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I have read some articals about design of Hi-Fi power amps and from what i understand most of the amplifiers use 2-3 or 4 stage amplification while 3 stage is most common (trans-admitance -> trans-impedance -> unity power stage)

why in most amplifiers circuits i have seen the design implements the trans-admitance stage from descrete op-amp and current source (4-6 BJTs) and not IC op-amp?

ie what are the benefits of desinging it from descrete transistors and not ICs?

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    \$\begingroup\$ Think about the power and how this relates to the supply voltage. Do you know any good high voltage and high power IC? \$\endgroup\$ – G36 Apr 5 '20 at 8:51
  • \$\begingroup\$ thanks for the quick answer! actualy i have seen some but i guess they are quite expencive, is that the only concideration? \$\endgroup\$ – Ron Vaisman Apr 5 '20 at 9:24
  • \$\begingroup\$ Also in audio, they do not like IC power amplifiers and discrete design, in general, can drive more current if needed and thermal management is also easier. \$\endgroup\$ – G36 Apr 5 '20 at 9:39
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Here is a very common audio amp topology, from Douglas Self:

enter image description here

It has the usual 3 stages :

  • Input stage is transconductance, converts error voltage into differential current which is then turned into push-pull current by current mirror

  • "VAS" or "voltage amplification stage" which is both a current gain stage and a transimpedance stage ; at frequencies above a few hundred Hz it behaves as transimpedance, this means the output voltage is the integration of input current via compensation cap C3.

  • Unity gain power stage

There are two main reasons for going discrete:

  • In order to have a marketable output power like 60W into 8 ohms or more, supply voltage has to be pretty high, which means opamps can't be used without special tricks which either bring in serious compromises wrt distortion, or would make the circuit more complicated than a fully discrete circuit anyway.

  • In a discrete circuit, compensation (C3 on the schematic) can be adjusted according to what the design requires.

This latter point is important, usually an opamp has an internal compensation that cannot be modified. Thus the opamp introduces one dominant pole which may not be where you want it. Especially, if you add an output stage with gain to increase output voltage swing beyond opamp supply rails, this will introduce another pole in your loop, thus extra phase shift. It is much simpler to use pole splitting and dominant pole compensation in a discrete amp.

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