I found the attached schematic on a site that is unfortunately now closed. Since I am unable to contact the designer, I was hoping someone here could help me with building the design.
1. Could someone please tell me if the JFETs in this buffer, or in the power supply, require selecting for a specific IDSS? If they do, roughly what IDSSs should I be selecting for?
2. At this time, I do not own a multimeter that can measure current. Is there still a way for me to measure IDSS? The only circuit I have been able to find to measure IDSS is on the Borbely website.
http://www.borbelyaudio.com/ae599bor.pdf
Thank you in advance.
1. Could someone please tell me if the JFETs in this buffer, or in the power supply, require selecting for a specific IDSS? If they do, roughly what IDSSs should I be selecting for?
2. At this time, I do not own a multimeter that can measure current. Is there still a way for me to measure IDSS? The only circuit I have been able to find to measure IDSS is on the Borbely website.
http://www.borbelyaudio.com/ae599bor.pdf
Thank you in advance.
Attachments
Interesting topology- and not that different than what I do with my tube-FET hybrid stuff. Yes, you'll want to match Idss and, if you can, Vp. One way to kill two birds with one stone is to just switch FETs in and out of this circuit and select for equal currents from both halves.
You can measure current by sticking a small value resistor in series with the drain and measuring the drop. Alternately, you can set the FET up as a current source on a breadboard for something near the current used in this circuit using a source resistor for adjustment, then switch FETs in and out, selecting for best current match (as determined by the voltage across the source resistor).
You can measure current by sticking a small value resistor in series with the drain and measuring the drop. Alternately, you can set the FET up as a current source on a breadboard for something near the current used in this circuit using a source resistor for adjustment, then switch FETs in and out, selecting for best current match (as determined by the voltage across the source resistor).
Hello -
To my eyes, the audio circuit is pretty straightforward. Basically you just have complementary source followers driving complementary emitter followers. So it is just a unity-gain buffer. The extra bipolar transistors act as current sources for the source followers.
If unmatched JFETs are used, there will be a DC offset, but that will be blocked by the coupling cap. Ideally, they will be matched, but another way to approach this problem would be to change R10 and R11 to trimpots. Then you could dial in the desired operating current for each FET.
The power supply is kind of wacky, in my opinion. He has a current source feeding a shunt regulator, which is a good approach. I don't really like zeners as they add a lot of noise. The other oddity is that the source followers are supplied through a pretty high resistor value, which is not the ideal approach.
Good luck,
Charles Hansen
To my eyes, the audio circuit is pretty straightforward. Basically you just have complementary source followers driving complementary emitter followers. So it is just a unity-gain buffer. The extra bipolar transistors act as current sources for the source followers.
If unmatched JFETs are used, there will be a DC offset, but that will be blocked by the coupling cap. Ideally, they will be matched, but another way to approach this problem would be to change R10 and R11 to trimpots. Then you could dial in the desired operating current for each FET.
The power supply is kind of wacky, in my opinion. He has a current source feeding a shunt regulator, which is a good approach. I don't really like zeners as they add a lot of noise. The other oddity is that the source followers are supplied through a pretty high resistor value, which is not the ideal approach.
Good luck,
Charles Hansen
For all of the circuit symmetry the design is very “unbalanced” in a engineering design sense:
Low noise fets with hot bias current of 15 mA, noise potential is wasted by 650 Ohm output resistance thermal noise (and some fets will not even have Idss of 15 mA)
1:1 current mirrors waste current and just copy supply noise rather than reduce it in the fet bias current sources
Complemetary fets are not in fact symmetrical enough for any reasonable distortion cancellation - real improvements would be cascoding fets and linearizing output
Bipolar output bias is a function of the sum of 6 different voltages, the 2 fet Vgs being the most variable by device, the output bias has no trim provision
With little clue to intended output stage bias and load it is possible that the bipolar transistors will dominate the distortion (or at least add odd harmonics) which would again seem to not fit philosophically with the rest
Low noise fets with hot bias current of 15 mA, noise potential is wasted by 650 Ohm output resistance thermal noise (and some fets will not even have Idss of 15 mA)
1:1 current mirrors waste current and just copy supply noise rather than reduce it in the fet bias current sources
Complemetary fets are not in fact symmetrical enough for any reasonable distortion cancellation - real improvements would be cascoding fets and linearizing output
Bipolar output bias is a function of the sum of 6 different voltages, the 2 fet Vgs being the most variable by device, the output bias has no trim provision
With little clue to intended output stage bias and load it is possible that the bipolar transistors will dominate the distortion (or at least add odd harmonics) which would again seem to not fit philosophically with the rest
J-fets vs bipolar
JCX- that's right.Greyhorse:I mean that better is do this circuit with bipolar transistors on positions of J-fets.If you will see on datasheets of high speed buffers(Burr-Brown,Harris etc.),you will get inspiration. Certainly is better "buildt" this circuit to the feetback loop of any operational amplifier-distortion will be much lower.
