unit.
- 2. Decibel - Power Gain Amplifier Pi Po Ap The power gain (Ap) of an amplifier is the logarithmic ratio between input power (Pi ) and output power (Po ) Ap = 10 log10 Po Pi dB
- 3. Decibel - Power Gain Pi 100mW Amplifier Ap Po 200mW Po 200 AP = = =2 Pi 100 200 Ap = 10 log10 = 3dB 100 A 3dB gain is a doubling of power.
- 4. Decibel - Attenuation Pi 100mW Amplifier Ap Po 1mW It is also possible to express a reduction of power in decibels – this is known as attenuation and the result takes a negative sign indicating a reduction. 0.001 Ap = 10 log10 = −20dB 0.10 A reduction of one hundred times is expressed as -20dB
- 5. Power Gain - Activity Input Power Amplifier Output A power amplifier is required to drive a 100 watt speaker system from a 5 watt signal source. Determine the gain of the amplifier in dB. It is required to double the output power to 200 watts, determine the new gain of the amplifier assuming the source signal is the same. Comment on your results.
- 6. Multi-stage Power Gain Pi 1st Stage (Pre-Amp) Ap1=10 2nd Stage (Power-Amp) Ap2=100 Po System Gain (Ap) is the sum of the individual stage gains. Ap = Ap1 + Ap2 = 10 + 100 = 110dB
- 7. Multi-stage Power Gain - Activity Pi 1st Stage (Pre-Amp) Ap1 2nd Stage (Power-Amp) Ap2 Po A hi-fi system is made up of a pre-amplifier and a power amplifier. To drive a 400W speaker system the power amplifier requires 2W. Determine the system power gain in dB if the pre-amp draws 20mW (0.02W) from a signal source when operating at full power. Investigate the system in two stages a) as a single system and b) as two separate systems in cascade. Comment on your results.
- 8. Decibel - Amplitude Gain Input Vi Ri Signal Amplifier Av Ro Vo Output It is also common to express the ratio of voltage or current in decibels Vo Av = 20 log10 Vi dB The output resistance (Ro ) must be equal to the input resistance (Ri ), ie the condition for maximum power transfer.
- 9. Decibel - Amplitude Gain Vi 20mV Signal Amplifier Vo 200mV Vo 200 = = 10 Vi 20 Av = 20 log10 200 = 20dB 20 A voltage gain of ten times is equal to +20dB
- 10. Amplitude Gain - Activity Input Signal Amplifier Av Output A signal amplifier provides a 12 volt output when a 2 volt signal is applied to its input. Determine the gain of the amplifier in dB. If the gain control is adjusted to provide a 24 volt signal at the output for the same input signal, determine the new gain of the amplifier. Comment on your results.
- 11. Gain and Loss Ratios To get a ‘feel’ for power gain and loss expressed in dB the following shows the corresponding ratios, A gain of 10,000 = 40dB 1000 = 30dB 100 = 20dB 10 = 10dB 1 = 0dB A loss (attenuation) of 0.5 = -3dB 0.1 = -10dB 0.01 = -20dB 0.001 = -30dB 0.0001 = -40dB
- 12. dB Ratios dBm is an abbreviation for the power ratio in decibels (dB) of the measured power referenced to one milliwatt (mW). It is used in radio, microwave and fibre optic networks as a convenient measure of absolute. dBW is referenced to one watt (1000 mW) and is an absolute unit, used when measuring absolute power. In audio dBm is typically referenced relative to a 600 ohm impedance, while in radio frequency work dBm is typically referenced relative to a 50 ohm impedance.

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