Final stage amplifier

[Kevin Weddle]

Why do final stage amplifers tend to use discrete transistors? If the previous stage is an integrated IC, why can't the last stage be?

[Hero999]

It depends on the design.

Lots of amplifiers use ICs for the output stage, TDA2003, TDA2050, TDA7294 etc.

[Kevin Weddle]

Are they high power IC's? Then why do they have to add power transistors in final stage amplification.

[audioguru]

Barely clipping with low distortion:
1) The TDA2003 has an output of 4.5W into 4 ohms or 8.5W into 1.6 ohms.
2) The TDA2050 has an output of 25W into 8 ohms or 28W into 4 ohms.
3) The TDA7294 has an output of 70W into 4 ohms or 8 ohms.

The amount of heat in these linear amplifiers is the same as the output power. If separate output transistors are used then there are at least two transistors so the heating can be doubled and then the output power can also be doubled. Or paralleled output transistors can share even more power.

[Hero999]

Barely clipping with low distortion:

That's got nothing to do with an IC being used rather than discretes for the output stage and all to do with the overly optimistic claims made on the datasheets.

If the power is kept low enough then there will be no more clipping and distortion than a transistor amplifier.

I know you know this but your statement might confuse people who don't into thinking IC amplifiers are worse for distortion.

[audioguru]

If the power is kept low enough then there will be no more clipping and distortion than a transistor amplifier.

I know you know this but your statement might confuse people who don't into thinking IC amplifiers are worse for distortion.

Most IC amplifiers have higher distortion than discrete amplifiers because the dynamic heating in the output transistors is passed to the input transistors and resistors which causes the distortion.
A half-decent amplifier made with discrete transistors or a very good IC amplifier has distortion of only 0.003% (LM3886 IC) at 1kHz.
1) The distortion of the TDA2003 below clipping is 0.15%.
2) The distortion of the TDA2050 below clipping is 0.03%.
3) The distortion of the TDA7294 below clipping is 0.005% to 0.01%.

[Kevin Weddle]

The amount of heat in these linear amplifiers is the same as the output power. If separate output transistors are used then there are at least two transistors so the heating can be doubled and then the output power can also be doubled. Or paralleled output transistors can share even more power.

That's right, you have to be able to match the IC amplifiers output with load. Some IC data sheets recommend additional circuitry.