JCX- that's right.Greyhorse:I mean that better is do this circuit with bipolar transistors on positions of J-fets.If you will see on datasheets of high speed buffers(Burr-Brown,Harris etc.),you will get inspiration. Certainly is better "buildt" this circuit to the feetback loop of any operational amplifier-distortion will be much lower.
You should keep the jfets. The reason that you will not see them in IC's is that they can't make them complementary. Almost all discrete high end circuits use comp. jfets, rather than bipolar transistors. Matching is recommended, but it would probably work without close matching. The circuit is either idealized or compromised, depending on your position, by leaving out self leveling gate-source resistor in the current sources in the power supply.
actually regardless of the general desirability of jfets, you can safely toss them out in this circuit, J1,2 will always be forward biased (ie acting as diodes) for the power supply and bias resistor values shown, R18,19 will drop more than half the supply volts from the Q1,2 and Q3,4 current source current flowing through them with the given R1,2,10 and R3,4,11 values
This is annoying...
Greyhorse originally asked about whether the FETs needed to be matched in this circuit. Somehow the thread devolved instead into a critique of the circuit and JFETs. Many of the opinions expressed here are unfounded and without merit. But when jcx posts that "you can safely toss [the JFETs] out" because "they will be ... acting as diodes", that is just plain wrong and needs to be corrected.
There is nothing so annoying as an instant "internet expert" that spouts off meaningless nonsense. The original poster may or may not be able to distinguish the helpful advice from the useless BS. I've got better things to do than to try and monitor the dispensation of useless information. So my request is to keep from posting unless you *really* know what you're talking about.
Charles Hansen
Greyhorse originally asked about whether the FETs needed to be matched in this circuit. Somehow the thread devolved instead into a critique of the circuit and JFETs. Many of the opinions expressed here are unfounded and without merit. But when jcx posts that "you can safely toss [the JFETs] out" because "they will be ... acting as diodes", that is just plain wrong and needs to be corrected.
There is nothing so annoying as an instant "internet expert" that spouts off meaningless nonsense. The original poster may or may not be able to distinguish the helpful advice from the useless BS. I've got better things to do than to try and monitor the dispensation of useless information. So my request is to keep from posting unless you *really* know what you're talking about.
Charles Hansen
Re: I agree with Charlie........
Hey, I don't claim to be perfect like Charles here. And I don't go around like Charles trying to tell people not to post anything unless they "*really* know" what they're talking about.
The running down was already done by Charles in his reply to jcx.
I'm simply pointing out that those who talk about open-loop emitter followers shouldn't be going around impugning others for posting something which may be erroneous and trying to tell them not to post anything unless they *really* know what they're talking about.
As for building something, I did try to build me one of them open-loop emitter followers that Charles recommended only to find out that they don't exist.
se
Jocko Homo said:
Hey, I don't claim to be perfect like Charles here. And I don't go around like Charles trying to tell people not to post anything unless they "*really* know" what they're talking about.
The running down was already done by Charles in his reply to jcx.
I'm simply pointing out that those who talk about open-loop emitter followers shouldn't be going around impugning others for posting something which may be erroneous and trying to tell them not to post anything unless they *really* know what they're talking about.
As for building something, I did try to build me one of them open-loop emitter followers that Charles recommended only to find out that they don't exist.
se
Re: Re: I agree with Charlie........
Steve, for a guy with encyclopedic knowledge of everything from Cooper pairs to altered states of conciousness, you sure seem to have a hard time reading. Let me help you out with this one. For an example of an open-loop follower, please refer to the data sheet for the Burr-Brown BUF634:
http://focus.ti.com/lit/ds/symlink/buf634.pdf
The first sentence in the section "Application Information" reads as follows:
"Figure 1 is a simplified circuit diagram of the BUF634 showing its open-loop complementary follower design."
They even give the schematic, so if you *really* want to build an open-loop follower, you could do so. Or you could just buy one from DigiKey.
On the other hand if you wanted a closed-loop follower, please refer to the data sheet for a National LMH6559 for an example:
http://www.national.com/ds/LM/LMH6559.pdf
On the first page you will find the information, "The LMH6559 is a high-speed, closed-loop buffer...configured internally for a loop gain of one."
Now they don't offer a schematic, so if you want to build one of these but are having trouble figuring out how this thing works, just post on this forum and I'm sure that somebody with some knowledge like Jocko or John Curl (Fred unfortunately left in disgust) will be glad to help you out.
