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Old 28th October 2007, 08:12 AM   #1
borges is offline borges  Norway
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Default Best emitter / source / cathode follower topology

What are your opinions on the best topology for a line driver follower? I'm designing the output of a CD player and want to do it with no feedback in the system.

Goals are for high linearity, low noise and relatively low output impedance. The power supply can be designed around the right follower.

There are options aplenty, NPN or PNP emitter followers, PMOS or NMOS, P/NJFETs, tubes etc. I'd like to learn about successfull output follower stages at line level.

Thanks for your contributions.

BÝrge
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Old 30th October 2007, 08:19 PM   #2
Jeb-D. is offline Jeb-D.  United States
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This probably belongs in the tube forums but,

Cathode follower has low output impedance and is highly linear. You probably won't get the tube effect though, as it will add the least amount of coloration. This may or may not be a good thing for you.

If you want tube sound but low Zout I'd do one of the following.

-Use a common tube wired in the conventional grounded cathode stage using it's own dedicated High voltage line. Then use a small step down audio transformer. You could cap-couple it, but the tube wouldn't swing enough voltage at line-level to extract it's characteristics. The step down transformer will also lower it's output impedance and noise floor.

- Try building a low voltage one using these type of tubes.
http://www.duntemann.com/12vtubes/12vtubesindex.htm
Pick one with a low Rp. Do a grounded cathode or SRPP cap-coupled. I've been eager to mess with these myself but haven't got around to it.
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Old 30th October 2007, 08:53 PM   #3
MRupp is offline MRupp  Germany
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There are lot's of options but probably no "best" topology, so do some more search! John Broskie's tubecad site has plenty of information on cathode followers and their variations, e.g. Aikido CF, White CF, Super Linear CF, etc. ...

For a JFET source follower discussion see part 2 of the Borbely article JFETS: THE NEW FRONTIERS. You can download his articles from his WEB pages.

P.S. 12 V tubes are a very good idea, especially since you can more easily build versions with a bipolar power supply.
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Old 30th October 2007, 09:15 PM   #4
borges is offline borges  Norway
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Thanks guys!

I'm not that familiar with tubes. As an alternative to the cathode follower I may work with anything that has a bit of gain and high input impedance. As I said in the first post I'm trying to get by without feedback.

I was also hoping for some transistor based suggestions to reduce the hi-voltage supplies.

If you have any suggestions for tubes that work well for line-level output stages, please let me know.

I'm trying to introduce as little distortion as possible. Hence the no-feedback approach.

BÝrge
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Old 30th October 2007, 10:05 PM   #5
Jeb-D. is offline Jeb-D.  United States
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How much input voltage will the stage get? What supply voltages are available?
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Old 31st October 2007, 08:02 AM   #6
borges is offline borges  Norway
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The supply has not been designed yet. But I should be able to crank it up to 100V if need be. However, I'm more comfortable below, say, +-30V.

The output of the follower (or output amp) is 2Vrms, +-2.82V. So in the follower case the input voltage will be about the same.

Alternatively, I have been considering making a class-A output stage with a gain in the neighborhood of 1-5. The important thing there, again, is very good linearity, low noise, high output impedance and low output impedance for line driving.


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Old 31st October 2007, 08:41 AM   #7
EC8010 is offline EC8010  United Kingdom
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Quote:
Originally posted by borges
I'm trying to introduce as little distortion as possible. Hence the no-feedback approach.
Feedback reduces distortion. An emitter/source/cathode follower operates with 100% negative feedback.
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Old 31st October 2007, 09:26 AM   #8
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I'm going to prove my ignorance now!
Quote:
An emitter/source/cathode follower operates with 100% negative feedback.
Does it? I've only real experience of emitter followers, but (unless its a cfp/sziklai arrangement, or maybe there is a cap in parallel with the emitter resistor) surely an emitter follower is just adding current gain, and the voltage output is Vbe less than the input voltage? Because of the current gain, you have the ability to 'match impendances'. Therefore, its open loop, and there is no feedback.

I may well have the wrong end of the stick there.
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Old 31st October 2007, 01:29 PM   #9
EC8010 is offline EC8010  United Kingdom
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You're right about Vbe, but if you think a little harder, you'll realise that can't be exactly right, because if it was, there would be nothing to drive the transistor. The fuller way to look at an emitter follower is to move the emitter load resistor up to the collector, calculate the voltage gain under those conditions, then apply 100% feedback to see what the emitter follower gain would be:

As an example, suppose you had a 20V supply, a 10k load resistance and you biased to drop half the supply voltage across that load resistance.

Ic would be 10V/10k = 1mA.
gm = 35Ic = 35mA/V
Av = gmRL = 35 x 10 = 350.

Your common emitter stage would have an open-loop gain of 350. But you slide the 10k load resistance through the transistor back into the emitter circuit, causing 100% feedback (B=1). You calculate the new gain:

A = A0/1+BA0 = 350/(1+1x350) = 350/351 = 0.997

Which is very nearly 1, but not quite.
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Old 31st October 2007, 01:54 PM   #10
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I follow you until you get to ...
" But you slide the 10k load resistance through the transistor back into the emitter circuit, causing 100% feedback (B=1). "

I need to understand that. I'll go away and investigate. Otherwise, it looks like a different way to get to the same result.
Thanks for the time to describe it. I need to read up.

Anyway, back on thread, I've just finished an IV stage for my DAC, and I also need to add some form of buffer. I was probably going to use a complimentary feedback pair acting as an emitter follower, with a two transistor current source instead of a load resistor. I've used a similar circuit in a headphone amp and it seemed to work well. Are there any obvious ways to improve it? I suspect cascoding would improve it, but I've never looked into it.
I guess thats an answer but also more questions.


Cheers,
Phil
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