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Old 4th December 2012, 04:00 PM   #1381
Salas is offline Salas  Greece
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From 100R to 50R when talking the ground return Fb ones for +6dB?
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Old 4th December 2012, 04:11 PM   #1382
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Two important points. Don't forget that, in a CFB amp, changing the serial feedback resistance value will change the slew rate. Changing the emitter resistance value will change the current in the input transistors.
So if you want to add some gain, the best is to replace the serial resistances by an attenuator, keeping the impedance seen by the input transistor near the same.

By example, if your original serial resistance is 1K and the emitter 50 Ohms, from the output, do 1Kserial +1k to gound and 500 Ohm to the 50Ohm.
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Last edited by Esperado; 4th December 2012 at 04:27 PM.
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Old 4th December 2012, 04:31 PM   #1383
vgeorge is offline vgeorge  Greece
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Quote:
Originally Posted by Salas View Post
From 100R to 50R when talking the ground return Fb ones for +6dB?
Yes, these ones.
Sonny supplied both values with the kit.
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Old 4th December 2012, 07:49 PM   #1384
sonnya is offline sonnya  Denmark
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Quote:
Originally Posted by Esperado View Post
Two important points. Don't forget that, in a CFB amp, changing the serial feedback resistance value will change the slew rate. Changing the emitter resistance value will change the current in the input transistors.
So if you want to add some gain, the best is to replace the serial resistances by an attenuator, keeping the impedance seen by the input transistor near the same.

By example, if your original serial resistance is 1K and the emitter 50 Ohms, from the output, do 1Kserial +1k to gound and 500 Ohm to the 50Ohm.
Hi Esperado.

With a reasonable bandwidth, reducing the emitter resistor will not cause any high currents in the audio range. But i have and will recommend any one to have a lowpass input filter.

For two reasons:
  1. Reduce emi from the line signal - cellphones in particular
  2. Reduce slewrate allowed even if the amp is capable of more. It stresses the transistor and zobel network a lot with "extreme" slewrates.

By leaving the Rf value and reducing the emitter resistor you will raise the loop gain... So by adding +6dB system gain, this way the loopgain stays the same.
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Old 4th December 2012, 07:59 PM   #1385
sonnya is offline sonnya  Denmark
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Quote:
Originally Posted by Salas View Post
The 200kHZ square waves pics are with no bypasses at all. Not on rails, not on feedback. Perfectly stable. I don't know if the bypasses speed it up more, but with just the FC lytics, I found the -3dB point at circa 2.5MHZ (6 vertical divisions reduced to 4 rough check point).
I will maybe change the DOC to say that they should start out without the bypass caps in the feedback network.

They can always add them later.
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Old 4th December 2012, 08:05 PM   #1386
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HWith a reasonable bandwidth, reducing the emitter resistor will not cause any high currents in the audio range. But i have and will recommend any one to have a lowpass input filter.
I too, of course. But keeping the ratio signal speed/amp speed as little as possible is the target. If no stability issue, i would reduce the serial feedback resistance instead. Keeping a low-pass filter at 200Khz before the input.
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Last edited by Esperado; 4th December 2012 at 08:09 PM.
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Old 4th December 2012, 08:29 PM   #1387
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May-be i need to precise my point. Decreasing the emitter resistance of the first stage will increase the gain of the input stage, as its collector one will stay the same. If the amp has been carefully designed for each stage produce the same distortion in open loop, you will increase-it. And the quiescent current will be increased as well. Right ?
That's why i recommended to not touch this resistance.
On the contrary, reducing the serial feed-back resistance will stay the input stage unchanged, and reduce the influence of parasitic influence near the emitter of the input transistor because impedance will be reduced.
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Old 4th December 2012, 08:34 PM   #1388
sonnya is offline sonnya  Denmark
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Agree. I tried that too. But then millercaps should be larger.

That was what real life tests showed.

Secondly its thermal stability gets worse when Rf was reduced. Not as bad as is the SSA design.

2.3mV/K across 1K instead of 100R in the SSA. That is a 10 factor.
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Old 4th December 2012, 08:39 PM   #1389
sonnya is offline sonnya  Denmark
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Quote:
Originally Posted by Esperado View Post
May-be i need to precise my point. Decreasing the emitter resistance of the first stage will increase the gain of the input stage, as its collector one will stay the same. If the amp has been carefully designed for each stage produce the same distortion in open loop, you will increase-it. And the quiescent current will be increased as well. Right ?
No the quiescent current will not increase, It is controlled by the current source and Rf. The emitter resistor is AC coupled to GND.

Quote:
Originally Posted by Esperado View Post
That's why i recommended to not touch this resistance.
On the contrary, reducing the serial feed-back resistance will stay the input stage unchanged, and reduce the influence of parasitic influence near the emitter of the input transistor because impedance will be reduced.
That is correct but you will raise the bandwidth. And my test showed that this is NOGO . 33pF and Rf=1K is the limit. Reducing Rf means larger Millercaps ....
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Old 4th December 2012, 08:52 PM   #1390
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That is correct but you will raise the bandwidth. And my test showed that this is NOGO . 33pF and Rf=1K is the limit. Reducing Rf means larger Millercaps ....
Can't you add a // cap with the feedback resistance to keep the closed loop bandwitch the same ??
For the quiescent, my amp use a ccs referenced to rail, so we are not in the same situation, you are right.
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Last edited by Esperado; 4th December 2012 at 08:55 PM.
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