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Old 15th June 2018, 01:43 PM   #41
EUVL is offline EUVL  Europe
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What frequency do you need your gate stopper to work at ?


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Old 15th June 2018, 03:00 PM   #42
Mark Johnson is offline Mark Johnson  United States
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Discrete Headphone Amp.
Quote:
Originally Posted by EUVL View Post
What frequency do you need your gate stopper to work at ?
Anyone eager to find this out, can look at the "Before ferrite" photo (post #19) and the "After ferrite" photo (post #25). Use the risetime-bandwidth equivalency formula, to find the bandwidths of the two different circuits. The ferrite bead needs to have a cutoff frequency well above BW#19 but only slightly above BW#25.
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Old 17th June 2018, 08:10 PM   #43
Shrut is offline Shrut
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Default Gain Phase Plot

Here is the Gain Phase plot:
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File Type: jpg SHR_HP-G-P.jpg (138.8 KB, 126 views)
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Old 17th June 2018, 09:10 PM   #44
Mark Johnson is offline Mark Johnson  United States
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Discrete Headphone Amp.
Looks like a closed loop bandwidth of ~ 550 kHz and a gain-bandwidth product (off the right end) around 5.5 MHz. Higher than the GBW of numerous IC opamps. Congratulations!
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Old 18th June 2018, 05:24 AM   #45
wrenchone is offline wrenchone  United States
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Shrut's recent plot shows the gain-phase with ferrite beads still in place on the output mosfets. It turns out that not all beads are created equal (Orwell, anyone?), and I by chance appear to have picked beads appropriate for the job (honest, a strip of beads-on-leads were sitting on my desk after I saved them from the e-waste bin, and I checked them out on the 4194A for inductance and slotted them in on a whim). They weren't even the ferrite formulation I suspected, but I'll expound on that later. In short, there are 3 ferrite formulations commonly used for beads - only one appears suitable, and it's not the most common variety, either, though still readily available from on-line distributors.
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Old 18th June 2018, 10:02 AM   #46
EUVL is offline EUVL  Europe
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Someone finally got the message.


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Old 18th June 2018, 06:04 PM   #47
wrenchone is offline wrenchone  United States
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For the last three square wave responses shown, the changes were due to chopping down the values of the two compensation caps. As the bandwidth of the amp increased, I wanted to be sure I wouldn't be running into the pole formed by the output stopper resistors, and the gate + Miller capacitance, hence a smallish bead. An even smaller bead might be appropriate, or a tiny lossy toroid with 2-3 turns of wire. The fastest response was obtained with 12pF for C4, and 680 pF for C2. I still want to see the G-P plot using a more capable instrument than the Venable, as I have seen amps with lovely square wave response display squirreley behavior at or beyond gain crossover . A favorite trick is for the amp response to cross zero, then climb back above zero at a higher frequency. I also want to make sure I'm crossing zero with the proper slope.

Last edited by wrenchone; 18th June 2018 at 06:19 PM.
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Old 2nd July 2018, 06:03 PM   #48
wrenchone is offline wrenchone  United States
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Attached are inductance vs. frequency plots for two ferrite beads -on-leads made by Fair-Rite corporation (a US vendor readily available from distributors) of identical size, different composition. The Fair-Rite type 43 mix (or a similar knock-off from another vendor) is the most commonly encountered. This is a Ni-Zn ferrite mix with a wider hysteresis loop and much higher electrical resistivity than the Mn-Zn ferrites used for power duty. The type 61 mix is also Ni-Zn, has much lower permeability than 43, and is used at higher frequencies. Both ferrite mixes display substantial resistive character in the upper ranges of their respective operating frequencies, useful for EMI suppression.

The actual beads shown in the plots are Fair-Rite 2743001112 ans 2761001112, with ferrite beads 3.5mm dia by 4.5 mm long, a convenient size to plonk down in place of s resistor for gate stopper duty. The type 43 bead clearly has too much inductance for the job at hand. The 61 bead is just right. One could also use a much smaller 43 mix bead to get the same inductance as the 61 material. One possibility is Fair-Rite 2643000501, a teensy bead that is 1.95mm dia by 1.5mm long. Two pieces slid on to a piece of bus wire will get the job done. Characteristics for the two beads-on-leads were measured using an HP4194A impedance/gain-phase analyzer with a 16047 test fixture to minimize lead length.

Here are the data sheets for the two beads showing impedance characteristics and behavior with DC bias.

Beads-on-Leads (2743001112) - Fair Rite

Beads-on-Leads (2761001112) - Fair Rite
Attached Images
File Type: gif 43_bead.GIF (35.9 KB, 42 views)
File Type: gif 61_bead.GIF (34.8 KB, 40 views)

Last edited by wrenchone; 2nd July 2018 at 06:10 PM. Reason: more info
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