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Why are BJTs common in output stages of power amplifiers?
I don't quite understand this FET-BJT preamp circuitOscillator Driven LED - Not OscillatingOscillation won't pass voltage bufferCan Emitter Follower BJTs work with large signal?BJT Capacitance at RF limmitationsWhy is my amplifiers frequency response unexpected?Why does the emitter follower clip?What is meant by “input/output impedence” of a transistor stage in amplifiers?Biasing high frequency BJT cascode amplifierEmitter follower and input impedance
.everyoneloves__top-leaderboard:empty,.everyoneloves__mid-leaderboard:empty,.everyoneloves__bot-mid-leaderboard:empty margin-bottom:0;
$begingroup$
From my understanding, the role of the output stage is to decrease the output impedance to almost 0. For that, MOSFETs seem better suited since they've got way lower $R_ds$.
Yet I see quite often BJTs as buffer in discrete design, often in a Darlington configuration to increase input impedance, while only one MOSFETs would have a high enough input impedance.
My thoughts were that it was either cheaper or simpler. Power BJTs are indeed a bit cheaper than power MOSFETs, and it seems to me that it is simpler to make a relatively linear buffer with a BJT emitter follower, while a MOSFET source follower may require some feedback.
mosfet bjt buffer
asked May 13 at 14:29
Jonas DaverioJonas Daverio
18119
$endgroup$
add a comment |
$begingroup$
From my understanding, the role of the output stage is to decrease the output impedance to almost 0. For that, MOSFETs seem better suited since they've got way lower $R_ds$.
Yet I see quite often BJTs as buffer in discrete design, often in a Darlington configuration to increase input impedance, while only one MOSFETs would have a high enough input impedance.
My thoughts were that it was either cheaper or simpler. Power BJTs are indeed a bit cheaper than power MOSFETs, and it seems to me that it is simpler to make a relatively linear buffer with a BJT emitter follower, while a MOSFET source follower may require some feedback.
mosfet bjt buffer
asked May 13 at 14:29
Jonas DaverioJonas Daverio
18119
$endgroup$
$begingroup$
I think the knee voltage is sharper and lower for BJT than mosfet, and that is the main reason why.
$endgroup$
– Harry Svensson
May 13 at 14:38
2
$begingroup$
The problem is linear DC biasing complementary MOSFETs in the linear zone with cross-conduction and the R*Ciss effects. BJT's are much more predictable for Vbe than MOSFETs for Vgs ( 1A)
$endgroup$
– Sunnyskyguy EE75
May 13 at 14:40
$begingroup$
However we're not stopping you from trying to make it linear with dual negative feedback loops to reduce shoothru losses and eliminate quadratic VI effects
$endgroup$
– Sunnyskyguy EE75
May 13 at 15:00
$begingroup$
BJT designs are definitely cheaper overall. I wouldn't say they are so common because they sound superior to other designs.
$endgroup$
– Todd Wilcox
May 14 at 6:18
add a comment |
$begingroup$
From my understanding, the role of the output stage is to decrease the output impedance to almost 0. For that, MOSFETs seem better suited since they've got way lower $R_ds$.
Yet I see quite often BJTs as buffer in discrete design, often in a Darlington configuration to increase input impedance, while only one MOSFETs would have a high enough input impedance.
My thoughts were that it was either cheaper or simpler. Power BJTs are indeed a bit cheaper than power MOSFETs, and it seems to me that it is simpler to make a relatively linear buffer with a BJT emitter follower, while a MOSFET source follower may require some feedback.
mosfet bjt buffer
asked May 13 at 14:29
Jonas DaverioJonas Daverio
18119
$endgroup$
From my understanding, the role of the output stage is to decrease the output impedance to almost 0. For that, MOSFETs seem better suited since they've got way lower $R_ds$.
Yet I see quite often BJTs as buffer in discrete design, often in a Darlington configuration to increase input impedance, while only one MOSFETs would have a high enough input impedance.
