Hi, I am busy makeing one with the contacts taken from an IC socket, the type with the leaf contacts, I think it will do the mechanical part just fine. I'm still a little unsure of what I need to match though, so far I know how to measure the VGS... 😛
Well, the basic matching is Idss matching, witch stands for I_drain_source_saturation, and it's the maximum current (under given circumstances) that will flow through a FET when you open it. Ideally FETs are matched for linearity, where you match pairs that have the same current at different Vgs voltages. Maybe AndrewT can chime in on this one.
Ok, so I need to plot and record the current through the fet at a few diffirent Vgs points? Do I just pot an ammeter in series wit the fet or do I calculate based on voltage measurement?
you need to thermally couple the device under test (DUT) to the device used as a reference (REF). set up the pair so that both have the same Vgs, both have the same Vds, both are at the same Tc, and check if the current Id is the same in both. set aside DUT with marking of reading Id.
do next DUT, set aside.
eventually you have groups of DUTs that have same Idss and same Id at Vgs.
Now pick a group. remove the old REF and insert a new REF from the selected group. Go through the whole group comparing DUT to REF. You may find a matching pair. Set them aside. Select a new REF from the group. Insert a dut and compare go through the whole group to find another matching pair. Keep changing the REF and compare all the DUTs until you have exhausted that group.
Now move on to the next group.
Set aside a few days to do this.
do next DUT, set aside.
eventually you have groups of DUTs that have same Idss and same Id at Vgs.
Now pick a group. remove the old REF and insert a new REF from the selected group. Go through the whole group comparing DUT to REF. You may find a matching pair. Set them aside. Select a new REF from the group. Insert a dut and compare go through the whole group to find another matching pair. Keep changing the REF and compare all the DUTs until you have exhausted that group.
Now move on to the next group.
Set aside a few days to do this.
Thanks Andrew I'm sure once I get started I can figure out what you said, line by line.
The transformer just arrived!!! and I raided my old myref C for 10000uf caps to cobble together a psu of sorts 10000uF 470mR 10000uF, preceded by a filter, of 2k2R from ground to pole 1n5 cap from pole connected to 0R47 with its free end back to ground.
Duncan's PSU doesn't cater for the filter but shows about 160mV 100Hz ripple from the CRC into 16 ohm, with centre voltage of 15.09V
The transformer just arrived!!! and I raided my old myref C for 10000uf caps to cobble together a psu of sorts 10000uF 470mR 10000uF, preceded by a filter, of 2k2R from ground to pole 1n5 cap from pole connected to 0R47 with its free end back to ground.
Duncan's PSU doesn't cater for the filter but shows about 160mV 100Hz ripple from the CRC into 16 ohm, with centre voltage of 15.09V
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Nothing new to report, yesterday was deicated to LED lighting project.
I have some questions about the PSU capacitors.
I did a basic version of my PSU rails in Duncan PSU. It claims a 6.02A spike (I assume that is the turn on spike). Should the PSU bulk capacitors be rated for a ripple in excess of this, or am I confusing two things? I know the average ripple will be much less as caps are only topped off every cycle after the thing is at its operating point. It just struck me now, that I will runn two channels off the same transformer, so I should probably double the load (half the resistance), and that will mean even higher spike.
Second question; What would be an acceptable amount of Voltage ripple in a 15VDC class-a supply like this , in percentage or mV?
I have some questions about the PSU capacitors.
I did a basic version of my PSU rails in Duncan PSU. It claims a 6.02A spike (I assume that is the turn on spike). Should the PSU bulk capacitors be rated for a ripple in excess of this, or am I confusing two things? I know the average ripple will be much less as caps are only topped off every cycle after the thing is at its operating point. It just struck me now, that I will runn two channels off the same transformer, so I should probably double the load (half the resistance), and that will mean even higher spike.
Second question; What would be an acceptable amount of Voltage ripple in a 15VDC class-a supply like this , in percentage or mV?
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I think if you look at the PsudII report you can see the voltage ripple and the current ripple in each component.
In an rCRC supply, you can compare ripple current and rms current between the two types of C. Even when identical Cs are used, the operating conditions of those two caps (before and after the R) are very different.
