One nice thing about the MOSFET is that the gate will tolerate a lot more reverse voltage than the base-emitter junction of a BJT will. One nice thing about the Darlingtons is that they have much better behaviour in the linear region, for when a saturated switch isn't what you want.
The last time I used parts like this on a new design was around 2000. Used them to drive the coils on 25A power relays and they were driven from 5V logic in a 12v automotive type power controller. In addition to the high saturation voltage another commenter noted, these are relatively slow switching if that matters in the application. When driving from logic, one also needs to be careful to have a base pull down resistor when the product may need to operate over a wide (including hot) temperature as leakage current from the driver can turn these partly on when you want them off.
I like bipolar in general for output stages because they are more robust for esd and overvoltage. At least they do not need a lot of protrction circuit compared to mosfet...
Resistors are put on mosfet gates to reduce the ringing when hard switching the FET on/off. A mosft gate has some capacitance and there is some inductance in the gate wire that makes a nice LC oscillator. A resistor in series dampens the oscillations.
The 2N7000 is possibly a 40+ year old part, and there are much better options now. It's gate threshold could be as high as 3.0V, so even 5V logic may struggle to drive it to saturation if the ID current demand is too high. Its RDS is also high by modern standards.
I think a fairer comparison, if we want to talk modern parts and low costs, almost has to look at the SMD only offerings at this point, I'd start with whatever small mosfet every other cheap gizmo defaults to using these days. I'm not sure when new parts stopped coming out in a TO-92 package, but I'm fairly certain it was a quite a while ago, especially in terms of semiconductor development.
BJT darlingtons have quite a bit more linearity than MOSFETs, so they might be better in amplifier circuits - that said, a JFET is probably a better choice for a very high sensitivity amplifier, since they are lower noise. Another application well suited to a darlington BJT is a sensitive current-amplifier (nobody uses those any more) where you have some sort of current source that varies slightly, such as in a circuit where the active element has variable resistance/conductance, where you want to amplify slight variations in current (resistance) to a larger linear signal. Again, a JFET transconductance amplifier would probably be better, but might use more parts. Great video, thanks for all the information about the fiddly bits that make technology work. Cheers!
The big problem with Darlingtons is their large saturation voltage of around 1V. At today's supply voltages of 3.3V or 5V, that's a big chunk. I've seen people use ULN2003 Darlington arrays in a 4 alkaline cell system for relay coil drive - and it conks out much earlier than a FET-based design would.
Take care if you compare data sheet frequencies. The on / off switching time means that the device really carries a considerable part (90%) of its specified drain source current. A transit frequency means its gain bandwidth product. Especially for Darlingtons it means that at the given frequency the beta is only 1 and no more 10000 or more. Darlington transistors are relatively slowly. If you want use a beta of 1000 from the original 10000 the maximum frequency is only 125 kHz. If you want to use the full beta of 10000 you can only operate at 12.5 kHz. You can't even use the full beta within the audio frequency range.
The price comparison is nonsense, a reel of 2N7002 can be very low cost when purchased in quantity, i.e. on LCSC they trade below $0.01. Few MOSFETs are sold in TO92 (Darlingtons score a point here), but even the 2N7000 can be bought for much less than the price discussed here. At realistic prices, MOSFETs are cheap enough to not worry about their cost.
I know in general for audio signal path purposes a bipolar/darlington is generally preferred to avoid the larger parasitic capacitance of a MOSFET. I think JFETs tend to have better capacitance as well but that’s generally a more specialized part as well and not perfect for every application.
Darlingtons have drawbacks too which may or may not be important depending on application. Saturation voltage is increased compared to a single transistor, and they are also much slower than one. Because of that they are best suited for low-frequency switching applications. Forget anything above 100kHz or so. :)
Back in the days of empire of evil as Reagan put it almost all of eastern european countries have their semiconductor production. Some of them developed their own, some of them licensed western designs and soviets mostly stole from the west. As I remember czechs were doing quite a lot of their own, pols were licensing newest western designs, and in my native Bulgaria they were licensing older designs and "developed" from them. In the 80s transistors were not a tecnilogy that needed buying know how, but they reflected the development history in their part designation codes, i.e. poles used BC500 small signal transistor designation, czechs used their own, and bulgarians used a mix of soviet commonwealth designation standard mixed with western origins, i.e. I have some 40 years old 2T3109 which are further development from BC109 and 2T3109C have their beta ranging from 800 to 1200 not being darligton, but a regular design. Used microamp or less biased these trasitors have not only high gain, but were also low noise with higher bandwith product (than needed for sound frequency range they were mostly intended for and worked nice on AM and IF), so I gues here is a good answer to the question about transistor gain in itself. Darlingtons have their role back in the days, and can still do the same today, though having good alternatives.
I wish they made a TO-92 version of the AO3400/3401 SMD Mosfet's. The 00 is n-channel and the 01 is p and they both are rated for 30V, 5.8A 😲 and turn on fully from as little as 1V, so you can be sure they will be hard on even at 3.3V. They're also dirt cheap. You could probably fit 20 of em inside a TO-92 package but that's kinda the problem -- some assembly is gonna be required to get them on a breadboard.
Assuming base resistors for both transistor types, you'll also need a pull down resistor for the FET to discharge the gate capacitance, so that's one extra part.
A 10 MOhm base resistor would perform just as well. Also, turn-on time is a digital (switching) spec. Analog applications would always have the gate biased above the threshold voltage making the "turn-on time" much less.
I had to revisit this one now that I got some work done with single package Darlingtons.