My thoughts were that it was either cheaper or simpler. Power BJTs are indeed a bit cheaper than power MOSFETs, and it seems to me that it is simpler to make a relatively linear buffer with a BJT emitter follower, while a MOSFET source follower may require some feedback.
mosfet bjt buffer
mosfet bjt buffer
asked May 13 at 14:29
Jonas DaverioJonas Daverio
18119
asked May 13 at 14:29
Jonas DaverioJonas Daverio
18119
asked May 13 at 14:29
Jonas DaverioJonas Daverio
18119
asked May 13 at 14:29
Jonas DaverioJonas Daverio
18119
asked May 13 at 14:29
Jonas DaverioJonas Daverio
18119
18119
$begingroup$
I think the knee voltage is sharper and lower for BJT than mosfet, and that is the main reason why.
$endgroup$
– Harry Svensson
May 13 at 14:38
2
$begingroup$
The problem is linear DC biasing complementary MOSFETs in the linear zone with cross-conduction and the R*Ciss effects. BJT's are much more predictable for Vbe than MOSFETs for Vgs ( 1A)
$endgroup$
– Sunnyskyguy EE75
May 13 at 14:40
$begingroup$
However we're not stopping you from trying to make it linear with dual negative feedback loops to reduce shoothru losses and eliminate quadratic VI effects
$endgroup$
– Sunnyskyguy EE75
May 13 at 15:00
$begingroup$
BJT designs are definitely cheaper overall. I wouldn't say they are so common because they sound superior to other designs.
$endgroup$
– Todd Wilcox
May 14 at 6:18
add a comment |
$begingroup$
I think the knee voltage is sharper and lower for BJT than mosfet, and that is the main reason why.
$endgroup$
– Harry Svensson
May 13 at 14:38
2
$begingroup$
The problem is linear DC biasing complementary MOSFETs in the linear zone with cross-conduction and the R*Ciss effects. BJT's are much more predictable for Vbe than MOSFETs for Vgs ( 1A)
$endgroup$
– Sunnyskyguy EE75
May 13 at 14:40
$begingroup$
However we're not stopping you from trying to make it linear with dual negative feedback loops to reduce shoothru losses and eliminate quadratic VI effects
$endgroup$
– Sunnyskyguy EE75
May 13 at 15:00
$begingroup$
BJT designs are definitely cheaper overall. I wouldn't say they are so common because they sound superior to other designs.
$endgroup$
– Todd Wilcox
May 14 at 6:18
$begingroup$
I think the knee voltage is sharper and lower for BJT than mosfet, and that is the main reason why.
$endgroup$
– Harry Svensson
May 13 at 14:38
$begingroup$
I think the knee voltage is sharper and lower for BJT than mosfet, and that is the main reason why.
$endgroup$
– Harry Svensson
May 13 at 14:38
2
2
$begingroup$
The problem is linear DC biasing complementary MOSFETs in the linear zone with cross-conduction and the R*Ciss effects. BJT's are much more predictable for Vbe than MOSFETs for Vgs ( 1A)
$endgroup$
– Sunnyskyguy EE75
May 13 at 14:40
$begingroup$
The problem is linear DC biasing complementary MOSFETs in the linear zone with cross-conduction and the R*Ciss effects. BJT's are much more predictable for Vbe than MOSFETs for Vgs ( 1A)
$endgroup$
– Sunnyskyguy EE75
May 13 at 14:40
$begingroup$
However we're not stopping you from trying to make it linear with dual negative feedback loops to reduce shoothru losses and eliminate quadratic VI effects
$endgroup$
– Sunnyskyguy EE75
May 13 at 15:00
$begingroup$
However we're not stopping you from trying to make it linear with dual negative feedback loops to reduce shoothru losses and eliminate quadratic VI effects
$endgroup$
– Sunnyskyguy EE75
May 13 at 15:00
$begingroup$
BJT designs are definitely cheaper overall. I wouldn't say they are so common because they sound superior to other designs.
$endgroup$
– Todd Wilcox
May 14 at 6:18
$begingroup$
BJT designs are definitely cheaper overall. I wouldn't say they are so common because they sound superior to other designs.
$endgroup$
– Todd Wilcox
May 14 at 6:18
add a comment |
3 Answers
3
active
oldest
votes
$begingroup$
To make an Audio Voltage source, you want the crossover voltage distortion to be null which requires some quiescent DC current > 1% of the max current. This modest distortion and output impedance is reduced further by negative feedback or excess open loop gain. The active diode bias DC voltage can be predicted in mV for the differential Darlington output stage.
