Bias in a classical current feedback amp - influence how? - diyAudio
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Old 26th June 2003, 11:34 AM   #1
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Default Bias in a classical current feedback amp - influence how?

Since I'm not a SPICE guy yet, how does current of the input stage influence on the performance of a classical current feedback amp (noise, speed, distortion)? Should I have low, (low noise) or high current in the input stage? With high I mean 1-5 mA.
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Old 27th June 2003, 07:11 AM   #2
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Noone knows, not even the experts here?
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Old 27th June 2003, 07:31 AM   #3
sonnya is offline sonnya  Denmark
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Hi Per!

if i remember right, my own amp which (was a mind offspring of the Alexander).. i have set up the current in the input stage.

The reason was that i could se a small difference on the slewrate.

With a current of 1 mA the input bjt will have a re = ~ 25Ohm.

by setting the current 5 times higher it will slew better.

But have to take into consideration that the BJT was low beta types (2n3904/06).

I hope this was what you were looking for.

I have not run any sim on this.

Sonny
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Old 27th June 2003, 11:19 AM   #4
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Default Noise

The noise curves are a function of the source impedance and collector current of the transistor. Its like a "u". So you will have to use a device that is suited for your source impedance for the lowest noise performance. A good example is the Phono cartridge . MM and MC need different input devices for lowest noise performance. I think lower source impedance usually have higher collector currents. The curve is a U and so the Noise Figure is lowest only in a small band . My memory is rusty. I will dig out the curves and put up a clearer post.
Check out the noise figure curves for the BC549 or similar and you can see what I mean.
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Old 27th June 2003, 11:34 AM   #5
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Guys, I know about noise figures in general. I have dived deep into it but the question now is more like if it's a difference in performance with different currents. Yes, obviously, but how much?

One parameter is critical and that is Rbe f the input transistors, should be as low as possible => high current
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Old 27th June 2003, 12:35 PM   #6
dimitri is offline dimitri  United States
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I'm not a SPICE guy yet...
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Old 27th June 2003, 02:11 PM   #7
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I messed with that topology years ago. I didn't remember it having any noticeable impact.

Are you making one from scratch, or are you just asking in general???

Jocko
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Old 27th June 2003, 02:32 PM   #8
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I got inspired to make one amp (headphone for starters) with a classic current feedback topology like in integrated ones. Just for fun and for pure technical reasons only.

Right now I'm just interested in the theory (mirrored over to real thing amp), don't care how good it sounds.
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Old 27th June 2003, 03:27 PM   #9
andy_c is offline andy_c  United States
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Hi Per,

I have a complete SPICE simulation of a power amplifier circuit using that topology with the freeware LTSpice from Linear Technology. If you like, I can provide you with all files and models needed to run it, or I can try some things in the simulation if you provide some specific suggestions. I'm at work now, and won't be home for another 11 hours or so. I can provide more specific information then (schematics, etc.).
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Old 27th June 2003, 05:31 PM   #10
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Quote:
Originally posted by andy_c
Hi Per,

I have a complete SPICE simulation of a power amplifier circuit using that topology with the freeware LTSpice from Linear Technology. If you like, I can provide you with all files and models needed to run it, or I can try some things in the simulation if you provide some specific suggestions. I'm at work now, and won't be home for another 11 hours or so. I can provide more specific information then (schematics, etc.).
Maybe I have to download LTSpice but it have to wait until Monday (just Mac at home ). Do I have regular parts as BC5xx and some common BD139 like parts?

Maybe this shall be my first SPICE project? I became a little bit burned when a mate of mine use a SPICE simulation of his oscilloscope amp (YES, he built a 2" oscillscope, with batteries!!!) and the real world was very cruel to him. He had a long print which came out from a VAX computer. But I suppose the SPICE models have been developed and also the computers....
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