One nice thing about the MOSFET is that the gate will tolerate a lot more reverse voltage than the base-emitter junction of a BJT will. One nice thing about the Darlingtons is that they have much better behaviour in the linear region, for when a saturated switch isn't what you want.
When both BJTs have a gain of 1 the configuration is called Simpleton, not Darlington.
The Darlington is the better choice for linear designs (audio and such).
The last time I used parts like this on a new design was around 2000. Used them to drive the coils on 25A power relays and they were driven from 5V logic in a 12v automotive type power controller. In addition to the high saturation voltage another commenter noted, these are relatively slow switching if that matters in the application. When driving from logic, one also needs to be careful to have a base pull down resistor when the product may need to operate over a wide (including hot) temperature as leakage current from the driver can turn these partly on when you want them off.
I like bipolar in general for output stages because they are more robust for esd and overvoltage. At least they do not need a lot of protrction circuit compared to mosfet...
Resistors are put on mosfet gates to reduce the ringing when hard switching the FET on/off. A mosft gate has some capacitance and there is some inductance in the gate wire that makes a nice LC oscillator. A resistor in series dampens the oscillations.
The 2N7000 is possibly a 40+ year old part, and there are much better options now. It's gate threshold could be as high as 3.0V, so even 5V logic may struggle to drive it to saturation if the ID current demand is too high. Its RDS is also high by modern standards.
I think a fairer comparison, if we want to talk modern parts and low costs, almost has to look at the SMD only offerings at this point, I'd start with whatever small mosfet every other cheap gizmo defaults to using these days. I'm not sure when new parts stopped coming out in a TO-92 package, but I'm fairly certain it was a quite a while ago, especially in terms of semiconductor development.
BJT darlingtons have quite a bit more linearity than MOSFETs, so they might be better in amplifier circuits - that said, a JFET is probably a better choice for a very high sensitivity amplifier, since they are lower noise. Another application well suited to a darlington BJT is a sensitive current-amplifier (nobody uses those any more) where you have some sort of current source that varies slightly, such as in a circuit where the active element has variable resistance/conductance, where you want to amplify slight variations in current (resistance) to a larger linear signal. Again, a JFET transconductance amplifier would probably be better, but might use more parts. Great video, thanks for all the information about the fiddly bits that make technology work. Cheers!
The big problem with Darlingtons is their large saturation voltage of around 1V. At today's supply voltages of 3.3V or 5V, that's a big chunk. I've seen people use ULN2003 Darlington arrays in a 4 alkaline cell system for relay coil drive - and it conks out much earlier than a FET-based design would.
Take care if you compare data sheet frequencies. The on / off switching time means that the device really carries a considerable part (90%) of its specified drain source current. A transit frequency means its gain bandwidth product. Especially for Darlingtons it means that at the given frequency the beta is only 1 and no more 10000 or more. Darlington transistors are relatively slowly. If you want use a beta of 1000 from the original 10000 the maximum frequency is only 125 kHz. If you want to use the full beta of 10000 you can only operate at 12.5 kHz. You can't even use the full beta within the audio frequency range.
The price comparison is nonsense, a reel of 2N7002 can be very low cost when purchased in quantity, i.e. on LCSC they trade below $0.01. Few MOSFETs are sold in TO92 (Darlingtons score a point here), but even the 2N7000 can be bought for much less than the price discussed here. At realistic prices, MOSFETs are cheap enough to not worry about their cost.
I know in general for audio signal path purposes a bipolar/darlington is generally preferred to avoid the larger parasitic capacitance of a MOSFET. I think JFETs tend to have better capacitance as well but that’s generally a more specialized part as well and not perfect for every application.
Darlingtons have drawbacks too which may or may not be important depending on application. Saturation voltage is increased compared to a single transistor, and they are also much slower than one. Because of that they are best suited for low-frequency switching applications. Forget anything above 100kHz or so. :)
Back in the days of empire of evil as Reagan put it almost all of eastern european countries have their semiconductor production. Some of them developed their own, some of them licensed western designs and soviets mostly stole from the west. As I remember czechs were doing quite a lot of their own, pols were licensing newest western designs, and in my native Bulgaria they were licensing older designs and "developed" from them. In the 80s transistors were not a tecnilogy that needed buying know how, but they reflected the development history in their part designation codes, i.e. poles used BC500 small signal transistor designation, czechs used their own, and bulgarians used a mix of soviet commonwealth designation standard mixed with western origins, i.e. I have some 40 years old 2T3109 which are further development from BC109 and 2T3109C have their beta ranging from 800 to 1200 not being darligton, but a regular design. Used microamp or less biased these trasitors have not only high gain, but were also low noise with higher bandwith product (than needed for sound frequency range they were mostly intended for and worked nice on AM and IF), so I gues here is a good answer to the question about transistor gain in itself. Darlingtons have their role back in the days, and can still do the same today, though having good alternatives.
I wish they made a TO-92 version of the AO3400/3401 SMD Mosfet's. The 00 is n-channel and the 01 is p and they both are rated for 30V, 5.8A 😲 and turn on fully from as little as 1V, so you can be sure they will be hard on even at 3.3V. They're also dirt cheap. You could probably fit 20 of em inside a TO-92 package but that's kinda the problem -- some assembly is gonna be required to get them on a breadboard.
Assuming base resistors for both transistor types, you'll also need a pull down resistor for the FET to discharge the gate capacitance, so that's one extra part.
Excellent. I'm on Ebay to get me a few of these.
A 10 MOhm base resistor would perform just as well. Also, turn-on time is a digital (switching) spec. Analog applications would always have the gate biased above the threshold voltage making the "turn-on time" much less.