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John Curl's Blowtorch preamplifier part II
John Curl's Blowtorch preamplifier part II
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Old 20th November 2017, 03:23 PM   #97621
jan.didden is offline jan.didden  Europe
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Originally Posted by keantoken View Post
Jan, the last paragraph in my post explains why Cds doesn't load the circuit as a whole, only the MOSFET. You get the same output gain until the MOSFET saturates enough to disrupt the bias system.
But looking from the point of view of quiescent bias current for the MOSFET, you need to account for ther 'load current' throuhgh Cds, right?

So, if I want to calculate the quiescent current (class A) that I need to swing the load cap through the full voltage range, I have to calculate for Cload + Cds + Cdg, right?

Jan
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Old 20th November 2017, 03:42 PM   #97622
scott wurcer is online now scott wurcer  United States
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Quote:
Originally Posted by jan.didden View Post
So, if I want to calculate the quiescent current (class A) that I need to swing the load cap through the full voltage range, I have to calculate for Cload + Cds + Cdg, right?

Jan
The current charging Cds subtracts from the drain current and Cds ends up getting charged by some of the pullup current (to the first order). I thought this was intended as an open loop circuit? I either case the DC bias needs to be enough to supply the max displacement current in Cds to remain well behaved.
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Old 20th November 2017, 06:13 PM   #97623
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by scott wurcer View Post
The current charging Cds subtracts from the drain current and Cds ends up getting charged by some of the pullup current (to the first order). I thought this was intended as an open loop circuit? I either case the DC bias needs to be enough to supply the max displacement current in Cds to remain well behaved.
That's what I thought. Interesting case when the load capacitance is largely caused by the intrinsic Cds and Cgs. I was thinking, well I just put two in parallel but obviously that has a very small advantage only in such cases, not worth the effort.

And it will be closed loop.

Jan
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Old 21st November 2017, 04:23 AM   #97624
RNMarsh is offline RNMarsh  United States
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John Curl's Blowtorch preamplifier part II
Quote:
Originally Posted by jan.didden View Post
That's what I thought. Interesting case when the load capacitance is largely caused by the intrinsic Cds and Cgs. I was thinking, well I just put two in parallel but obviously that has a very small advantage only in such cases, not worth the effort.

And it will be closed loop.

Jan
some depends on the cascode series gate resistor R11 value..... compare with zero Ohms and 1 meg. The degree of effect of the Cdg and Cgs and that R11 ---- assuming gate bias source Z is zero.


THx-RNMarsh

Last edited by RNMarsh; 21st November 2017 at 04:28 AM.
 
Old 21st November 2017, 05:16 PM   #97625
dimitri is offline dimitri  United States
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No, professional division (BSS, dbx, Lexicon, DigiTech).
I would like to stay corrected: Crown also Crown Audio files notice of first half of layoffs
Only four years ago: Crown Audio announced plans to expand its operations that will lead to 44 new jobs in Elkhart
 
Old 22nd November 2017, 02:38 AM   #97626
RNMarsh is offline RNMarsh  United States
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John Curl's Blowtorch preamplifier part II
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Originally Posted by dimitri View Post
I would like to stay corrected: Crown also Crown Audio files notice of first half of layoffs
Only four years ago: Crown Audio announced plans to expand its operations that will lead to 44 new jobs in Elkhart
It seems from experience that plans like expanding all go out the window when the parent company gets bought out. Especially affected are any duplications.

-RNM
 
Old 22nd November 2017, 02:18 PM   #97627
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by scott wurcer View Post
The current charging Cds subtracts from the drain current and Cds ends up getting charged by some of the pullup current (to the first order). I thought this was intended as an open loop circuit? I either case the DC bias needs to be enough to supply the max displacement current in Cds to remain well behaved.
OK, got further and accomodated the Cds & Cdg current. Circuit runs on the sim as expected.
But (there always is a but) I assume that that displacement current does not cause dissipation in the device. Is that correct? To calculate the dissipation I should account for the total device current but subtract the part that is due to the 'shunting' parasitic capacitance current to get at the current that needs to be multiplied by the device voltage (RMS fashion) to determine the dissipation. Yes?

Jan
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Old 22nd November 2017, 02:30 PM   #97628
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Originally Posted by jan.didden View Post
OK, got further and accomodated the Cds & Cdg current. Circuit runs on the sim as expected.
But (there always is a but) I assume that that displacement current does not cause dissipation in the device. Is that correct? To calculate the dissipation I should account for the total device current but subtract the part that is due to the 'shunting' parasitic capacitance current to get at the current that needs to be multiplied by the device voltage (RMS fashion) to determine the dissipation. Yes?

Jan
Jan, do you use Spice (LTspice) simulator. It should do all that if models are correct.
Damir
 
Old 22nd November 2017, 02:47 PM   #97629
scott wurcer is online now scott wurcer  United States
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Not necessarily, you want the true power in the FET which will depend on the phase angle of the net current and voltage. As Damir said this should be built into any simulator, it's fundamental.

Power in AC circuits
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Old 22nd November 2017, 02:58 PM   #97630
jan.didden is offline jan.didden  Europe
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Damir & Scott, beg to differ. Yes, LTspice gives me dissipation in any color I want, but the way I see this is as follows.

The current LTspice 'measures' is based on the mesh current flowing (in the case of my MOSFET) from D to S. But intrinsically to the device are the capacitances like Cds. So if I ask LTspice to .measure the dissipation as the product of device current and device voltage (RMS fashion) the Cds current is included, although it does not (I believe) contribute to heating.


Edit: do you mean that the phase shift of the device current to the device voltage, as result of Cds, is automagically taking care of this through the complex calculation?

Jan
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Last edited by jan.didden; 22nd November 2017 at 03:02 PM.
 

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