However for MOSFETs the conduction threshold can vary 50% e.g. 1 to 2V or 2 to 4V, so the biasing for cross conduction to eliminate crossover distortion is not easily done with low voltage gain linear power amps.
answered May 13 at 15:21
Sunnyskyguy EE75Sunnyskyguy EE75
73.9k228104
$endgroup$
add a comment |
$begingroup$
MOSFETS used to be more common in power amplifiers, but they were often the lateral type power MOSFETS.
Most modern MOSFETs (Vertical MOSFETs / HEXFETs) are highly optimized for switching and require very careful design in a linear amp design. For example these modern switching types have a large nonlinear gate capacitance that is difficult to drive.
In addition the likes of HEXFETs can suffer from localized heating effects that can cause thermal runaway in a linear application.
A good description of these issues can be found here
Lateral MOSFETs are still available but are more quite expensive. See here
So really it's not a case that MOSFETs cannot be used, but it's often more difficult and less cost effective to achieve the same performance and reliability for a given price point.
answered May 13 at 15:35
TheMachinistTheMachinist
31613
$endgroup$
$begingroup$
Old school MOSFET designs definitely sound different from BJT designs. Some say they sound better, and I wouldn't argue with them, but it's all a matter of taste.
$endgroup$
– Todd Wilcox
May 14 at 6:16
add a comment |
$begingroup$
Second Breakdown
(Many) Audio amplifiers operate the output stage in their linear region.
Modern power MOSFETs are not designed to operate in the linear region. Many of them (HEXFETS) are composed of a grid of hundreds of thousands of smaller FET elements to increase power density and switching speed. Other switching-optimized MOSFET families have similar constructions, with large die areas and/or arrays of smaller elements.
For MOSFETs, the threshold voltage has a negative temperature coefficient. As a particular area of the die / FET element gets hotter, it's threshold voltage decreases and since the MOSFET is operating in it's linear region, that area conducts a larger portion of the current, so it gets even hotter. Before long, the localized heating on a tiny fraction of the die has resulted in a short circuit, often called "Second Breakdown".
But...
A relatively new type of amplifier, the "Class D" amplifier, works by switching the output stage transistors on and off rapidly, at a frequency much higher than the speaker is expected to reproduce. A low-pass filter filters out the high-frequency noise, and amplification is achieved through varying the duty cycle.
MOSFETs are extremely common in such designs, as class D amplifiers either have the output stage elements fully on or fully off. As power MOSFETs are optimized for that, that's what they are used for.
answered May 13 at 16:01
Thor LancasterThor Lancaster
33115
$endgroup$
$begingroup$
-1 because it sounds like you are talking about MOSFET thermal runaway (when it's not an actual issue with MOSFETs, but is an issue with BJTs). You word it in a strange way though which makes it sound like you are talking about something else, therefore clarification would be required to delineate between these two effects.
$endgroup$
– Toor
May 13 at 17:24
4
$begingroup$
@toor the tempco of FET threshold voltage is negative just like a BJT. Thermal runaway is still a concern with FETs in linear mode.
$endgroup$
– sstobbe
May 13 at 18:00
2
$begingroup$
@Toor I dunno, his story seems to check out...
$endgroup$
– marcelm
May 13 at 20:36
add a comment |
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3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
To make an Audio Voltage source, you want the crossover voltage distortion to be null which requires some quiescent DC current > 1% of the max current. This modest distortion and output impedance is reduced further by negative feedback or excess open loop gain. The active diode bias DC voltage can be predicted in mV for the differential Darlington output stage.
However for MOSFETs the conduction threshold can vary 50% e.g. 1 to 2V or 2 to 4V, so the biasing for cross conduction to eliminate crossover distortion is not easily done with low voltage gain linear power amps.
answered May 13 at 15:21
Sunnyskyguy EE75Sunnyskyguy EE75
73.9k228104
$endgroup$
add a comment |
$begingroup$
To make an Audio Voltage source, you want the crossover voltage distortion to be null which requires some quiescent DC current > 1% of the max current. This modest distortion and output impedance is reduced further by negative feedback or excess open loop gain. The active diode bias DC voltage can be predicted in mV for the differential Darlington output stage.
