Lets define for this post that a class B is a a NPN back-to-back with a PNP in push-pull with no diodes or VBE multiplers so that it will have the 1.2V of deadzone and xover distortion.
Since the transistors aren't biased on with a fixed voltage, and say I use feedback instead to correct the xover, will it have thermal runaway?
--
Danny
Since the transistors aren't biased on with a fixed voltage, and say I use feedback instead to correct the xover, will it have thermal runaway?
--
Danny
thermal run-away is a characterisics of biplor transistors and have no regard for topology. so the answer in this case is a "yes".
It is that with the cross-over section unbiased, you reduce the risk of thermal run away.
But if you run the transistors too hot (driving heavy load for example), you will get thermal run-away.
It is that with the cross-over section unbiased, you reduce the risk of thermal run away.
But if you run the transistors too hot (driving heavy load for example), you will get thermal run-away.
Re: Re: Will a true class B have thermal runaway?
what will the front stages do if they are driving an unbiased ops (class C) within a feedback loop? assume of course that all stages are perfect other than the cross-over region for the OPS.
if you apply a small voltage to the input that's within the cross-over at the OPS, there will be zero output. so the front stages will output more until and unless the ops is generating the right output.
This would suggest that feedback indeed corrected the cross-over distortion by working in a non-linear fashion.
Of course, how that works in real life with less-than-perfect front stages will be much more complicated.
sreten said:Just note the xover cannot be "corrected" by feedback,
sreten.
what will the front stages do if they are driving an unbiased ops (class C) within a feedback loop? assume of course that all stages are perfect other than the cross-over region for the OPS.
if you apply a small voltage to the input that's within the cross-over at the OPS, there will be zero output. so the front stages will output more until and unless the ops is generating the right output.
This would suggest that feedback indeed corrected the cross-over distortion by working in a non-linear fashion.
Of course, how that works in real life with less-than-perfect front stages will be much more complicated.
millwood said:
Wrong!...in this case you will have high dissipation !And not thermal run away.
If you stop to drive the heavy load the transistors will return
to near zero standing current...and will cool down!
Thermal run away is when the current in the transistor increase...the transistor become hoter ...as the transistor become hoter the current increase...and the process ends with the self destruction of the device.
Class C?
John, can you give a reference for the 120 deg. figure you mentioned? I have read an article in which an audio designer who was classifying all the different types of audio amplifiers (Class A, AB, B, D, G etc.) has referred to a an output stage with no bias, as Danny mentioned in the first post, as class C. They defined Class B as an output stage in which each device conducts for 180 deg, i.e. optimally biased. Using that definition, any output stage in which each device conducts for less than 180 deg. would be considered as Class C. From a quick glance at my bookself Duncan's, Self's and Slone's books would not contradict this definition, though none of them state it outright in these terms.
James
dhaen said:
John, can you give a reference for the 120 deg. figure you mentioned? I have read an article in which an audio designer who was classifying all the different types of audio amplifiers (Class A, AB, B, D, G etc.) has referred to a an output stage with no bias, as Danny mentioned in the first post, as class C. They defined Class B as an output stage in which each device conducts for 180 deg, i.e. optimally biased. Using that definition, any output stage in which each device conducts for less than 180 deg. would be considered as Class C. From a quick glance at my bookself Duncan's, Self's and Slone's books would not contradict this definition, though none of them state it outright in these terms.
James
oops...
I was getting mixed up between the optimum conduction angle for some RF class C circuitry and a real definition.
You (and Millwood) are of course right that class C is defined as "Less than 180 degrees conduction angle".
Aplogies to all concerned
Maybe I should have bid harder on 7V's auction
I was getting mixed up between the optimum conduction angle for some RF class C circuitry and a real definition.
You (and Millwood) are of course right that class C is defined as "Less than 180 degrees conduction angle".
Aplogies to all concerned
Maybe I should have bid harder on 7V's auction
Attachments
Tube_Dude said:
so you don't get thermal run-away if the amp cools down at idle?
[/B][/QUOTE]Thermal run away is when the current in the transistor increase...the transistor become hoter ...as the transistor become hoter the current increase...and the process ends with the self destruction of the device. [/B][/QUOTE]
yeah. that's my understanding as well and it has no mentioning of being conditional on cool idling.
and from it a class C amp can certainly enter into a thermal run-away.
I think you are talking about two different types of thermal
runaway.
I think I see millwoods point here. If the transistors have
insufficient cooling for the signal we feed them, they will not
cool down entirely during the idle half cycle, so their average
temperature will slowly rise and they will amplify more and
more making them run hotter and hotter. This may or may
not lead to a runaway, I guess. Since the amplification increases
feedback will try to counteract this. Beta droop in the transistors
will eventually start limiting the current. So, I suppose it depends
on how good the heatsinking is. However, I don't think this
phenomenon is covered by the term "thermal runaway" as it
is commonly used, although it certainly is a kind of thermal
runaway.
