Electronics > Projects, Designs, and Technical Stuff
Power Amplifier AB class
dzseki:
--- Quote from: David Hess on May 16, 2019, 03:22:44 pm ---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.
--- End quote ---
I am still not convinced. I did a simulation on a circuit excerpt of an amplifier I am currently restoring. In this circuit the emitter degeneration is 220 Ohm and the bias is at 2.7mA (with proper current source) -see pictures.
If there is no degeneration on the emitter the simulation shows 62nV/sqrt(Hz) output noise, with 220 Ohm emitter degeneration this is only about 21nV/sqrt(Hz). This is of course because the gain of the stage is higher with no degeneration. Nevertheless this noise will propagate through the VAS regardless of the loop gain, leaving the non degenerated case noisier when everything else is the same.
strawberry:
when poking shcassis with screwdriver amplifier make crackling noise, zobel network is stopping from lachup oscilation but is not eliminating problem
David Hess:
--- Quote from: dzseki on May 17, 2019, 07:05:40 am ---If there is no degeneration on the emitter the simulation shows 62nV/sqrt(Hz) output noise, with 220 Ohm emitter degeneration this is only about 21nV/sqrt(Hz). This is of course because the gain of the stage is higher with no degeneration. Nevertheless this noise will propagate through the VAS regardless of the loop gain, leaving the non degenerated case noisier when everything else is the same.
--- End quote ---
The gain makes all of the difference when measuring output noise. The output noise must be divided by the gain to find the input noise. Where the gain matters is dividing the noise contribution of the following stage which is why the gain of the differential input stage is typically low and most of the gain is provided by a following stage.
The noise from the emitter resistance is in series with the base-emitter junction voltage so it shows up as the input noise voltage directly, with the added complication that with a differential stage, the noise is actually sqrt(2) greater which is one of the disadvantages of a differential amplifier. On the other hand, the differential configuration has common mode rejection which attenuates the noise from the tail current.
Just doing a back of the notepad calculation, I get 2.8nV/SqrtHz of input noise and a gain of 6.9 for your first example and 0.81nV/SqrtHz input noise and a gain of 85.5 for your second. That turns into 19.3nV/SqrtHz and 69 nV/SqrtHz of output noise which is pretty close to your results. I have no idea what the bulk resistance of a BC182 is but SPICE might.
--- Quote from: strawberry on May 16, 2019, 07:20:37 pm ---Differential stage can go into weird clipping
--- End quote ---
The only time that should happen is when the output slew rate is exceeded preventing the feedback network from balancing the input which shows why full power bandwidth is important in an audio power amplifier.
Besides the mentioned benefit of allowing an increase in full power bandwidth, degeneration does increase linearity but in the examples I have studied, the resulting decrease in distortion was marginal and better accomplished in other ways. For simple designs however, this is the way to go and the increased noise is hardly a disadvantage in a power amplifier except in special cases.
strawberry:
How to calculate Long tailed pair and VAS stage amplification and slewrate?
David Hess:
--- Quote from: strawberry on May 26, 2019, 09:25:20 am ---How to calculate Long tailed pair and VAS stage amplification and slewrate?
--- End quote ---
The easy way is simply the tail current into the Miller compensation capacitance of the VAS stage. This points to why Gm reduction allows an increase in slew rate. With a lower Gm, either less capacitance can be used for compensation or the tail current can be increased.
Of course if there is a fixed voltage amplification after the operational amplifier like in the examples I gave, it directly multiplies the slew rate at the cost of increasing the minimum closed loop gain. This is essentially sticking a current feedback amplifier with a fixed gain on the output of a conventional voltage feedback operational amplifier.
For detailed information on this subject of tail current, compensation, and slew rate:
Designing Audio Power Amplifiers - Bob Cordell
Audio Power Amplifier Design Handbook - Douglas Self
and
AN-A The Monolithic Operational Amplifier - National Semiconductor
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