However for MOSFETs the conduction threshold can vary 50% e.g. 1 to 2V or 2 to 4V, so the biasing for cross conduction to eliminate crossover distortion is not easily done with low voltage gain linear power amps.
answered May 13 at 15:21
Sunnyskyguy EE75Sunnyskyguy EE75
73.9k228104
$endgroup$
add a comment |
$begingroup$
To make an Audio Voltage source, you want the crossover voltage distortion to be null which requires some quiescent DC current > 1% of the max current. This modest distortion and output impedance is reduced further by negative feedback or excess open loop gain. The active diode bias DC voltage can be predicted in mV for the differential Darlington output stage.
However for MOSFETs the conduction threshold can vary 50% e.g. 1 to 2V or 2 to 4V, so the biasing for cross conduction to eliminate crossover distortion is not easily done with low voltage gain linear power amps.
answered May 13 at 15:21
Sunnyskyguy EE75Sunnyskyguy EE75
73.9k228104
$endgroup$
To make an Audio Voltage source, you want the crossover voltage distortion to be null which requires some quiescent DC current > 1% of the max current. This modest distortion and output impedance is reduced further by negative feedback or excess open loop gain. The active diode bias DC voltage can be predicted in mV for the differential Darlington output stage.
However for MOSFETs the conduction threshold can vary 50% e.g. 1 to 2V or 2 to 4V, so the biasing for cross conduction to eliminate crossover distortion is not easily done with low voltage gain linear power amps.
answered May 13 at 15:21
Sunnyskyguy EE75Sunnyskyguy EE75
73.9k228104
answered May 13 at 15:21
Sunnyskyguy EE75Sunnyskyguy EE75
73.9k228104
answered May 13 at 15:21
Sunnyskyguy EE75Sunnyskyguy EE75
73.9k228104
answered May 13 at 15:21
Sunnyskyguy EE75Sunnyskyguy EE75
73.9k228104
73.9k228104
add a comment |
add a comment |
$begingroup$
MOSFETS used to be more common in power amplifiers, but they were often the lateral type power MOSFETS.
Most modern MOSFETs (Vertical MOSFETs / HEXFETs) are highly optimized for switching and require very careful design in a linear amp design. For example these modern switching types have a large nonlinear gate capacitance that is difficult to drive.
In addition the likes of HEXFETs can suffer from localized heating effects that can cause thermal runaway in a linear application.
A good description of these issues can be found here
Lateral MOSFETs are still available but are more quite expensive. See here
So really it's not a case that MOSFETs cannot be used, but it's often more difficult and less cost effective to achieve the same performance and reliability for a given price point.
answered May 13 at 15:35
TheMachinistTheMachinist
31613
$endgroup$
$begingroup$
Old school MOSFET designs definitely sound different from BJT designs. Some say they sound better, and I wouldn't argue with them, but it's all a matter of taste.
$endgroup$
– Todd Wilcox
May 14 at 6:16
add a comment |
$begingroup$
MOSFETS used to be more common in power amplifiers, but they were often the lateral type power MOSFETS.
Most modern MOSFETs (Vertical MOSFETs / HEXFETs) are highly optimized for switching and require very careful design in a linear amp design. For example these modern switching types have a large nonlinear gate capacitance that is difficult to drive.
In addition the likes of HEXFETs can suffer from localized heating effects that can cause thermal runaway in a linear application.
A good description of these issues can be found here
Lateral MOSFETs are still available but are more quite expensive. See here
So really it's not a case that MOSFETs cannot be used, but it's often more difficult and less cost effective to achieve the same performance and reliability for a given price point.
answered May 13 at 15:35
TheMachinistTheMachinist
31613
$endgroup$
$begingroup$
Old school MOSFET designs definitely sound different from BJT designs. Some say they sound better, and I wouldn't argue with them, but it's all a matter of taste.
