# Is a power amplifier able to generate power? Does a power amplifier draw power from a supply?

• 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?

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.

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.

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.