But all of this is really silly. In case you haven't realized it, you've "hijacked" this thread. The original poster had a specific question regarding the construction of a specific circuit. None of this is helpful or illuminating to him (or anyone else, for that matter).
For you to drag in one of your old arguments from a thread on a completely different forum (Audio Asylum) is out of line and uncalled for. This argumentative behavior that generates heat and no light is exactly why you have been banned from some forums. It is exactly why new rules have been created on other forums. It is exactly why so many people on this forum wish the moderators would ban you.
If you have something helpful to add to the original poster's question, please do so. Otherwise, I'm sure everyone would appreciate it if you refrained from posting altogether.
Charles Hansen
Steve Eddy said:
As for building something, I did try to build me one of them open-loop emitter followers that Charles recommended only to find out that they don't exist.
se
Steve, for a guy with encyclopedic knowledge of everything from Cooper pairs to altered states of conciousness, you sure seem to have a hard time reading. Let me help you out with this one. For an example of an open-loop follower, please refer to the data sheet for the Burr-Brown BUF634:
http://focus.ti.com/lit/ds/symlink/buf634.pdf
The first sentence in the section "Application Information" reads as follows:
"Figure 1 is a simplified circuit diagram of the BUF634 showing its open-loop complementary follower design."
They even give the schematic, so if you *really* want to build an open-loop follower, you could do so. Or you could just buy one from DigiKey.
On the other hand if you wanted a closed-loop follower, please refer to the data sheet for a National LMH6559 for an example:
http://www.national.com/ds/LM/LMH6559.pdf
On the first page you will find the information, "The LMH6559 is a high-speed, closed-loop buffer...configured internally for a loop gain of one."
Now they don't offer a schematic, so if you want to build one of these but are having trouble figuring out how this thing works, just post on this forum and I'm sure that somebody with some knowledge like Jocko or John Curl (Fred unfortunately left in disgust) will be glad to help you out.
But all of this is really silly. In case you haven't realized it, you've "hijacked" this thread. The original poster had a specific question regarding the construction of a specific circuit. None of this is helpful or illuminating to him (or anyone else, for that matter).
For you to drag in one of your old arguments from a thread on a completely different forum (Audio Asylum) is out of line and uncalled for. This argumentative behavior that generates heat and no light is exactly why you have been banned from some forums. It is exactly why new rules have been created on other forums. It is exactly why so many people on this forum wish the moderators would ban you.
If you have something helpful to add to the original poster's question, please do so. Otherwise, I'm sure everyone would appreciate it if you refrained from posting altogether.
Charles Hansen
Oh, I almost forgot...
That was the last time I will ever reply to one of your posts on the DIY Audio forum. Since all you like to do is argue, making any replies just drags the argument out. Nobody comes to these places to listen to arguments (except apparently you). So go ahead and argue away, all by yourself.
Charles Hansen
That was the last time I will ever reply to one of your posts on the DIY Audio forum. Since all you like to do is argue, making any replies just drags the argument out. Nobody comes to these places to listen to arguments (except apparently you). So go ahead and argue away, all by yourself.
Charles Hansen
Knowledge doesn't count..........
I got an Email from a very knowledgable
poster saying he was tired of arguing with people who knew nothing and were confrontational. He said he doesn't plan to post anymore and this a great loss as i have downloaded every circuit he posted.
Do I see a trend here?
I got an Email from a very knowledgable
poster saying he was tired of arguing with people who knew nothing and were confrontational. He said he doesn't plan to post anymore and this a great loss as i have downloaded every circuit he posted.
Do I see a trend here?
Will someone point out what I did wrong?
I calculated the +Vcc2 DC value with the 2.2K series R18, assuming J2_Id = Q4_Ic ~ 15 mA (guessing that HZ15 is a 15 V zener - but since the current is set by (|Vcc1|-Q3_Vbe)/(R11+R3), everything is nearly ratiometric)
when I try to put 15 mA through the 2.2K R18 I get 33V, which is greater than the total supply voltage so I assume something must be satrutated, like J2, with the excess current comming from the forward biased gate-source diode
I calculated the +Vcc2 DC value with the 2.2K series R18, assuming J2_Id = Q4_Ic ~ 15 mA (guessing that HZ15 is a 15 V zener - but since the current is set by (|Vcc1|-Q3_Vbe)/(R11+R3), everything is nearly ratiometric)
when I try to put 15 mA through the 2.2K R18 I get 33V, which is greater than the total supply voltage so I assume something must be satrutated, like J2, with the excess current comming from the forward biased gate-source diode
Sorry JCX....