$endgroup$
– Todd Wilcox
May 14 at 6:16
add a comment |
$begingroup$
MOSFETS used to be more common in power amplifiers, but they were often the lateral type power MOSFETS.
Most modern MOSFETs (Vertical MOSFETs / HEXFETs) are highly optimized for switching and require very careful design in a linear amp design. For example these modern switching types have a large nonlinear gate capacitance that is difficult to drive.
In addition the likes of HEXFETs can suffer from localized heating effects that can cause thermal runaway in a linear application.
A good description of these issues can be found here
Lateral MOSFETs are still available but are more quite expensive. See here
So really it's not a case that MOSFETs cannot be used, but it's often more difficult and less cost effective to achieve the same performance and reliability for a given price point.
answered May 13 at 15:35
TheMachinistTheMachinist
31613
$endgroup$
MOSFETS used to be more common in power amplifiers, but they were often the lateral type power MOSFETS.
Most modern MOSFETs (Vertical MOSFETs / HEXFETs) are highly optimized for switching and require very careful design in a linear amp design. For example these modern switching types have a large nonlinear gate capacitance that is difficult to drive.
In addition the likes of HEXFETs can suffer from localized heating effects that can cause thermal runaway in a linear application.
A good description of these issues can be found here
Lateral MOSFETs are still available but are more quite expensive. See here
So really it's not a case that MOSFETs cannot be used, but it's often more difficult and less cost effective to achieve the same performance and reliability for a given price point.
answered May 13 at 15:35
TheMachinistTheMachinist
31613
answered May 13 at 15:35
TheMachinistTheMachinist
31613
answered May 13 at 15:35
TheMachinistTheMachinist
31613
answered May 13 at 15:35
TheMachinistTheMachinist
31613
31613
$begingroup$
Old school MOSFET designs definitely sound different from BJT designs. Some say they sound better, and I wouldn't argue with them, but it's all a matter of taste.
$endgroup$
– Todd Wilcox
May 14 at 6:16
add a comment |
$begingroup$
Old school MOSFET designs definitely sound different from BJT designs. Some say they sound better, and I wouldn't argue with them, but it's all a matter of taste.
$endgroup$
– Todd Wilcox
May 14 at 6:16
$begingroup$
Old school MOSFET designs definitely sound different from BJT designs. Some say they sound better, and I wouldn't argue with them, but it's all a matter of taste.
$endgroup$
– Todd Wilcox
May 14 at 6:16
$begingroup$
Old school MOSFET designs definitely sound different from BJT designs. Some say they sound better, and I wouldn't argue with them, but it's all a matter of taste.
$endgroup$
– Todd Wilcox
May 14 at 6:16
add a comment |
$begingroup$
Second Breakdown
(Many) Audio amplifiers operate the output stage in their linear region.
Modern power MOSFETs are not designed to operate in the linear region. Many of them (HEXFETS) are composed of a grid of hundreds of thousands of smaller FET elements to increase power density and switching speed. Other switching-optimized MOSFET families have similar constructions, with large die areas and/or arrays of smaller elements.
For MOSFETs, the threshold voltage has a negative temperature coefficient. As a particular area of the die / FET element gets hotter, it's threshold voltage decreases and since the MOSFET is operating in it's linear region, that area conducts a larger portion of the current, so it gets even hotter. Before long, the localized heating on a tiny fraction of the die has resulted in a short circuit, often called "Second Breakdown".
But...
A relatively new type of amplifier, the "Class D" amplifier, works by switching the output stage transistors on and off rapidly, at a frequency much higher than the speaker is expected to reproduce. A low-pass filter filters out the high-frequency noise, and amplification is achieved through varying the duty cycle.
MOSFETs are extremely common in such designs, as class D amplifiers either have the output stage elements fully on or fully off. As power MOSFETs are optimized for that, that's what they are used for.
answered May 13 at 16:01
Thor LancasterThor Lancaster
33115
$endgroup$
$begingroup$
-1 because it sounds like you are talking about MOSFET thermal runaway (when it's not an actual issue with MOSFETs, but is an issue with BJTs). You word it in a strange way though which makes it sound like you are talking about something else, therefore clarification would be required to delineate between these two effects.
