Mr.Pass writes that to get better performance out of a mosfet we can give more voltage and more current to it. This will result in more dissipation in a mosfet.
What really makes the operation linear and low distortion? Is it the high current or high voltage?
I dont have any simulator, could someone with simulator simulate this ?
1. Differential with mosfet (9610) working at +/-60V at 20mA CCS
2. The same differential as above working at +-20V at 60mA CCS
Both examples will dissipate the comparable heat, but one in high voltage, and one in high current.
Which gives more linear operation?
Which gives less distortion?
Which gives better audible performance?
What really makes the operation linear and low distortion? Is it the high current or high voltage?
I dont have any simulator, could someone with simulator simulate this ?
1. Differential with mosfet (9610) working at +/-60V at 20mA CCS
2. The same differential as above working at +-20V at 60mA CCS
Both examples will dissipate the comparable heat, but one in high voltage, and one in high current.
Which gives more linear operation?
Which gives less distortion?
Which gives better audible performance?
Off hand, I am able to answer part of the question. Just my two cents: Increasing both will result in more linearity, but on a different scale.
If the voltage increases, the gate voltage of the mosfets increases. This results in a higher Vgs (in DC) since the source voltage stays the same because of the constant drain current. As a result, the transconductance gm increases, and does so linearly with Vgs.
If Ids increases, the transconductance also increases, but in the power of 0.5, i.e. g_m = (constant) * (Ids)^0.5.
When g_m increases, the overall gain of the amplifier increases. This gives us more linearity from the overall -tive feedback. (The overall negative feedback works better when the open-loop gain is high).
Please note that I did not talk about the linearity of the mosfets themselves when Ids increases. But generally, they tend to be more linear when Ids increases (in the sense that Vgs and Id curve is more straight).
-Sean
If the voltage increases, the gate voltage of the mosfets increases. This results in a higher Vgs (in DC) since the source voltage stays the same because of the constant drain current. As a result, the transconductance gm increases, and does so linearly with Vgs.
If Ids increases, the transconductance also increases, but in the power of 0.5, i.e. g_m = (constant) * (Ids)^0.5.
When g_m increases, the overall gain of the amplifier increases. This gives us more linearity from the overall -tive feedback. (The overall negative feedback works better when the open-loop gain is high).
Please note that I did not talk about the linearity of the mosfets themselves when Ids increases. But generally, they tend to be more linear when Ids increases (in the sense that Vgs and Id curve is more straight).
-Sean
What is the temp's role in this?
Another question I'd really like to know more about is what does the working temperature have to say in all this.
The reason for asking this is Nelsson's saying: Run'em hot.
Now is this because the increased voltage/current gives better linearity, or is the temperature in itself also a contributing reason.
The reason for asking is that I'm in the starting process of building several AlephX' now. And I'm doing some considerations on how efficient cooling system to make (see my thread about it here ).
I'm able to make the transistors really cool if I want (by phase-change or water), but do I really want that? If one is able to control the temperature in a higher degree that normal, what temperature do I really want to aim for? Lower temps means the devices are able to handle more energy.
I know you can't think the same way as in computers where lower temps are better, no matter what temp.
Anyone with experiences on the sound on the same amps with different temps? I also know about the warm up time, and that the amp sounds better after warming up, but is it the increased temp that is the reason for this?
Anders
Another question I'd really like to know more about is what does the working temperature have to say in all this.
The reason for asking this is Nelsson's saying: Run'em hot.
Now is this because the increased voltage/current gives better linearity, or is the temperature in itself also a contributing reason.
The reason for asking is that I'm in the starting process of building several AlephX' now. And I'm doing some considerations on how efficient cooling system to make (see my thread about it here ).
I'm able to make the transistors really cool if I want (by phase-change or water), but do I really want that? If one is able to control the temperature in a higher degree that normal, what temperature do I really want to aim for? Lower temps means the devices are able to handle more energy.
I know you can't think the same way as in computers where lower temps are better, no matter what temp.
Anyone with experiences on the sound on the same amps with different temps? I also know about the warm up time, and that the amp sounds better after warming up, but is it the increased temp that is the reason for this?