Do a report for 500ms after a delay of 2000ms. That will show you the continuous conditions after start up has completed.
In an rCRC supply, you can compare ripple current and rms current between the two types of C. Even when identical Cs are used, the operating conditions of those two caps (before and after the R) are very different.
Do a report for 500ms after a delay of 2000ms. That will show you the continuous conditions after start up has completed.
Thanks Andrew, one more tiny question, does the amp only draw the amount of current for biasing or does it also need more current to pass the actual music signal?
I.e. do I need to look at higher current draws in simulation or only the 1A fixed bias ?
I.e. do I need to look at higher current draws in simulation or only the 1A fixed bias ?
This F..ing Forum software logged me out while typing up a detailed explanation of quiescent and bias and output currents. The Forum lost my post.
B...r me, if I'm re-typing it.
Hahah, that happens sometimes to all of us.
I was busy reading the JHL PHL paper thingy today and it mentioned current there fluctuates from 100% to about 130 to 150% of bias current
On simulating this I get a voltage of about 15.5V at idle and 14.4V at full power (two channels, one PSU). In a AB amp this would not bother me, but is this ok for class A?
I hooked up a simple filter as discussed before as well as two pairs of 10000uf 26m ohm caps in parallel through two bridges today just to confirm the transformer is working. With no load it shows a voltage just shy of 17VDC, which if I divide it by the sqaure root of 2, is 12.0066VAC or something.... It is supposed to be a 225VA 12VAC transformer. So things look fairly OK. Nothing went boom and nothing got hot, no buzzing etc....
Can I just hook my scope up to the PSU without any protection or special method? I'd like to see what the ripple looks like in real life.
I was busy reading the JHL PHL paper thingy today and it mentioned current there fluctuates from 100% to about 130 to 150% of bias current
On simulating this I get a voltage of about 15.5V at idle and 14.4V at full power (two channels, one PSU). In a AB amp this would not bother me, but is this ok for class A?
I hooked up a simple filter as discussed before as well as two pairs of 10000uf 26m ohm caps in parallel through two bridges today just to confirm the transformer is working. With no load it shows a voltage just shy of 17VDC, which if I divide it by the sqaure root of 2, is 12.0066VAC or something.... It is supposed to be a 225VA 12VAC transformer. So things look fairly OK. Nothing went boom and nothing got hot, no buzzing etc....
Can I just hook my scope up to the PSU without any protection or special method? I'd like to see what the ripple looks like in real life.
Hey Andrew,
With the current forum software the likelyhood of this can be reduced by consequently using the preview function. If you're logged out while typing, the preview attempt fails but the editor content is preserved and you can navigate back after logging back in (in order to attempt a preview again). Also, the preview windows will explicitly tell you so in case you got logged out in the meantime.
On top of that I made it a habit to select and copy the whole editor content before posting (just a quick and routined ⌘+A and ⌘+C or ^+A and ^+C, according to your OS).
With the current forum software the likelyhood of this can be reduced by consequently using the preview function. If you're logged out while typing, the preview attempt fails but the editor content is preserved and you can navigate back after logging back in (in order to attempt a preview again). Also, the preview windows will explicitly tell you so in case you got logged out in the meantime.
On top of that I made it a habit to select and copy the whole editor content before posting (just a quick and routined ⌘+A and ⌘+C or ^+A and ^+C, according to your OS).
The scope input is of very high impedance and capable of sustaining a couple hundred volts (look at the input connector warnings). Therefore it can be connected to the power supply and amplifier directly via it's probe tips.
But be careful when selecting your measurement function. Ground referencing while measuring something ground referenced can go wrong (i.e. the scope is connected to your mains PE safety, your power supply under test may or may not be). Safety can be maximized by supplying the scope off an isolating transformer.
Thanks sek, I got the mains earth hooked up to power ground. Will check it out as soon as I clear my desk. Will start with a small 5V supply first that I have lying around.
& for fun
That would make it ready for measuring up to mains voltage levels (and slightly beyond) and safety insulated. If it has a three prong mains plug this means it's internally insulated but still offers a (liftable) ground connection for measurement purposes. Never toggle this one without careful thinking, instead think about it's setting before you start to measure. Never use the scope on an ungrounded mains outlet, though.