$endgroup$
– Toor
May 13 at 17:24
4
$begingroup$
@toor the tempco of FET threshold voltage is negative just like a BJT. Thermal runaway is still a concern with FETs in linear mode.
$endgroup$
– sstobbe
May 13 at 18:00
2
$begingroup$
@Toor I dunno, his story seems to check out...
$endgroup$
– marcelm
May 13 at 20:36
add a comment |
$begingroup$
Second Breakdown
(Many) Audio amplifiers operate the output stage in their linear region.
Modern power MOSFETs are not designed to operate in the linear region. Many of them (HEXFETS) are composed of a grid of hundreds of thousands of smaller FET elements to increase power density and switching speed. Other switching-optimized MOSFET families have similar constructions, with large die areas and/or arrays of smaller elements.
For MOSFETs, the threshold voltage has a negative temperature coefficient. As a particular area of the die / FET element gets hotter, it's threshold voltage decreases and since the MOSFET is operating in it's linear region, that area conducts a larger portion of the current, so it gets even hotter. Before long, the localized heating on a tiny fraction of the die has resulted in a short circuit, often called "Second Breakdown".
But...
A relatively new type of amplifier, the "Class D" amplifier, works by switching the output stage transistors on and off rapidly, at a frequency much higher than the speaker is expected to reproduce. A low-pass filter filters out the high-frequency noise, and amplification is achieved through varying the duty cycle.
MOSFETs are extremely common in such designs, as class D amplifiers either have the output stage elements fully on or fully off. As power MOSFETs are optimized for that, that's what they are used for.
answered May 13 at 16:01
Thor LancasterThor Lancaster
33115
$endgroup$
$begingroup$
-1 because it sounds like you are talking about MOSFET thermal runaway (when it's not an actual issue with MOSFETs, but is an issue with BJTs). You word it in a strange way though which makes it sound like you are talking about something else, therefore clarification would be required to delineate between these two effects.
$endgroup$
– Toor
May 13 at 17:24
4
$begingroup$
@toor the tempco of FET threshold voltage is negative just like a BJT. Thermal runaway is still a concern with FETs in linear mode.
$endgroup$
– sstobbe
May 13 at 18:00
2
$begingroup$
@Toor I dunno, his story seems to check out...
$endgroup$
– marcelm
May 13 at 20:36
add a comment |
$begingroup$
Second Breakdown
(Many) Audio amplifiers operate the output stage in their linear region.
Modern power MOSFETs are not designed to operate in the linear region. Many of them (HEXFETS) are composed of a grid of hundreds of thousands of smaller FET elements to increase power density and switching speed. Other switching-optimized MOSFET families have similar constructions, with large die areas and/or arrays of smaller elements.
For MOSFETs, the threshold voltage has a negative temperature coefficient. As a particular area of the die / FET element gets hotter, it's threshold voltage decreases and since the MOSFET is operating in it's linear region, that area conducts a larger portion of the current, so it gets even hotter. Before long, the localized heating on a tiny fraction of the die has resulted in a short circuit, often called "Second Breakdown".
But...
A relatively new type of amplifier, the "Class D" amplifier, works by switching the output stage transistors on and off rapidly, at a frequency much higher than the speaker is expected to reproduce. A low-pass filter filters out the high-frequency noise, and amplification is achieved through varying the duty cycle.
MOSFETs are extremely common in such designs, as class D amplifiers either have the output stage elements fully on or fully off. As power MOSFETs are optimized for that, that's what they are used for.
answered May 13 at 16:01
Thor LancasterThor Lancaster
33115
$endgroup$
Second Breakdown
(Many) Audio amplifiers operate the output stage in their linear region.
Modern power MOSFETs are not designed to operate in the linear region. Many of them (HEXFETS) are composed of a grid of hundreds of thousands of smaller FET elements to increase power density and switching speed. Other switching-optimized MOSFET families have similar constructions, with large die areas and/or arrays of smaller elements.
For MOSFETs, the threshold voltage has a negative temperature coefficient. As a particular area of the die / FET element gets hotter, it's threshold voltage decreases and since the MOSFET is operating in it's linear region, that area conducts a larger portion of the current, so it gets even hotter. Before long, the localized heating on a tiny fraction of the die has resulted in a short circuit, often called "Second Breakdown".