Anders
Hello Anders,
As I remember, Nelson suggested running the amp so that the temperature of the heatsink (transistor) is about 50deg Celcius. I read it in another thread that Nelson wrote (don't ask me the exact link... I forgot). He also mentioned that Passlabs products were aimed at that temperature.
Correct me if I'm wrong....
As I remember, Nelson suggested running the amp so that the temperature of the heatsink (transistor) is about 50deg Celcius. I read it in another thread that Nelson wrote (don't ask me the exact link... I forgot). He also mentioned that Passlabs products were aimed at that temperature.
Correct me if I'm wrong....
SeanPool said:
Sean, while the rest of your comments are correct, I would have
to disagree with this part. Higher Vds does not typically result
in higher Vgs at the same current, rather the opposite is usually
true.
Higher Vds gives greater linearity, particularly if you are starting
out at low voltages, but you get decreasing returns above, say,
5 volts or so. Then the heat builds up.
Higher Ids similarly - when you look at the curves, you see that
the linearity quickly improves as you increase from 0, but at some
(usually high) current, improvement ceases.
What makes a Mosfet Linear? They are not all that linear, as
is true of the rest of the gain devices.
lumanauw said:
It's the current... but that's not the end of it.
FETS (and BJTs) have 3-regions of operation. cut-off, triode, saturation. For an amplifier, you need the FET to be in the saturation region. However, looking at the Vds/Ids graph, you'll see that it's smoothly curved. The operating region you want to be in is where the curve is 'flat' (saturation). This corresponds to a higher Ids. If you set your operating point so that you'll stay nicely in the saturation region for your input conditions, then it won't matter too much what current you choose.
Since FETS (and BJTs) aren't predictable when they are created, these curves can be slightly different for each FET. So, to be safer and assured that you're within the Saturation region, you'll want to run them with more current.
Lastly, gM the transconductance (or gain) of the FET changes with the current going through it. At higher currents, the gain is more constant, this leads to less distortion.
Refer to fig 1. and fig 6.
http://www.irf.com/product-info/datasheets/data/irf9610.pdf
Sianturi, apa kabar? Banyak dari Tanah Air yang ikut di website ini?
Mosfets comes in different sizes, TO220, TO247. For small signal stages (Differential and VAS), what is the general rule or indication up to how big standing current will give improvement? Will the body temperature gives a good indication (like said 50-55deg?). If this body temperature gives good indication that a certain mosfet is in the optimal working point, wouldn't it be that the current be different in case of we put heatsink or not to the mosfet?
The question rises after reading the article Balanced Zen Line Stage by Nelson Pass. In this article, Mr.Pass gives measurement result of the cct running in +/-30V, 40V, 60V.
This cct is working with R only, so if we rise the voltage, it automaticly will rise the standing current.
If the final answer is =ONLY HIGH CURRENT that gives improvement, wouldn't it be that we could make a mosfet works in relatively smaller voltage rails, but be forced by CCS to whatever current we want, to get max performance of this mosfet?
But if the final answer still includes factor of HIGH VOLTAGE to get good performance out of a mosfet, forcing the current by CCS wouldn't help so much. We just have to stick in high voltage rail, without nothing can be changed.
Why is this question rises? Making high voltage is more difficult and more expensive than making high current. Especially in the transformer and Elko's. Most of the Elkos I can find here, the good ones, only up to 50V. More than that like 63V rating or 80V rating or 100V rating are hard to find, especially if we look for good ratings (computer grades).
Mosfets comes in different sizes, TO220, TO247. For small signal stages (Differential and VAS), what is the general rule or indication up to how big standing current will give improvement? Will the body temperature gives a good indication (like said 50-55deg?). If this body temperature gives good indication that a certain mosfet is in the optimal working point, wouldn't it be that the current be different in case of we put heatsink or not to the mosfet?
The question rises after reading the article Balanced Zen Line Stage by Nelson Pass. In this article, Mr.Pass gives measurement result of the cct running in +/-30V, 40V, 60V.
This cct is working with R only, so if we rise the voltage, it automaticly will rise the standing current.
