Electronics > Projects, Designs, and Technical Stuff
Power Amplifier AB class
dzseki:
--- Quote from: strawberry on May 15, 2019, 08:37:04 am ---Do amplifier can be designed without over current protection?
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Of course, for example the schematic you posted is one such design.
David Hess:
--- Quote from: dzseki on May 15, 2019, 10:53:56 am ---
--- Quote from: David Hess on May 13, 2019, 04:03:56 pm ---To just give one example, both of these designs use emitter degeneration to increase full power bandwidth which is common in audio power amplifiers however this raises the input noise by enough that it will be audible in the speaker. A discrete design, although not this one, could avoid this to make a "noiseless" power amplifier.
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Without emitter degeneration the open loop amplification of the differential stage will be much higher, which may lead to Transient intermodulation (TIM) problems, in other words emitter degenerated differential pairs are more linear in behaviour when examined alone less demanding for the global feedback loop to get the things "right"
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Lack of linearity of the input stage does not cause TIM; lack of full power bandwidth (slew rate) does.
Adding Gm reduction of any type allows for a higher tail current and a smaller Miller capacitance (C5) without closed loop instability resulting in a higher slew rate so recovery is faster. The easiest and most straightforward way to add Gm reduction is emitter degeneration and the added input noise is seldom a disadvantage in a power amplifier. It is likely that without this, the full power bandwidth would not be sufficient anyway so something has to be done and it might as well be this.
dzseki:
Why emitter degeneration affects noise so severely? I mean the typical value of emitter degeneration is a few 10 Ohms, the differential pair is usualy biased at a few milliamps. Where does the noise comes then? The induced current and voltage noise of the resistor is negligable here as I see, also without emitter degeneration the stage would have higher gain, therefore it would amplify its own noise more too.
David Hess:
--- Quote from: dzseki on May 16, 2019, 07:11:54 am ---Why emitter degeneration affects noise so severely? I mean the typical value of emitter degeneration is a few 10 Ohms, the differential pair is usualy biased at a few milliamps. Where does the noise comes then? The induced current and voltage noise of the resistor is negligable here as I see, also without emitter degeneration the stage would have higher gain, therefore it would amplify its own noise more too.
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You just gave the exact reason. In the example above, the tail current is 1.8 milliamps yielding an emitter resistance of 28 ohms making the 47 ohm resistor the largest noise source.
This particular design is one of the quieter ones. Most are much worse with the emitter degeneration being 10 times larger than the emitter resistance.
strawberry:
Differential stage can go into weird clipping
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