Decibel Gain Power Gain The power gain G of an amplifier is the ratio of output power to the input power If the output power is 15W and input power is 0.5W then The output power is 30 times greater then the input power

Decibel Gain The power gain of an audio amplifier is measured in the units of ‘BEL’ in the honor of Graham Bell the inventor of Telephones. 1 Bel= 10 deciBel As a unit the Bel was actually devised as a convenient way to represent power loss in he telephone system wiring rather then gain in amplifiers. It is defined in logarithmic scale since human ear is sensitive to sound energy only in logarithmic scale. The decibel power gain (G) is defined in terms of the normal gain (G) dB

Decibel Gain Decibels: If the circuit has a power gain of 100, its decibels power gain is dB

Decibel Gain 10 dB for each factor of 10: If the power gain is 10; then the decibel power gain is If the power gain is 100; then the decibel power gain is If the power gain is 1000; then the decibel power gain is

Decibel Gains G G/ 1 10 100 1000 10000 0dB 10dB 20dB 30dB 40dB Similar results applies to power gain of less than 1. When the power gain is 0.1; the decibel gain is -10dB

Decibel Gain 3 dB for each factor of 2: If the power gain is 2, then the decibel power gain is G=4, the decibel power gain is G=8, the decibel power gain is When the normal power gain doubles, the decibel power gain increases by approximately 3dB

Decibel Gains Negative Decibels: If power gain is less than 1 there is a power loss (attenuation), and the decibel power gain is negative. For instance, if the output power is 1.5W and the input power is 3W, then And the decibel power gain is When the power gain is 0.25, When the power gain is 0.125,

Decibel Gain

Ordinary Gain Multiply The figure shows two stage s in an amplifier Ordinary Gain Multiply The figure shows two stage s in an amplifier. The first stage has an input power of P1, an output power of P2, and a power gain The second stage has an input power of p2,an output power of P3, and a power gain of The total power gain of the two stages is

For example, a first stage power gain is 100 and the second stage power gain is 200, the total power gain is

Decibel Gain Decibel Gains Add: Since the total gain of two cascaded stages is We can take the logarithm of both sides to get Multiplying both sides by 10 gives Which can be written as

Relation of power and voltage gains The rms voltage across the amplifier input terminal equals V1 and the rms voltage across the amplifier output terminals equals V2. therefore, the input power to the amplifier is And the output power is The power gain from input to out equals

Decibel voltage gain (A/) is For example if A is 40, then Voltage measurements are more common than power measurements. Therefore it is not surprising that decibels are also used to specify voltage gain. Decibel voltage gain (A/) is For example if A is 40, then A 1 0.5 0.25 0.125 0dB -6dB -12dB -18dB A 1 2 4 8 0dB 6dB 12dB 18dB

Factors of 2 and 10: When A=2 When A=4 When A=8 When the voltage gain doubles the decibel gain increases by 6dB. If the voltage gain decreases by a factor of 2, hen the decibel gain decreases by 6dB. Also when the voltage gain increases by a factor of 10 , the decibel gain increases by 20 dB.

A 1 10 100 1000 0dB 20dB 40dB 60dB

Half power point: The frequency at which the output power is half of the input power is called the half power point. This is also called the cutoff frequency At the cut off frequency A=0.707 As For R1 =R2