If the final answer is =ONLY HIGH CURRENT that gives improvement, wouldn't it be that we could make a mosfet works in relatively smaller voltage rails, but be forced by CCS to whatever current we want, to get max performance of this mosfet?
But if the final answer still includes factor of HIGH VOLTAGE to get good performance out of a mosfet, forcing the current by CCS wouldn't help so much. We just have to stick in high voltage rail, without nothing can be changed.
Why is this question rises? Making high voltage is more difficult and more expensive than making high current. Especially in the transformer and Elko's. Most of the Elkos I can find here, the good ones, only up to 50V. More than that like 63V rating or 80V rating or 100V rating are hard to find, especially if we look for good ratings (computer grades).
- I guess this is what I don't quite understand (although irrelavant to the quote above): Let's say we have a CS, with the source connected to ground, and the gate gets the bias voltage from a voltage divider (R1&R2), which connects from the rail to the ground, tapped to the gate. In this setting, the gate will see the voltage dropped across R2. Since gate current is zero, this voltage drop may not be 4 volts. Actually, it may be a lot higher/lower. But you usually say that Vgs of a mosfet is 4 volts. This quite confuses me.
One explanation that comes into my mind is that the Vgs=4V is really because of the DC feedback from the Rs?
- This explains why different amps are sold at different prices: we are simply trying to make the most out of it. Actually, this is why Passlabs, Krell, and the likes, and this forum exist.
lumanauw
I forgot to mention: I remember from a thread (don't know exactly where) in which NP talked about how to improve the 30: Run the 30 with the output stage of the 60, with cranked up bias. Now the 30 has about 25 V rail, while the 60 has 34 V. So, *I think* he suggested that lower rail voltage with higher idle current should give you a better performance. (since the voltage is dropped to 24 V, you can run them at a higher current per mosfet for the same dissipated watts) -- please correct me if I'm wrong.
In addition to greater overall transconductance (means more linear in some frequency band, in general), what he suggested will result in a lower output impedance of the amp as well ---> you get more control over the load.
The catch: need to sacrifice the power as clippings will occur at lower power.
-Sean
SeanPool said:
In your scenario you'll have one of two things happen:
a) If the max current "allowed" to flow through the FET is larger than the CCS is providing, then the full current will go through the FET. Even though the FET would for the same Vgs allow more current to flow.
b) If the max current "allowed" to flow through the FET is less than the CCS is providing, then only the allowed current will flow, the rest will flow through another path or not at all. It is possible for a CCS to source less current than it was designed for if the loading doesn't allow for it.
I suppose that is one thing. There are some BJTs and FETs out there that have more "linear" saturation regions. Basically that comes down to larger "early voltages" or the MOSFET equivelent of it. While these are more expensive, sometimes 3 or 4x more, it doesn't really account for the major difference in a lot of the prices.
I think that a lot more of it has to do with a well designed, maybe proprietary, topology and tighter tolerances in the other components as well. Well matched parts is a hands-on adventure that you're probably paying for too.
Re: Re: What makes a mosfet linear?
It's not just the 'flatness' of the curved, but also the distance between the Vds/Ids curve for each Vgs value that makes the mosfet linear. If the mosfet has uniform distance, then the linearity will be good. In term of uniformity of the distance between each curve, BJT is better.
Hallo lumanauw....
Saya perhatikan ada beberapa dari tanah air yang ikutan forum ini. Tapi tidak terlalu banyak kayaknya.
I saw your website, it seems that you are really an audio addict. I went to college in Bandung, so maybe we could exchange some info on diyaudio parts source.... Jaya plaza is my favorite place though.
Cheers,
azira said:
It's not just the 'flatness' of the curved, but also the distance between the Vds/Ids curve for each Vgs value that makes the mosfet linear. If the mosfet has uniform distance, then the linearity will be good. In term of uniformity of the distance between each curve, BJT is better.
lumanauw said:
Hallo lumanauw....
Saya perhatikan ada beberapa dari tanah air yang ikutan forum ini. Tapi tidak terlalu banyak kayaknya.
