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Old 28th June 2002, 08:52 AM   #11
sonnya is offline sonnya  Denmark
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Quote:
Originally posted by peranders
You have a potential problem here: high output impedance. Collector out gives you 1-10 kohm. You should have << 1 ohm output impedance in order to drive speaker without colouring the sound. I sucpect that your open-loop gain is too low. You can't feedback away the high impedance.

You must find a trick to reduce the output impedance (before you feedback the signal).
I could very easy find myself design an amp a bit this way if it is stable... As Nelson says, the feedback forms the outputimpedance.

Sonny
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Old 28th June 2002, 11:17 AM   #12
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Default my amp design

Another thing to be carefull about is the standing current in the output stage. This topology will be very hard (impossible) to stably bias. It is very very difficult to tweak the resistor values for say 100mA in the output stage in a physical implemenation, whatever Spice will say. Look away for 10 secs and it may go down to 10mA, or, more likely, to 10 A. At the very least, put a small resistor in Q5 and Q8 emitters.

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Old 28th June 2002, 12:43 PM   #13
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I can't tell you what the output impedance is but think of it this way - the output transistors are arranged basically as current sources which by definition have infinite output impedance. You know how on the characteristic curves for a transistor for a given base current you wind up the collector voltage and plot the graph? The graph of collector current goes up pretty steep initially and then goes almost horizontally across the page, uphill ever so slowly even though the collector voltage gets higher and higher. That is the "resistance" of the transistor, a *change* of 50 volts might make a *change* of collector current from 1 amp to 1.01 amps so it is 50/0.01 = 5000 ohms! I think this is called slope resistance. So you got high output impedance, man.

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Old 28th June 2002, 02:14 PM   #14
sonnya is offline sonnya  Denmark
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Okay you guys do not get it!

Without feedback it will have a high output impedance, but with feedback it is another story!

It is basicly a RR output like the ones designed in a RR output opamp. And they can have a outputimpedance as low as milliohms because of the feedback.

I think you should go read some books about feedback and opampdesign. Sorry if i am rude! But i mean it.

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Old 28th June 2002, 03:35 PM   #15
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Here is a link to an amp that uses a collector output:
http://www.gmweb.btinternet.co.uk/mf-a1.htm
Also, follow this link and look at section 4.4 - Complementary Sziklai Output Stage.
http://www.ee.und.ac.za/Courses/dne4...#_Toc515065899
What I'm attempting to do is refine this stage to be a complete amp if possible.
Look at section 8 of this document for an amp using this output configuration.
Another familiar implementation:
http://w1.521.telia.com/~u52107110/tr_monster.html
Here is one using a mosfet output, but i think a bjt could be used with some changes.
http://home.zonnet.nl/tschrama/owndesign.html
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Old 28th June 2002, 04:56 PM   #16
sonnya is offline sonnya  Denmark
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You could add a emitter resistor to the output devices. Not only could this help getting a stable idlecurrent, but also be usefull for a currentlimiter.

But still you would need some way to get control of the idlecurrent. Design a circuit with bias a extra current to the bases on the output devices depending on the temp. of the heatsink!?

Sonny
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Old 28th June 2002, 04:58 PM   #17
Electrons are yellow and more is better!
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Quote:
Originally posted by sonnya
Okay you guys do not get it!

Without feedback it will have a high output impedance, but with feedback it is another story!

It is basicly a RR output like the ones designed in a RR output opamp. And they can have a outputimpedance as low as milliohms because of the feedback.

Sonny
If you have feedback of 60 dB (rather much!), it means that you reduce the output impedance with a factor of 1000. Let's say the transistors output impedance is 10 kohm (I don't know the real value, maybe down to 1 kohms iat higher currents) this gives 10 to 1 ohm out. 1 ohm may work but will colouring the sound especially if the speaker is "difficult". 10 ohms out is a total catastrophy! You lose 3/4 of the output power for starters, it's only 6 dB but it surely will colouring the sound.

A friend of mine built a Hiraga amp (class A) with collectors out and with moderate feedback. The amp didn't sound good with my friend's speaker (with difficult charateristics).

Listen to the old boys, use emitters out. "Grundtips" (basic tips) as we say in swedish.
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Old 28th June 2002, 05:49 PM   #18
sonnya is offline sonnya  Denmark
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Where did you find power BJT with a VA of more than 1000V? I would like to know.

More realistic : VA = 200 + half supply voltage = 250V.

ro ~= va/ic => 250Ohm

Two BJT(NPN+PNP) ro = ro/2 = 125Ohm.

Openloop gain of 1000times.

125mOhm.

By the way : Monster is more like a inverted compound (emitter follower?)

Sonny
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Old 28th June 2002, 06:03 PM   #19
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Thanks for the lesson on ro.
How about this:
Attached Images
File Type: png amp9.png (5.5 KB, 293 views)
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Old 28th June 2002, 07:06 PM   #20
Electrons are yellow and more is better!
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Quote:
Originally posted by sonnya


Openloop gain of 1000times.

125mOhm.

What do you mean?
Openloop gain of at 1000. How much feedback do you suggest?

Wanted gain must be 20-30 dB. 60 dB feedback gives 80-100 dB open-loop gain. Anybody will no fix that. Oscillations or a slow amp.
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