But...
A relatively new type of amplifier, the "Class D" amplifier, works by switching the output stage transistors on and off rapidly, at a frequency much higher than the speaker is expected to reproduce. A low-pass filter filters out the high-frequency noise, and amplification is achieved through varying the duty cycle.
MOSFETs are extremely common in such designs, as class D amplifiers either have the output stage elements fully on or fully off. As power MOSFETs are optimized for that, that's what they are used for.
answered May 13 at 16:01
Thor LancasterThor Lancaster
33115
edited May 14 at 1:44
answered May 13 at 16:01
Thor LancasterThor Lancaster
33115
answered May 13 at 16:01
Thor LancasterThor Lancaster
33115
answered May 13 at 16:01
Thor LancasterThor Lancaster
33115
33115
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-1 because it sounds like you are talking about MOSFET thermal runaway (when it's not an actual issue with MOSFETs, but is an issue with BJTs). You word it in a strange way though which makes it sound like you are talking about something else, therefore clarification would be required to delineate between these two effects.
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– Toor
May 13 at 17:24
4
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@toor the tempco of FET threshold voltage is negative just like a BJT. Thermal runaway is still a concern with FETs in linear mode.
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– sstobbe
May 13 at 18:00
2
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@Toor I dunno, his story seems to check out...
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– marcelm
May 13 at 20:36
add a comment |
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-1 because it sounds like you are talking about MOSFET thermal runaway (when it's not an actual issue with MOSFETs, but is an issue with BJTs). You word it in a strange way though which makes it sound like you are talking about something else, therefore clarification would be required to delineate between these two effects.
$endgroup$
– Toor
May 13 at 17:24
4
$begingroup$
@toor the tempco of FET threshold voltage is negative just like a BJT. Thermal runaway is still a concern with FETs in linear mode.
$endgroup$
– sstobbe
May 13 at 18:00
2
$begingroup$
@Toor I dunno, his story seems to check out...
$endgroup$
– marcelm
May 13 at 20:36
$begingroup$
-1 because it sounds like you are talking about MOSFET thermal runaway (when it's not an actual issue with MOSFETs, but is an issue with BJTs). You word it in a strange way though which makes it sound like you are talking about something else, therefore clarification would be required to delineate between these two effects.
$endgroup$
– Toor
May 13 at 17:24
$begingroup$
-1 because it sounds like you are talking about MOSFET thermal runaway (when it's not an actual issue with MOSFETs, but is an issue with BJTs). You word it in a strange way though which makes it sound like you are talking about something else, therefore clarification would be required to delineate between these two effects.
$endgroup$
– Toor
May 13 at 17:24
4
4
$begingroup$
@toor the tempco of FET threshold voltage is negative just like a BJT. Thermal runaway is still a concern with FETs in linear mode.
$endgroup$
– sstobbe
May 13 at 18:00
$begingroup$
@toor the tempco of FET threshold voltage is negative just like a BJT. Thermal runaway is still a concern with FETs in linear mode.
$endgroup$
– sstobbe
May 13 at 18:00
2
2
$begingroup$
@Toor I dunno, his story seems to check out...
$endgroup$
– marcelm
May 13 at 20:36
$begingroup$
@Toor I dunno, his story seems to check out...
$endgroup$
– marcelm
May 13 at 20:36
add a comment |
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I think the knee voltage is sharper and lower for BJT than mosfet, and that is the main reason why.
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– Harry Svensson
May 13 at 14:38
2
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The problem is linear DC biasing complementary MOSFETs in the linear zone with cross-conduction and the R*Ciss effects. BJT's are much more predictable for Vbe than MOSFETs for Vgs ( 1A)
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– Sunnyskyguy EE75
May 13 at 14:40
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However we're not stopping you from trying to make it linear with dual negative feedback loops to reduce shoothru losses and eliminate quadratic VI effects
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– Sunnyskyguy EE75
May 13 at 15:00
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BJT designs are definitely cheaper overall. I wouldn't say they are so common because they sound superior to other designs.
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– Todd Wilcox
May 14 at 6:18