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Old 9th September 2013, 08:43 PM   #1001
bcarso is offline bcarso  United States
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I shall soon --- have to run some errands.
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Old 9th September 2013, 10:47 PM   #1002
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Default Noise in Current Mirrors

Quote:
Originally Posted by sonnya View Post
Then please...
Until Brad returns - Read this Link from Samuel. Excellent comments about noise towards the front.

Best wishes
David
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Old 9th September 2013, 11:09 PM   #1003
bcarso is offline bcarso  United States
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Quote:
Originally Posted by Dave Zan View Post
Until Brad returns - Read this Link from Samuel. Excellent comments about noise towards the front.

Best wishes
David
Heh, yes, I was just looking at one of the references I suggested to Samuel.

Waldhauer, in his excellent book Feedback, after analyzing the unballasted mirror, writes: "Figure 11.6a shows a way to quiet the current mirror by adding equal resistors in series with the diode and the transistor emitter. On the surface it seems paradoxical that we can quiet the current mirror by adding resistors that are themselves noisy..." [my italics].

Attached are a couple of examples to show the effect.

As far as the amplification of noise in the input current signals, mirrors are mirrors, and will pass that noise to the output, ideally unchanged.
Attached Images
File Type: jpg two 10mA current sinks 09-09-13 001.jpg (204.6 KB, 325 views)
File Type: jpg two current mirrors 09-09-13 001.jpg (275.5 KB, 311 views)
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Old 10th September 2013, 12:08 AM   #1004
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Default IPS & VAS for jan.didden

This is actually for Mr. Didden who would have us compare IPS & VAS with 'ideal' OPSs .. with apologies to Belaji & also Guru bcarso.

It in fact has less Unobtanium parts than Belaji's #856 circuit and half its THD at 20kHz 50W 8R. I added a 8R load which he left out.

The THD is almost pure 2nd for yus Golden Pinnae but at such a low level that its academic.(????) Other products are below 10 ppb.

Models are standard Cordell & LTspice.

You can see its slighly simpler than Belaji's ... and I would have it even simpler but then I couldn't claim its a CFA.
______________

I stand by my assertion that you can't look at dis stuff in isolation. How the IPS/VAS interfaces with the OPS is probably its most important feature.
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File Type: gif SimpleTHD.gif (14.5 KB, 286 views)
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Old 10th September 2013, 12:10 AM   #1005
Bonsai is offline Bonsai  Taiwan
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There has been a lot written on this subject and it does seem paradoxical.

In a practical amplifier, and especially one where the input current is quite high (mA rather than uA) the resistor value is unfortunately limited by practical considerations. In a practical power amplifier however, you can for the most part get away with it, except of course where you may be driving very efficient speakers.
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Old 10th September 2013, 02:16 AM   #1006
Bonsai is offline Bonsai  Taiwan
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Some more sim work I completed last night.

I have added Brad's latest design - we are down to ~2.5ppm at 50V pk to pk at 20 kHz. However, variant one that I posted up yesterday is now at 700 ppb for the same input and output signal conditions.

Loop gains and closed loop bandwidths are almost identical.

I've used 'Alexander' comp for these sims.
Attached Files
File Type: pdf CFA FE+TIS Study.pdf (282.8 KB, 94 views)
File Type: asc fx-Amp_advanced_5_LG_Plots.asc (17.2 KB, 36 views)
File Type: asc fx-Amp_advanced_5.asc (17.7 KB, 29 views)
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Old 10th September 2013, 02:30 AM   #1007
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Quote:
Originally Posted by bcarso View Post
Heh, yes, I was just looking at one of the references I suggested to Samuel.

Waldhauer, in his excellent book Feedback, after analyzing the unballasted mirror, writes: "Figure 11.6a shows a way to quiet the current mirror by adding equal resistors in series with the diode and the transistor emitter. On the surface it seems paradoxical that we can quiet the current mirror by adding resistors that are themselves noisy..." [my italics].

Attached are a couple of examples to show the effect.

As far as the amplification of noise in the input current signals, mirrors are mirrors, and will pass that noise to the output, ideally unchanged.
I had the pleasure of working with Fred Waldhauer at Bell Labs in the early 1970's. I still remember his excitement in describing many feedback systems with ABCD parameters. He also designed a gigahertz op amp in the 1970s.

He was a brilliant character.

During that time he led the development of key functions in the T4M digital coaxial transmission system, operating at 274 Mb/s circa 1974.

He went on to do some pioneering work in hearing aids.

Cheers,
Bob
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Old 10th September 2013, 03:17 AM   #1008
Bonsai is offline Bonsai  Taiwan
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In my sims above I appear to have forgotten that the BVceo of the 547/557 is 45V . . .
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Old 10th September 2013, 05:19 AM   #1009
sonnya is offline sonnya  Denmark
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Quote:
Originally Posted by bcarso View Post
Heh, yes, I was just looking at one of the references I suggested to Samuel.

Waldhauer, in his excellent book Feedback, after analyzing the unballasted mirror, writes: "Figure 11.6a shows a way to quiet the current mirror by adding equal resistors in series with the diode and the transistor emitter. On the surface it seems paradoxical that we can quiet the current mirror by adding resistors that are themselves noisy..." [my italics].

Attached are a couple of examples to show the effect.

As far as the amplification of noise in the input current signals, mirrors are mirrors, and will pass that noise to the output, ideally unchanged.
Thanks for the analyze.
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Old 10th September 2013, 05:26 AM   #1010
sonnya is offline sonnya  Denmark
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Quote:
Originally Posted by Bonsai View Post
Some more sim work I completed last night.

I have added Brad's latest design - we are down to ~2.5ppm at 50V pk to pk at 20 kHz. However, variant one that I posted up yesterday is now at 700 ppb for the same input and output signal conditions.

Loop gains and closed loop bandwidths are almost identical.

I've used 'Alexander' comp for these sims.
Some practical values.

If you do not use matched pair like BCM847BS and BCM857BS the value of 33R will give you an large imbalance in the current transfer as you easily can have 10mV or more difference on the VBE voltage creating an imbalance of at least 330uA. This offset will affect the system initial DC offset.

In circuit 1 you have C2 and C3 across R3 and R4. It is good to speed up the slewrate and raise the openloop gain, but an 1mF capacitor will have its resonance point below 1MHz in real life. And we try to make an an with high bandwidth.

Sorry .. It is not to be harsh, but that is my experience.
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