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Old 5th May 2012, 02:58 PM   #1391
flg is offline flg  United States
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Originally Posted by buzzforb View Post
Are you saying that Rs value is related to rail voltage. Could you explain further? I thought it was mainly for stability against current hogging and linearization.
buzzforb, I beleive you are mostly correct. The Rs is based on the Vgs and the parts own R. With current and temp the linearity and bias point will change. The Rs has a portion of the total V gate-gnd (in Common Source) and bias through it and the voltage drop across it. The Rs has more stability and it's own temp coefficient. When these changes in temp and current do happen to the FET, the resistor imparts stability to the circuit. The bigger the resistor, the more it swamps the FETs ability to shift the circuit parametors non liearly. But, the more it reduces gain also. I have seen where designers of some type of circuits just choose an R that drops about 20-30% of Vgs. We would proabaly be around 15% with our typical Rs's?
Patrick has chosen a FET with roughly 1/2 the Vgs of the original F5 deices. Bias current etc. being roughly equal would support an Rs being roughly 1/2 the value Increasing bias current would support the use of even lower Rs for the same effects. Just my 2c
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Old 5th May 2012, 03:02 PM   #1392
AndrewT is offline AndrewT  Scotland
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My reading of the Cordell interviews says otherwise.

Vds and Rs are critical to Thermal Stability.
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Old 5th May 2012, 03:11 PM   #1393
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Tea,
What numbers did you get?
Andrew,
How do you suggest we arrive at this ratio?
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Old 5th May 2012, 03:21 PM   #1394
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Tea,
What numbers did you get?
What numbers am I looking for?
I hope to spend more time getting mini V3 version with mini-Diode heatsinks, and the V2 version with direct attached diodes to heatsinks in comparison. Going to use some low hz sine wave loops from old test disk to see what results. That's the plan anyway. The wife suddenly wants to change all that.
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Old 5th May 2012, 03:24 PM   #1395
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Many of N.P.s articles discuss Rs size directly related to bias through the device. Useing more devices with less bias he will suggest maybe a 1 ohm rather than a .47. More than an amp bias he might suggest less than a .47 ohm Rs. I'm not saying there is no Vds effect better/worse regarding Rs big/small. But, Rs significantly effects other parameters of the circuit with only a minute change in Vds?
Regarding DC/Thermal stability, when a gate voltage refrenced to ground is applied to the FET, the FETs tempco becomes a significant variable in stability. A large source R can overcome the significant Vgs shift with temp vs. the applied gate to ground voltage and improve stability.
I beleive Patrick has mentioned that the Fuku Rs we are using in the F5X have a TempCo that may be the opposite of the FET offering some automated control assistance???
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Old 5th May 2012, 03:40 PM   #1396
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Speaking of automated control assistance, adding a low value power resistor in series with the diode. (a relic pot smoker would dream of high current silistors)
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Old 5th May 2012, 03:45 PM   #1397
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Originally Posted by flg View Post
Many of N.P.s articles discuss Rs size directly related to bias through the device. Useing more devices with less bias he will suggest maybe a 1 ohm rather than a .47. More than an amp bias he might suggest less than a .47 ohm Rs. I'm not saying there is no Vds effect better/worse regarding Rs big/small. But, Rs significantly effects other parameters of the circuit with only a minute change in Vds?
Regarding DC/Thermal stability, when a gate voltage refrenced to ground is applied to the FET, the FETs tempco becomes a significant variable in stability. A large source R can overcome the significant Vgs shift with temp vs. the applied gate to ground voltage and improve stability.
I beleive Patrick has mentioned that the Fuku Rs we are using in the F5X have a TempCo that may be the opposite of the FET offering some automated control assistance???
This is the theory i was going on. The introduction of the diode creates another level of complexity as it it adds its on dynamic characterstics.


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Speaking of automated control assistance, adding a low value power resistor in series with the diode. (a relic pot smoker would dream of high current silistors)
This is either a hint or an older joke than my internal reference material, although some of the statement was relevant, personally.
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Old 5th May 2012, 03:58 PM   #1398
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Here is how I am going to test them. I amusing small sinks intentionally, so that i can make sure the heat is high. One will have diodes mounted to sink, the other freestanding. I will drive parallel 4 ohm speakers to give it a nice tough load. If it gets too hot, I'll put them on larger sinks.
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Old 5th May 2012, 06:31 PM   #1399
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Great job buzzforb, I'm looking forward to the results.
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Old 5th May 2012, 07:05 PM   #1400
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...........I do not know if the F5 would work well with Rs=0r3.

That's why I put out a call for advice on what to do with the F5T biasing.

I am still waiting. Where have all the experts gone?
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........How do you suggest we arrive at this ratio?
That was a genuine question. I am asking.
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