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Old 19th February 2006, 11:28 AM   #1
Did it Himself
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Default Output Stage Comparison

I've been playing around with amp designs for a while now, messing about with various implementations. I thought it would be nice to gather all the common output stages together comparitively in terms of output swing. So here they are:

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The same schematic was used for all stages. Output stage quiescent current was adjusted in each case to be about 22mA. Voltage rails were 33.5V and no 'emitter' resistors were used. Load driven was 8 ohms resistive.

These results say to me that there is no major winner in output stage efficiency, despite what some people may claim. Driver 'emitter' resistors were 150R and I was able to reduce swing by increasing this resistance, so this is clearly a factor to consider when designing output stages.
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Old 19th February 2006, 11:41 AM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
I think you have chosen a set of output conditions that do not exaggerate the differences between the devices.

But, I can see some differences in the plots.

Look at the length of the flat top/bottom for each type.

By far the longest is the Nch FET and you did not show Pch.

It would appear from this scant information that the FET would have a lower output than any of the others if you reduced the voltage to just at or below clipping.

If you reduced the load impedance, you may find that the other devices will also show some difference.

But try to find a better way to plot your information and the differences may jump out at you. eg. the two diode drop of the Darlington should stand out.
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Old 19th February 2006, 12:22 PM   #3
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Andrew, I should have been a little clearer about the information on the plots. The clipping points are measured by markers and if you look at the bottom of the plots the cursor points are listed.

I was fully aware that the n-channel vertical MOSFET was the least efficient of the bunch. My point was that there is barely a volt between the best and worst performer, at 8 ohms load.

I did not show a pure p-channel vertical MOSFET output stage because nobody uses them.
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Old 19th February 2006, 12:24 PM   #4
AndrewT is offline AndrewT  Scotland
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But it forms the other half of a complementary output stage.
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Old 19th February 2006, 12:42 PM   #5
Eva is offline Eva  Spain
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How did you connect the MOSFETs? as source followers?
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Old 19th February 2006, 12:53 PM   #6
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Greetings from Norfolk

For those busy re-learning, what is a Sziklai circuit ?

Thanks

Richard
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Old 19th February 2006, 01:01 PM   #7
Eva is offline Eva  Spain
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Googling I found some CFP a.k.a. "Sziklai pair" figures from Douglas Self site:

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Old 19th February 2006, 01:02 PM   #8
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Quote:
Originally posted by AndrewT
But it forms the other half of a complementary output stage.
No you have misinterpreted things. The plots show the output node when driving an 8 ohm load, i.e. the available output swing. The output stages are built as follows:

Plot 1: complementary Darlington, i.e. NPN upper and PNP lower
Plot 2: complementary Sziklai (also known as complementary feedback pair, CFP), i.e. PNP upper and NPN lower
Plot 3: pure NPN, i.e. Darlington upper and Sziklai lower
Plot 4: pure N V-MOSFET, i.e. as Plot 3 but NPN replaced with N V-MOSFET.

Hope that clears things up.
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Old 19th February 2006, 01:04 PM   #9
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My comment on the test is that an output stage circuit should be designed around the chosen audio devices. If so you can have the same efficiency out of almost any type of power semiconductor.
You would probably never connect different devices in a similar circuit in practical use.

It's like saying, which motor is best a car engine or a boat engine, by putting both in a car.
My guess is that the car engine will win, simply because it's doing what it's designed to do.

So the test doesn't really show anything.
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Old 19th February 2006, 01:10 PM   #10
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Depending on V-MOSFET Rds-on, you may get rail-to-rail output with less than 1V drop if the gates are driven in such a way that the output won't be limited by Vgs. Replacing a bipolar device with a MOSFET is not an optimum solution due to the Vgs drop.
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