The usual meaning of thermal runaway, as I understand it, is
that if the transistor gets hotter for some reason, be it because
of the signal or the room temperature increasing or whatever,
the bias current will increase. More precisely, suppose we keep
the bias voltage fixed and heat the transistor somewhat. The
bias current will then increase slightly due to the positive temp.
coefficient of Ic vs. Vbe. This increased bias current will increase
the average temperature of the transistor, leading to yet
higher bias current, leading to yet higher current etc. etc.
This is why there usually is and should be a temperature
compensation circuit that senses the average temperature
and lowers the bias voltage accordingly when the temperature
rises. I cannot see how this type of thermal runaway could
occur in a class B amp, since the bias voltage is 0V.
runaway.
I think I see millwoods point here. If the transistors have
insufficient cooling for the signal we feed them, they will not
cool down entirely during the idle half cycle, so their average
temperature will slowly rise and they will amplify more and
more making them run hotter and hotter. This may or may
not lead to a runaway, I guess. Since the amplification increases
feedback will try to counteract this. Beta droop in the transistors
will eventually start limiting the current. So, I suppose it depends
on how good the heatsinking is. However, I don't think this
phenomenon is covered by the term "thermal runaway" as it
is commonly used, although it certainly is a kind of thermal
runaway.
The usual meaning of thermal runaway, as I understand it, is
that if the transistor gets hotter for some reason, be it because
of the signal or the room temperature increasing or whatever,
the bias current will increase. More precisely, suppose we keep
the bias voltage fixed and heat the transistor somewhat. The
bias current will then increase slightly due to the positive temp.
coefficient of Ic vs. Vbe. This increased bias current will increase
the average temperature of the transistor, leading to yet
higher bias current, leading to yet higher current etc. etc.
This is why there usually is and should be a temperature
compensation circuit that senses the average temperature
and lowers the bias voltage accordingly when the temperature
rises. I cannot see how this type of thermal runaway could
occur in a class B amp, since the bias voltage is 0V.
millwood said:
No because if you stop the suplying of current to the heavy load the transistors will cool down...and in a case of thermal run away even if you stop the suplying of current to the load the transistors will continue becoming hoter and self destruct(even if you have stoped the supliyng of current to the load!)...
I hope have been clear now...
For what it is worth, I agree with Tube_Dude.
A class C amplifier (or even a class B amplifier that is kept at class B for all temperatures, with bias voltage going down if temperature rises to compensate for lower Vbe) will not suffer from thermal runaway. This has nothing to do with loading or overloading the amplifier. Of course overloading may cause breakdown of the output transistors because of high temperature, but that high temperature was deliberately caused by the overloading itself (external cause).
Thermal runaway is a phenomenon where the dissipation and temperature increases due to the fact that the temperature just has been increased (internal cause). As an example, this might happen with bipolar class AB and class A amplifiers without proper temperature compensation. Here the bias current will increase when temperature increases and this higher bias current causes additional dissipation to increase the temperature even more. Thermal runaway is born.
We have to distinguish between load current and resulting dissipation that will not be the cause itself of thermal runaway, and the idle current (or bias current) that will flow allways and can cause thermal runaway when it is not kept within certain boundaries. Of course it can happen that thermal runaway is started by a big load current that pushes up the temperature with the result that the idle current increases (bad compensation). But then the compensation was already bad and still it is the continuously increasing idle current that causes the thermal runaway and not the (stable) load current.
In a class C amplifier, as described at the start of the thread, this will never happen because no temperature increase will move this output stage into a area where an idle current will flow and certainly not a big idle current.
Steven
A class C amplifier (or even a class B amplifier that is kept at class B for all temperatures, with bias voltage going down if temperature rises to compensate for lower Vbe) will not suffer from thermal runaway. This has nothing to do with loading or overloading the amplifier. Of course overloading may cause breakdown of the output transistors because of high temperature, but that high temperature was deliberately caused by the overloading itself (external cause).
Thermal runaway is a phenomenon where the dissipation and temperature increases due to the fact that the temperature just has been increased (internal cause). As an example, this might happen with bipolar class AB and class A amplifiers without proper temperature compensation. Here the bias current will increase when temperature increases and this higher bias current causes additional dissipation to increase the temperature even more. Thermal runaway is born.
We have to distinguish between load current and resulting dissipation that will not be the cause itself of thermal runaway, and the idle current (or bias current) that will flow allways and can cause thermal runaway when it is not kept within certain boundaries. Of course it can happen that thermal runaway is started by a big load current that pushes up the temperature with the result that the idle current increases (bad compensation). But then the compensation was already bad and still it is the continuously increasing idle current that causes the thermal runaway and not the (stable) load current.
In a class C amplifier, as described at the start of the thread, this will never happen because no temperature increase will move this output stage into a area where an idle current will flow and certainly not a big idle current.
Steven
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