I saw your website, it seems that you are really an audio addict. I went to college in Bandung, so maybe we could exchange some info on diyaudio parts source....
Jaya plaza is my favorite place though. Cheers,
Transfer curves
If you look at textbook(my 20 year old EE textbooks) transfer curves for FETs they do not look like useful devices at all for large signal amplification. Obviously the curves have flattened out somewhat with better more modern devices. Is using a lot of devices in parallel the strategy for keeping each device within its linear zone at maximum excursion? Also I think mindless cranking up of the bias make NO sense. Bias serves the exacting function of putting the FET in the middle of the most linear portion of its specific transfer curve ...doesn't it?
If you look at textbook(my 20 year old EE textbooks) transfer curves for FETs they do not look like useful devices at all for large signal amplification. Obviously the curves have flattened out somewhat with better more modern devices. Is using a lot of devices in parallel the strategy for keeping each device within its linear zone at maximum excursion? Also I think mindless cranking up of the bias make NO sense. Bias serves the exacting function of putting the FET in the middle of the most linear portion of its specific transfer curve ...doesn't it?
Re: Re: Re: What makes a mosfet linear?
Yes, this is true, but the "distance" you are talking about isn't exactly a parameter of the MOSFET. It is a function of the MOSFET "early voltage". If the early voltage is too low, then the saturation regions will be 1) not very flat and 2) have larger seperation with higher Vds for given Vgs's. Good audio MOSFETS and BJTs should have high VA.
You mean saturation zone The linear/triode zone is where a MOSFET acts like an on-off switch. I tend to agree that biasing up without reason makes no sense. Biasing serves to set a center point about which the amp is operating. But keep in mind that nearly all the Pass DIY amps are SE class-A or PP Class-A. In this case, you need the bias current to be 2x your peak transfer current. So for these amps, you want as high a current as possible for maximum power and so that you have headroom at your listening levels.
Lots of devices in parallel helps lower output resistance, drive more current, share the load between several devices so they aren't as strained.
on a side note, my Mom is Thai, been there several times, I love the country, wish I could speak more of the language though.
sianturi said:
Yes, this is true, but the "distance" you are talking about isn't exactly a parameter of the MOSFET. It is a function of the MOSFET "early voltage". If the early voltage is too low, then the saturation regions will be 1) not very flat and 2) have larger seperation with higher Vds for given Vgs's. Good audio MOSFETS and BJTs should have high VA.
You mean saturation zone
The linear/triode zone is where a MOSFET acts like an on-off switch. I tend to agree that biasing up without reason makes no sense. Biasing serves to set a center point about which the amp is operating. But keep in mind that nearly all the Pass DIY amps are SE class-A or PP Class-A. In this case, you need the bias current to be 2x your peak transfer current. So for these amps, you want as high a current as possible for maximum power and so that you have headroom at your listening levels. Lots of devices in parallel helps lower output resistance, drive more current, share the load between several devices so they aren't as strained.
on a side note, my Mom is Thai, been there several times, I love the country, wish I could speak more of the language though.
A couple of things: The Vgs is a characteristic of the Mosfet
itself and if you have more than a couple volts Drain-Source,
Vgs is primarily a function of the current, not the voltage, in
fact Vgs is the control voltage for current. At low Drain-Source
voltages, say below 2 volts, this starts falling apart, so I always
assume at least 2 volts loss in linear uses.
There is a place for "mindless cranking up the bias" in that the
distortion continues to decline in a device like a Mosfet, since
its transconductance increases and distortion continues to drop
up to a rather high limit. As the signal current becomes a smaller
percentage of the bias, the performance is seen to improve.
Of course, at some point this does becomes ridiculous.
itself and if you have more than a couple volts Drain-Source,
Vgs is primarily a function of the current, not the voltage, in
fact Vgs is the control voltage for current. At low Drain-Source
voltages, say below 2 volts, this starts falling apart, so I always
assume at least 2 volts loss in linear uses.
There is a place for "mindless cranking up the bias" in that the
distortion continues to decline in a device like a Mosfet, since
its transconductance increases and distortion continues to drop
up to a rather high limit. As the signal current becomes a smaller
percentage of the bias, the performance is seen to improve.
