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Old 25th October 2015, 08:11 AM   #1
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Default output impedance in shunt reg sim

I have been working on this shunt regulator sim and need some help. I started with the 2 transistor regulator of tnt-audio and modified it by adding a ccs everywhere.

Most of the stuff seems to be ok except for the high output impedance (100hz – 3.3R, 1khz – 340mR). I tried a lot of things to fix it, tweaked component values, changed mosfets, transistors, jfet ccs etc, but it just doesnt budge. Trying different stuff improved the ripple, but no change in impedance. Increasing the main ccs current also didnt help with the output impedance. My output load is at 1 khz and the spike at 1khz is also high. Zener current is a little more than the zener test current in the datasheet.

What decides the output impedance of this kind of circuits or how do I improve it.
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Old 25th October 2015, 08:26 AM   #2
AndrewT is offline AndrewT  Scotland
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I don't know your answer.
But I suspect your formula for calculating the output impedance is incorrect.
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Old 25th October 2015, 12:36 PM   #3
Ketje is offline Ketje  Belgium
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To start with, you'r driving the gate of the shunt fet from a high impedance.That is a current source an a collector output.Not the best way to get a fast response.
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Old 25th October 2015, 12:43 PM   #4
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Quote:
Originally Posted by doors666 View Post
...Most of the stuff seems to be ok except for the high output impedance (100hz – 3.3R, 1khz – 340mR). What decides the output impedance of this kind of circuits or how do I improve it.
A common sign that a parameter (in your case, output impedance) may be getting affected by the value of some capacitor (with respect to it's opposing resistance term) is when that parameter changes by a factor of ten with decade change in frequency.

You did mention that you've tried changing some components but didn't specify which. Did you try increasing the values of C9, C8 and C11?
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Old 25th October 2015, 01:15 PM   #5
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Originally Posted by AndrewT View Post
I don't know your answer.
But I suspect your formula for calculating the output impedance is incorrect.
I just tried Vout/1A and changed the response to linear from log to see it in ohms in the ac analysis response.

Quote:
Originally Posted by Ketje View Post
To start with, you'r driving the gate of the shunt fet from a high impedance.That is a current source an a collector output.Not the best way to get a fast response.
Mona
I tried an emitter follower between the two, didnt change the impedance.

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Originally Posted by Ken Newton View Post
A common sign that a parameter (in your case, output impedance) may be getting affected by the value of some capacitor (with respect to it's opposing resistance term) is when that parameter changes by a factor of ten with decade change in frequency.

You did mention that you've tried changing some components but didn't specify which. Did you try increasing the values of C9, C8 and C11?
Just tried that, changed all three to 1000uf, no change in impedance.
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Old 25th October 2015, 03:49 PM   #6
Ketje is offline Ketje  Belgium
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And another thing, how do you suppose current source Q3 will work with only 0V6 volts from the b-e junction of Q1 ?
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Old 25th October 2015, 07:22 PM   #7
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No - you cannot expect useful simulation results invoking ac-analysis with nonlinear components in your circuit. Transient analysis is the only way!
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Old 25th October 2015, 10:34 PM   #8
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The current source feeding the ref-zener has a dropout voltage twice that of the BE-threshold of following npn - this cannot work properly.

Played about your simu, my version simulates 0,16mVpp of 1kHz ripple.
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Old 25th October 2015, 11:24 PM   #9
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Btw the load current source should not swing with 70mA amplitude, that is +-70mA when feeding 100mA=. I set this to 10mA amplitude and 50mA DC offset, thus current swings now between 40 and 60mA (20mApp).

Measuring 30uVpp of 1 kHz ripple gives an output impedance of 1,5mOhm at 1kHz.
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Old 26th October 2015, 03:06 AM   #10
Pafi is offline Pafi  Hungary
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Originally Posted by voltwide View Post
No - you cannot expect useful simulation results invoking ac-analysis with nonlinear components in your circuit. Transient analysis is the only way!
Well, you can, if you set a DC operating point. Not easy in case of a bridge rectifier though... So I would simply replace it by a voltage source.

And AC analysis is needed for checking stability + freq. dependancy.
My version:
Shunt sim.gif

1.6 uVpp, 140 mApp -> 0.011 mOhm @ 1 kHz. I like this game. :-)

Last edited by Pafi; 26th October 2015 at 03:10 AM.
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