I was overly harsh in my previous posting. Please accept my apologies for over reacting.
When I look at a circuit, I primarily look at the overall topology and tend to ignore the component values. The topology is (as I previously posted) a complementary source follower driving a complementary emitter-follower. There is nothing whatsoever wrong with this circuit topology.
If we use the posted values, we do indeed find some oddities. Following your assumption (which seems a reasonable one) that HZ15 is a 15 volt zener, then it does appear that there will be something close to 15 mA of current coming from the current sources.
This is too much for these particular FETs, which are better off running in the range of 2 mA to 6 mA or so. There is also the problem that you correctly pointed out the voltage drop across the 2.2 k decoupling resistors in the power supply.
You reached the conclusion that the FETs had a reverse bias from gate-to-source, were only acting as forward-biased diodes, and should be removed from the circuit. I think this is jumping to an unwarranted conclusion. Even if the circuit were built with the values shown it wouldn't reverse-bias the FETs. It would, however, have some other problems.
I would instead assume that a typo was made somewhere along the way, and that the 750 resistors in the current sources should really be 7500 ohms. Or that the 2.2 k resistors in the power supply should really be 220 ohms. It's clear to me that the circuit was drawn by someone who has a good understanding of things. He may have made a typo, or the originator of this thread may have made a typo, but let's not throw out the baby with the bathwater.
I also disagree with many of the assertions you made in your earlier post, but I don't think getting into an argument of your opinion versus my opinion of the circuit is going to help the original poster get his circuit built.
Best regards,
Charles Hansen
I was overly harsh in my previous posting. Please accept my apologies for over reacting.
When I look at a circuit, I primarily look at the overall topology and tend to ignore the component values. The topology is (as I previously posted) a complementary source follower driving a complementary emitter-follower. There is nothing whatsoever wrong with this circuit topology.
If we use the posted values, we do indeed find some oddities. Following your assumption (which seems a reasonable one) that HZ15 is a 15 volt zener, then it does appear that there will be something close to 15 mA of current coming from the current sources.
This is too much for these particular FETs, which are better off running in the range of 2 mA to 6 mA or so. There is also the problem that you correctly pointed out the voltage drop across the 2.2 k decoupling resistors in the power supply.
You reached the conclusion that the FETs had a reverse bias from gate-to-source, were only acting as forward-biased diodes, and should be removed from the circuit. I think this is jumping to an unwarranted conclusion. Even if the circuit were built with the values shown it wouldn't reverse-bias the FETs. It would, however, have some other problems.
I would instead assume that a typo was made somewhere along the way, and that the 750 resistors in the current sources should really be 7500 ohms. Or that the 2.2 k resistors in the power supply should really be 220 ohms. It's clear to me that the circuit was drawn by someone who has a good understanding of things. He may have made a typo, or the originator of this thread may have made a typo, but let's not throw out the baby with the bathwater.
I also disagree with many of the assertions you made in your earlier post, but I don't think getting into an argument of your opinion versus my opinion of the circuit is going to help the original poster get his circuit built.
Best regards,
Charles Hansen
Re: Knowledge doesn't count..........
Hi Fred,
It was not me who wrote you that e-mail but I could have been it as I agree 100 % with the sender.
I stopped posting at 999 posts as I had enough of the quarrels here but did resume posting as there was some confusion about my clock and ASR.
I get enough of commercial postings like "my clock is better than yours" and guys knowing it better all the time, posing as an expert.
I was under the impression this forum was for exchanging ideas and experiences NOT for proving who is "right" or has the "best" circuit.
In closing I am looking at my equipment and trying to sum up which ideas I got from this forum in exchange to mines...
In fact the result is very meager as I can only think of ultra-soft recovery diodes. An idea inspired by your high-speed diode thread.
Fred Dieckmann said:
Hi Fred,
It was not me who wrote you that e-mail but I could have been it as I agree 100 % with the sender.
I stopped posting at 999 posts as I had enough of the quarrels here but did resume posting as there was some confusion about my clock and ASR.
I get enough of commercial postings like "my clock is better than yours" and guys knowing it better all the time, posing as an expert.
I was under the impression this forum was for exchanging ideas and experiences NOT for proving who is "right" or has the "best" circuit.
In closing I am looking at my equipment and trying to sum up which ideas I got from this forum in exchange to mines...
In fact the result is very meager as I can only think of ultra-soft recovery diodes. An idea inspired by your high-speed diode thread.
Guys, I'd really appreciate it if you'd stick to the technical issues and cut out the sniping. The technical portions are interesting and well-argued. The sniping back-and-forth is silly and childish. Let me add that if you want to argue about something that you posted in another forum, that other forum would be a more appropriate place for it.
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