Of course, at some point this does becomes ridiculous.
I read this as a strenthtening words on "more current" is the main factor in linear operation in mosfets. Higher Transconductance is the answer to why it is better to have higher standing current for mosfets.
But to dig deeper to the question,
Some designer in bipolar using cascode for gain transistors. The cascode is very low, in my opinion, like 2 of the cascoded bipolar transistor are distanced by a red LED. Wouln't it be that the working transistor is only have less than 1.5V for its Vce?
Is this implementable in mosfets? The Vgs of a mosfet is roughly 4 volts. What happens if we operate a mosfet in quite low voltage, like 5 or 6volt Vds (like if we use Cascode), but still running quite high standing current (forced by CCS) through it? Will it still shows lower distortion and linear operation? Or mosfets cannot be operated in too low Vds like bipolars in cascode?
I have obtained good results cascoding Mosfets and also
using them as cascode devices.
They like more voltage than Bipolars, and this is one of the
reasons cascoding a Mosfet works so well. I have cascoded
power Mosfets to as low as 2 volts D-S, but I wouldn't go
lower than that. The choice of Bipolar versus Mosfet as the
cascoding device does not seem to be critical at all.
Cascoding will be part of the Zen Variations conclusion, but I
can tell you at this time that one of the advantages you will
enjoy is that the distortion remains far more constant with
varying frequencies and source impedances.
Also, check out my patent #5,343,166. This is a unique
use of cascoding which I have yet to incorporate in a product.
(Saving something for my old age)
using them as cascode devices.
They like more voltage than Bipolars, and this is one of the
reasons cascoding a Mosfet works so well. I have cascoded
power Mosfets to as low as 2 volts D-S, but I wouldn't go
lower than that. The choice of Bipolar versus Mosfet as the
cascoding device does not seem to be critical at all.
Cascoding will be part of the Zen Variations conclusion, but I
can tell you at this time that one of the advantages you will
enjoy is that the distortion remains far more constant with
varying frequencies and source impedances.
Also, check out my patent #5,343,166. This is a unique
use of cascoding which I have yet to incorporate in a product.
(Saving something for my old age)
Mr. Pass,
Many say that the more importing part of building an audio amplifier is the total design. Cannot judge by part per part or transistor per transistor. For me this is half true. Total design and good PCB routing is indeed very important, but if one do not understand what happens inside 1 single transistor, what makes it operates good or bad, how come he can design a good audio amplifier?
From your writings I concluded 2 important things about transistors. I thank you, because this cannot be found in my audio textbooks or any other book that I have. First the distortions are generated by fluctuating current and fluctuating voltage. The remedy for fluctuating current is to make the standing current as big as possible, so the percentage of signal to the standing current is small (lead to class A)
The remedy for fluctuating voltage is to cascode, making the transistor works in steady small voltage.
For mosfets it is better to have bigger standing current, because it will lead to more transconductance. But I dont know about bipolars. What is the effect of big standing current to bipolars?
I'm a bit surprised that you said you cascode a mosfet about 2 volts. The Vgs is already about 4 volts, how come a mosfet can be cascoded to as low as this? Will it still works properly? My first imagination about mosfet cascoding is about 6 or 7volt, but never tought about lower than Vgs. Is cascoding mosfets to 2 or 3 volts is can made good operation too?
What is the guidance to determine the cascoding voltage for mosfets and bipolars?
Is this means we can cascode a bipolar lower than 0.6V?
The conclusion of this can be seen in your patent #4,107,619 or can be seen in the cascode.pdf. I understand this as I see the schematic and read the explenation.
But reading more about your patent is not easy. Very difficult to understand. Like the patent #5,343,166 you mentioned. After reading it many-many times, I come to conclusion that this is a cascode configuration, with 2 supplies (like class G?) Smaller supply feds the output transistor with small voltage, big current, and main supply feds the cascode transistor. This way we can get the output transistor working in class A, but the major heat dissipation in cascode transistor can still be small.
Is my understanding about patent #5,343,166 correct?
