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  • Is a power amplifier able to generate power?
  • Does a power amplifier draw power from a supply?
  • Does a power amplifier dissipate power itself?
  • If a power amplifier draws power from a supply, does it dissipate all of the indicated power drawn?
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schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. The output stage of a power amplifier and the loudspeaker.

Is a power amplifier able to generate power?

No. The amplifier uses a power supply.1

Does a power amplifier draw power from a supply?

Yes, V1 and V2 in Figure 1.

  • If Q1 is turned on current will flow from V1 through Q1 and into the speaker. This will push the speaker cone in one direction.
  • If Q1 is turned off and Q2 is turned on current will flow from the speaker through Q2 and into V2.

Does a power amplifier dissipate power itself?

Yes. The transistors behave a little like variable resistors. When the resistance decreases more current flows to the speaker. This current and the voltage across the transistor produces heat which has to be dissipated - usually on a heatsink at the back of the amplifier.2

If a power amplifier draws power from a supply, does it dissipate all of the indicated power drawn?

No. Power in = Power out + heat. Depending on the amplifier circuitry it could dissipate about half the power as heat and half to the speaker (which will also dissipate most of the power as heat).3


1 This explanation uses a dual rail power supply for simplicity. Other arrangements are possible and common.

2 To be more specific, \$ P_{Q} = V_{Q}I_{Q} \$ where \$ V_{Q} \$ is the voltage across the transistor.

3 Higher efficiency amplifiers such as Class-D minimise power loss by switching at high frequency between V+ and V-. In this method a transistor is either fully off so \$ I = 0 \$ and \$ P_Q = 0 \$ or the transistor is fully on, the voltage across it is low and the resultant power dissipated in the transistor is low.

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Is a power amplifier able to generate power?

No

Does a power amplifier draw power from a supply?

Yes

Does a power amplifier dissipate power itself?

Yes

If a power amplifier draws power from a supply, does it dissipate all of the indicated power drawn?

Only if it's supplying no power to a load. If it supplies a load, then it dissipates the difference between what it draws from the supply, and what it supplies to the load.

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  1. No, power comes from supply.
  2. Yes, amplifier uses power from supply.
  3. Yes, amplifier does consume power, there is no device that is 100% efficient.
  4. No, an amplifier would usually take only the power it needs to provide some power at output.
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