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Old 2nd November 2003, 06:21 PM   #261
andy_c is offline andy_c  United States
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I also tried the alternate connection of the electrolytic cap C3 which bypasses the zener diode string per Walt's suggestion. Unfortunately, this made the 50 MHz parasitic in the pass elements come back. I believe what's going on is that at VHF frequencies, the real part of the input impedance of Q4 is going negative. R7 seems to cure this, but connecting C3 to the base of Q4 provides a low impedance to the base of Q4, bypassing R7 and reintroducing the parasitic oscillation. I don't think the noise will be affected too badly, as there's so much gain ahead of this point from the op-amp that the front end should dominate. That is, the composite noise factor Ftot as below

Ftot = F1 + (F2-1)/G1

should be dominated by F1, since G1 is large in the audio band. At any rate, since this is for a power amp application, noise is not as critical as for the classic version of the super reg. Line rejection and clean, fast transient response to pulses of load current (from charging the gates of the MOSFET output stage) are the most important criteria for me.
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Old 2nd November 2003, 07:27 PM   #262
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Default Or maybe another approach

"I also tried the alternate connection of the electrolytic cap C3 which bypasses the zener diode string per Walt's suggestion"

At 50 MHz and electrolytic cap looks inductive due to parasistics. Keep the electrolytic bypass, add another 10 ohm resistor between the base of the pass transistor of and your filtered zener stack output. Add a 0.01uF cap directly from the output of the follower (at the other end of filtered zener stack) to the 10 ohm resistor (the end not going to the base of the output transistor). Use a noninductive cap like a ceramic or stacked film mylar and make path as short as possible (for low parasitic inductance) from the added emitter high frequency coupling cap (0.01uF) to the base resistor for output transistor. I'll bet you this will work.....
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Old 2nd November 2003, 09:57 PM   #263
jwb is offline jwb  United States
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Default Current sources compared

The banter between Jung and the rest was irresistible, so I was compelled to test these current source ideas out on the bench. I compared the canonical current source with an LED and transistor (Isrc 1), Isrc 1 with a J511 biasing the LEDs (Isrc 2), Isrc 1 with a cascode transistor (Isrc 3), and Isrc 2 with a cascode transistor (Isrc 4). I tested Zout of each current source with a 3.3V 120Hz ripple on the Vcc line. Isrc 4 was the winner by far, with a Zout at 5.6MΩ
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Old 2nd November 2003, 09:58 PM   #264
jwb is offline jwb  United States
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Too bad you can only include one image per message.
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Old 3rd November 2003, 05:04 AM   #265
Bricolo is offline Bricolo  France
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The 4th clearly looks like a winner here. And isn't soo much more complicated.

Jwb: have you also run an AC analysis?
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Old 3rd November 2003, 05:36 AM   #266
jwb is offline jwb  United States
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Quote:
Originally posted by Bricolo
The 4th clearly looks like a winner here. And isn't soo much more complicated.

Jwb: have you also run an AC analysis?
As I said, I tested the impedance at 120Hz. I'm not really equipped to test other frequencies, do sweeps, or what-have-you. Would be an interesting simulation for a SPICE-head.
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Old 3rd November 2003, 05:42 AM   #267
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Question Would you believe a ........

I believe that the Isrc 4 must have been using a J501 and not a J511. The current for the J511 is about 4.7 mA and that for the J501 is about 0.33 mA. The dynamic impedance for the J511 is around 300 K and that of the J511 typically 10 Meg. With I = (6-0.33) mA across 221 ohms the voltage across the resistor is around 1.2 volts. Add this to the base emitter drop and you get about 1.8 volts for the LED. which would make it a green one.

Sorry about that Chief,

Fred
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Old 3rd November 2003, 06:23 AM   #268
jwb is offline jwb  United States
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Sorry Fred I don't understand your point. Current diode used was in fact J511, 4.7mA. It was the only one I had laying about, as I used the last of the J506 on Saturday. LED used was LN342GP, a green one that drops 1.9V at 5mA. I used two ZTX869 because they were already on the breadboard.

Do you disagree that the fourth topology is the best one, or are you saying it would have been better had I used a more reasonable current diode?
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Old 3rd November 2003, 08:09 AM   #269
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Quote:
Originally posted by andy_c
[snip]
Per Jan's request, I've shown a plot with a load capacitor of 1 uF, having a series resistance of .01 Ohm and a series inductance of 5 nH. The attenuation at the notch is 33.3 dB, which would correspond to an output impedance of the pass transistor of 0.45 Ohms if it were purely resistive. This is consistent with the 61 mA operating current from the DC current source load. The behavior with the 100 uF cap with 0.25 Ohm ESR is also consistent with that value of pass transistor output impedance. I think this plot shows pretty clearly that a low ESR cap is not good. The notch can be so deep that it forces the loop gain magnitude to unity at a frequency below the notch where the phase is lagging considerably.[snip] I'm now beginning to wonder whether Jan wanted this or to add it in parallel with the 100 uF cap. The Zobel network has been removed for this simulation.
Andy,

Thanks very much for this. It neatly confirms my experiences that the "better" the output cap, the more problems with stability and in fact worse output Z in some freq areas. Counter-intuitive, a good, expensive film cap with low ESR is a nono on these supplies, EXCEPT close at the load, away from the supply, so that the wiring L can correct things. Then again, with remote sensing, it comes back again, UNLESS you use the lp filter in the sense line. It all makes sense (no pun intended).

As far as parallelling the two caps, I woulkd think that for this effect the small low ESR cap would dominate and determine the response.

Jan Didden
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Old 3rd November 2003, 08:17 AM   #270
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Hi,

Quote:
Then again, with remote sensing, it comes back again, UNLESS you use the lp filter in the sense line. It all makes sense (no pun intended).
Good to see some people understand the nature of the beast...

666....

Cheers,
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