But still I'm not clear about these methods. Cascoding may provide a better transistor operation. But once you writes that to built a good power amp, the circuit must employ as minimal component as possible. Even adding 1 transistor is advoided, if it is not necessary. You even only employ 2 stages in aleph amps.
So, cascoding is good. But minimal component is also good. What is the compromise of these 2? Or they are for different field, like minimal component is for class A/single ended + mosfets, and cascode is for class B or AB + bipolars?
Many say that the more importing part of building an audio amplifier is the total design. Cannot judge by part per part or transistor per transistor. For me this is half true. Total design and good PCB routing is indeed very important, but if one do not understand what happens inside 1 single transistor, what makes it operates good or bad, how come he can design a good audio amplifier?
From your writings I concluded 2 important things about transistors. I thank you, because this cannot be found in my audio textbooks or any other book that I have. First the distortions are generated by fluctuating current and fluctuating voltage. The remedy for fluctuating current is to make the standing current as big as possible, so the percentage of signal to the standing current is small (lead to class A)
The remedy for fluctuating voltage is to cascode, making the transistor works in steady small voltage.
For mosfets it is better to have bigger standing current, because it will lead to more transconductance. But I dont know about bipolars. What is the effect of big standing current to bipolars?
I'm a bit surprised that you said you cascode a mosfet about 2 volts. The Vgs is already about 4 volts, how come a mosfet can be cascoded to as low as this? Will it still works properly? My first imagination about mosfet cascoding is about 6 or 7volt, but never tought about lower than Vgs. Is cascoding mosfets to 2 or 3 volts is can made good operation too?
What is the guidance to determine the cascoding voltage for mosfets and bipolars?
Is this means we can cascode a bipolar lower than 0.6V?
The conclusion of this can be seen in your patent #4,107,619 or can be seen in the cascode.pdf. I understand this as I see the schematic and read the explenation.
But reading more about your patent is not easy. Very difficult to understand. Like the patent #5,343,166 you mentioned. After reading it many-many times, I come to conclusion that this is a cascode configuration, with 2 supplies (like class G?) Smaller supply feds the output transistor with small voltage, big current, and main supply feds the cascode transistor. This way we can get the output transistor working in class A, but the major heat dissipation in cascode transistor can still be small.
Is my understanding about patent #5,343,166 correct?
But still I'm not clear about these methods. Cascoding may provide a better transistor operation. But once you writes that to built a good power amp, the circuit must employ as minimal component as possible. Even adding 1 transistor is advoided, if it is not necessary. You even only employ 2 stages in aleph amps.
So, cascoding is good. But minimal component is also good. What is the compromise of these 2? Or they are for different field, like minimal component is for class A/single ended + mosfets, and cascode is for class B or AB + bipolars?
Hi lumanauw!
In any non-linear device, making the signal current small compared to the standing current, is the most simple and most effectice counter-measure in fighting THD. If you double the standing current, K2 will be reduced to 50%, K3 to 25%, K4 to 12% and K5 to 6% of the base values. Provided, you don't enter a less favorite point of operation of your device. And provided, that you can cope thermically with the dissipated power.
While the absolute values differ, in both MOSFETs and BJTs there is a "small" and a "large" voltage drop. In the case of BJTs Vbe is the large and Vce is the small voltage drop.
So, yes, operating a BJT as with Vce smaller than Vbe is perfectly sensibly for some few applications, but in contrast, operating MOSFETs with Vds smaller than Vgs is fairly normal.
You can have an interesting reading on the WWW when looking for linear MOS IC design given as exercise to students, where the solutions found are typically extensively commented.
E.g. googling for "wide swing cascode" will tell a lot of stories, about how to avoid losing Vgs from your available output swing.
Regards,
Peter Jacobi
lumanauw said:
In any non-linear device, making the signal current small compared to the standing current, is the most simple and most effectice counter-measure in fighting THD. If you double the standing current, K2 will be reduced to 50%, K3 to 25%, K4 to 12% and K5 to 6% of the base values. Provided, you don't enter a less favorite point of operation of your device. And provided, that you can cope thermically with the dissipated power.
lumanauw said:
While the absolute values differ, in both MOSFETs and BJTs there is a "small" and a "large" voltage drop. In the case of BJTs Vbe is the large and Vce is the small voltage drop.
