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Old 27th October 2006, 04:02 AM   #1
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Default Output stage with gain

Awhile back, I noticed a post regarding output stages (for class B or AB use) that have a voltage gain greater than 1. I was wondering if anyone has a link or reference to this topic, I am very interested in it.

If the output stage has gain, then the overall voltage swing at the output of the amp might not be constrained by the VAS.

Thank you!
-RT
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Old 27th October 2006, 04:32 AM   #2
clem_o is offline clem_o  Philippines
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It's been used on some amplifiers, including (if I recall) the Tigersaurus.

The issue is getting the stage to be free of oscillation. With a gain over 1 and an inductive emitter resistor, the result is often oscillation.

Modern times you can now get plate resistors, etc so it may be possible to implement...

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Old 27th October 2006, 04:38 AM   #3
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I did that long ago with good results.
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Old 27th October 2006, 04:40 AM   #4
clem_o is offline clem_o  Philippines
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Good for you! We tried it and junked it. The Tigersaurus was notorious for poofing its outputs... :-)

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Old 27th October 2006, 05:07 AM   #5
big jim is offline big jim  England
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The texan wireless world 1970s o/p stage gain of 10

Alba ua700 1970s british amp o/p stage gain of 10


jim
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Old 27th October 2006, 08:51 AM   #6
sreten is offline sreten  United Kingdom
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Hi,

In D.Selfs Power Amplifier Design book the is a chapter on op-amps
and amplifiers with high gain output stages that would appear to
have very high output impedance, certainly open loop performance
looks poor.
However closed loop performance is equivalent to a unity gain stage.

They are common as op-amps in MP3 type low voltage 32R output stages.

Self notes the configuration gives good voltage swing to the rails, but
as he was investigating c/o distortion in the article the configuration
is less linear than a conventional stage so its not examined further.

/sreten.
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Old 27th October 2006, 09:19 AM   #7
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Quote:
Originally posted by sreten

Self notes the configuration gives good voltage swing to the rails, but
as he was investigating c/o distortion in the article the configuration
is less linear than a conventional stage so its not examined further.

/sreten.

Doug is smart guy, but did he compare them on complex loads, or poorly resistive?
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Old 27th October 2006, 10:17 AM   #8
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Default Output stage with Voltage gain of 4 - the ETI480

hello sreten,

In Australia a very popular amplifier kit has been available since the mid 1970's that had an output stage having a voltage gain of about 4 which was set by a local feedback loop. It was called the ETI480. It used 2N3055 transistors in the output stage, but it did not have a very good reputation as an accurate amplifier, and from various accounts from different people it seemed to be quite prone to oscillation.

However, the fact that the kit is still available today must count for something, although recently Silicon Chip magazine has published a replacement kit for it called the SC480, which has reverted back to the more common configuration of an emiter follower output stage after the VAS. A brief discussion of the relative merits of the old ETI480 and the new SC480 can be found here:

http://www.siliconchip.com.au/cms/A_30377/article.html

Hope this helps you.

cheers,
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Old 27th October 2006, 11:07 AM   #9
jez is offline jez  United Kingdom
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A similar thread was around just a few days ago....
I have always found this idea quite interesting also, mainly because of the possibility of reduced swing from the VAS and easy regulation of the (all low voltage) pre output stage part of the amp using just off the shelf cheap regulators. I mentioned "The Texan" in the other thread.....this design was the inspiration for the Alchemist Products Axiom/Maxim (Maxim was Axiom with phono stage) amplifiers which I designed around 1994.
This used an output stage with a gain of ten driven by a simple FET diff amp and small signal MOSFET VAS which ran from +&- 15V. The FETs, MOSFETs and very low feedback were mainly attempts to capitalize on the marketing cache of these buzz words at the time Guilty as charged
I was never happy with this design for many reasons, (see above! not a good topology by my own admission...but novel in a budget amp) but it sold quite well and had very good reviews
Bias stabilisation is a problem with this type of output stage, as is distortion, there are several trade offs to be made but it can work very well with lateral MOSFETS.
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Old 27th October 2006, 12:39 PM   #10
mikeks is offline mikeks  United Kingdom
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