So, yes, operating a BJT as with Vce smaller than Vbe is perfectly sensibly for some few applications, but in contrast, operating MOSFETs with Vds smaller than Vgs is fairly normal.
You can have an interesting reading on the WWW when looking for linear MOS IC design given as exercise to students, where the solutions found are typically extensively commented.
E.g. googling for "wide swing cascode" will tell a lot of stories, about how to avoid losing Vgs from your available output swing.
Regards,
Peter Jacobi
Pjacobi, thanks for the explenation. Some things are not clear for me.
Many encourage to use mosfets in hot mode, as we know the reason why. But I seldom read that bipolars are tobe treated the same way too. This is my opinion, (and big chance that it is wrong). While mosfets don't suck current into their gate, bipolars have HFE figures. The more standing current will need bigger current to be fed to its base. I think this will result in bipolars will be less sensitive to a certain signal source, if we have big standing current passing that bipolar. Because the signal source have to fed more current to the transistors base. I have done experiment with bipolar audio power amp. If I raise the standing current in its differential, the sound tends tobe harsh, not enjoyable. The optimum standing current for bipolar differential for me is about 2.5-4mA. Smaller than that will blur the reproduction, while bigger than that will give harsh/not enjoyable reproduction.
While giving big standing current will reduce the distortion, in the same time it will give less sensitive operation for the bipolar. Maybe this distortion reduction is not as valuable as the transistor itself loses sensitivity. Maybe this is why we tend to use bipolar in cold mode, while mosfets in hot mode.
Again, I could be wrong.
About transistor cascode, is it really Vce and Vbe are independent to each other? From what I learn at school, transistor are made by silicon semiconductor.
Silicon atoms have 4 electrons on its outer skin. This skin is saturated with 8 electrons. That is why pure silicon (glass) will be an isolator, because each silicon atom have perfect junction with its friends, to make 8 perfect electrons in its outer skin.
To make a semiconductor material, we impurify this silicon with doping process. If we impurify with material with 5 electrons on its outer skin, it will produce a N semiconductor. If we dope this silicon with material with 3 electrons (or 1 hole) in its outer skin, it will produce P semiconductor. The doping itself is about 1:10million.
Bipolar transistor are made by 3 parts (like layer cake) N,P,N or P,N,P. This is the point that I dont understand. How could Vce and Vbe are independent to each other in case of cascoding?By its drawing, it should be not-independent to each other. Is the cake drawing I learn at school is not perfect?
Mosfets are made different way (In my school tutorial). It consist of 1 tubular semiconductor material, but have a small belt on its center. This belt produces field that controls how many current can pass that tubular semiconductor.
Based on my school tutorial, mosfets could have independent Vds and Vgs, due to its structure. But how come bipolars also have independent Vce and Vbe, looking at its cake configuration?
About optimal operating point
We are in the cascode discussion. What graph of the transistor datasheet should be analyzed and how can we pick this "good" operating point? All this time I only look into SOA without knowing how to pick this optimum operating point.
This discussion will really change me to look at transistors. Mr. NP has tell that he cascode mosfets to only 2volts or so. From this statement, I learn that it is the current that matters. But how low can we drop the Vds or Vce while still wanting very linear operation with bipolars or mosfets?
What I really want to know is how to operate mosfets and bipolars in small voltage cascode with big currents? How to choose the optimal operation point for different mosfets and bipolars, by reading the datasheet graph?
Many encourage to use mosfets in hot mode, as we know the reason why. But I seldom read that bipolars are tobe treated the same way too. This is my opinion, (and big chance that it is wrong). While mosfets don't suck current into their gate, bipolars have HFE figures. The more standing current will need bigger current to be fed to its base. I think this will result in bipolars will be less sensitive to a certain signal source, if we have big standing current passing that bipolar. Because the signal source have to fed more current to the transistors base. I have done experiment with bipolar audio power amp. If I raise the standing current in its differential, the sound tends tobe harsh, not enjoyable. The optimum standing current for bipolar differential for me is about 2.5-4mA. Smaller than that will blur the reproduction, while bigger than that will give harsh/not enjoyable reproduction.
While giving big standing current will reduce the distortion, in the same time it will give less sensitive operation for the bipolar. Maybe this distortion reduction is not as valuable as the transistor itself loses sensitivity. Maybe this is why we tend to use bipolar in cold mode, while mosfets in hot mode.
Again, I could be wrong.
About transistor cascode, is it really Vce and Vbe are independent to each other? From what I learn at school, transistor are made by silicon semiconductor.
Silicon atoms have 4 electrons on its outer skin. This skin is saturated with 8 electrons. That is why pure silicon (glass) will be an isolator, because each silicon atom have perfect junction with its friends, to make 8 perfect electrons in its outer skin.
To make a semiconductor material, we impurify this silicon with doping process. If we impurify with material with 5 electrons on its outer skin, it will produce a N semiconductor. If we dope this silicon with material with 3 electrons (or 1 hole) in its outer skin, it will produce P semiconductor. The doping itself is about 1:10million.
Bipolar transistor are made by 3 parts (like layer cake) N,P,N or P,N,P. This is the point that I dont understand. How could Vce and Vbe are independent to each other in case of cascoding?By its drawing, it should be not-independent to each other. Is the cake drawing I learn at school is not perfect?
Mosfets are made different way (In my school tutorial). It consist of 1 tubular semiconductor material, but have a small belt on its center. This belt produces field that controls how many current can pass that tubular semiconductor.
Based on my school tutorial, mosfets could have independent Vds and Vgs, due to its structure. But how come bipolars also have independent Vce and Vbe, looking at its cake configuration?
About optimal operating point
We are in the cascode discussion. What graph of the transistor datasheet should be analyzed and how can we pick this "good" operating point? All this time I only look into SOA without knowing how to pick this optimum operating point.
This discussion will really change me to look at transistors. Mr. NP has tell that he cascode mosfets to only 2volts or so. From this statement, I learn that it is the current that matters. But how low can we drop the Vds or Vce while still wanting very linear operation with bipolars or mosfets?
What I really want to know is how to operate mosfets and bipolars in small voltage cascode with big currents? How to choose the optimal operation point for different mosfets and bipolars, by reading the datasheet graph?
Only a short answer this time, when nobody gives the long version, I'll try at the weekend.
a) because of the base current, it is difficult to change the idle current of a BJT in isolation. Most likely the preceding stages must be adjusted, too.
b) The minimal Vbe is always given by semiconductor physics to be something around 0.65V (for silicon). For the minimal Vce no such limit exists, and weighting between several design goals, it can be made quite small, down to, say 10mV. See this Philips application note:
http://www.semiconductors.philips.com/acrobat/applicationnotes/AN10116_2.pdf
As to what cascode voltage would give good overall results, you can either
- listen
- measure
- simulate using advanced (Mextram, VBIC) BJT models
Regards,
Peter Jacobi
a) because of the base current, it is difficult to change the idle current of a BJT in isolation. Most likely the preceding stages must be adjusted, too.
b) The minimal Vbe is always given by semiconductor physics to be something around 0.65V (for silicon). For the minimal Vce no such limit exists, and weighting between several design goals, it can be made quite small, down to, say 10mV. See this Philips application note:
http://www.semiconductors.philips.com/acrobat/applicationnotes/AN10116_2.pdf
As to what cascode voltage would give good overall results, you can either
- listen
- measure
- simulate using advanced (Mextram, VBIC) BJT models
Regards,
Peter Jacobi
lumanauw said:
Sorry, I can't let that stand uncorrected.
Pure silicon isn't glass and it isn't an isolator, but a semiconductor.
There is always some concentration np of positive and some concentration nn of negative charge carriers free by spontanous dissociation of bonds.
np * nn = constant (but significantly temperature dependent)
By doping, either np or nn is increased and conductivity raises.
Sorry for sounding mentorish, but nothing beats reading a good book about this, if you want understand all the gory details.
Regards,
Peter Jacobi
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