| Electronics > Beginners |
| BJT/MOSFET as switch |
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| Moriambar:
--- Quote from: Zero999 on January 23, 2019, 09:22:30 am --- Many people just assume BJT when one refers to transistors, but MOSFETs are actually the most common form of transistor nowadays, especially in digital ICs. Perhaps it's because most discrete transistors are still BJTs, especially the smaller ones. --- End quote --- You know, I understand why MOSFETs are the most common transistor for digital switching etc. My question arises from reading on the internet and on some books about transistor and I'm curious why 80% of the transistor part of books is on BJTs. I understand they can have many other applications I have not insight about, but it seems to me that most people just ignores MOSFETs. Furthermore I always find "MOSFETs are only for high current load switching", but I use them for mainly logic/lowish (<100mA) loads. That's why I started this topic: to understand whether I was wrong using them, while the internet seems to think BJTs everywhere. --- Quote ---Regarding leakage current: yes MOSFETs are leaky, compared to BJTs, but circuits composed entirely of MOSFETs normally use less power, compared to the equivalent circuit made with BJTs, especially in low frequency, low power applications, because a MOSFET doesn't need any current to remain on, whilst a BJT requires a base current to stay on. A classic example is digital logic: compare the power consumption of a CMOS quad NAND gate IC such as the CD4011 or 74HC00 with a TTL IC such as the 74LS00 and note the huge difference in current draw. --- End quote --- This is interesting. Cheers |
| kosine:
--- Quote ---I'm curious why 80% of the transistor part of books is on BJTs. --- End quote --- It's a fair question, and I suspect part of the answer is simply because BJTs got there first and retain some of that inertia. Although the concept for the FET goes back to Lillienfeld in 1925, they weren't really commercialiised until the late 1960s. By then, BJTs had been around for nearly 20 years. They were cheap, readily available to hobbyist, well understood, and loads of clever circuits had been designed around them. (Plus their development won a Nobel Prize, which didn't hurt their popularity.) Even into the 80s and 90s (and probably still today), most electronics kits explain circuits using BJTs. They were/are likely favoured in education due to cost, robustness and also their legacy reputation and wealth of available application circuits, so that's what everyone learns on. (The teachers themselves probably learned on them when they were starting out.) Anyone writing a book (or publishing a web page), especially for beginners or students, is likely to focus on the BJT as a result. It's kind of self-fulfilling, and BJTs aren't the only example of this effect. |
| Doctorandus_P:
The < 100nA leakage "power off" circuit I meant is in the transistor tester circuit. The pdf in the link below shows at least 20 different display variants, and it also has the 3 BJT power switch circuit. https://github.com/Upcycle-Electronics/AVR-Transistor-Tester/blob/master/Final%20Draft%20LCD%20Master%20Transistor%20Tester.pdf Mine has been laying in a drawer for 2 or more years and the 9V battery is still good. (It shows battery voltage on startup :). BJT's do need a continuous base current to turn them on, but in applications such as the transistor tester the low leakage while off is the predominant factor. In the few seconds the transistor tester is on, the base current of those transistors is still < 1 % of the current draw and therefore pretty insignificant. Mine version is also one of the oldest / simplest versions with an HD44780 display. Battery life is so long that I simply glued it with hot snot to the PCB. I also glued a piece of corrugated cardboard to the bottom of the circuit: 1). First press the wires of the THT components in the cardboard. 2). Deform the cardboard a bit more where the wires are. 3). Glue the cardboard to the bottom of the PCB. 4). Cut cardboard to the size of the PCB. 5). Smear some more glue around the sides of the cardboard to make it more durable. You can also make "rubber feet" from a dot of hot snot. Once it's cooled and especially some dust has accumulated on the outside it does not stick anymore, and they hold better to the underside of your equipment than those "self adhesevive" rubber feet, which always fall off after some time. Last time I used my transistor tester was for some IR power MOSfets. For some kind of weird reason they do not specify the pinout of the TO220 power MOSfet's in their datasheets ??? and with this tester it's easy to check which pin is where. Some transistors also come in different pinouts, with the same part number, and some different suffix and then this tester also helps to get more confidence in the actual component you have in your hands. I always like to verify info from datasheets with relality. |
| David Hess:
--- Quote from: Moriambar on January 22, 2019, 07:26:58 am --- --- Quote from: David Hess on January 22, 2019, 03:41:32 am ---The large variation in threshold voltage for MOSFETs compared to the very consistent Vbe of a bipolar transistor makes them more difficult to use and less suitable at low voltages. The gate of a MOSFET is more susceptible to damage than the base of a bipolar transistor so extra precautions against over-voltage and ESD are required. MOSFETs cost more for the same voltage and current rating; this becomes especially pronounced at high voltages. Similar objections apply to JFETs. --- End quote --- Are you basically saying that ESD and the fact that to use a mosfet one has to read the datasheet are what make these a bit more complicated to use? Thanks! --- End quote --- The major difference is that every bipolar transistor can be driven and used in a switching application with less than 1 volt of compliance. Vbe is order of magnitude more predictable making design for variation much easier. Meanwhile FETs are graded into separate part numbers for different Vgs(th) the same way that some bipolar transistors are graded for hfe except Vgs(th) is more critical than hfe. The result is standard power MOSFETs, low threshold MOSFETs for 5 volt gate drive, lower threshold MOSFETs for 3.3 volt gate drive, even lower threshold MOSFETs for 1.8 volt gate drive, etc. And this extends to linear circuits where variation in Vgs(th) of a single type is much worse than Vbe of bipolar transistors. Try using parallel MOSFETs in a linear application with the same value of ballast resistor which bipolar transistors would require. |
| Moriambar:
--- Quote from: David Hess on January 23, 2019, 07:50:52 pm --- --- Quote from: Moriambar on January 22, 2019, 07:26:58 am --- --- Quote from: David Hess on January 22, 2019, 03:41:32 am ---The large variation in threshold voltage for MOSFETs compared to the very consistent Vbe of a bipolar transistor makes them more difficult to use and less suitable at low voltages. The gate of a MOSFET is more susceptible to damage than the base of a bipolar transistor so extra precautions against over-voltage and ESD are required. MOSFETs cost more for the same voltage and current rating; this becomes especially pronounced at high voltages. Similar objections apply to JFETs. --- End quote --- Are you basically saying that ESD and the fact that to use a mosfet one has to read the datasheet are what make these a bit more complicated to use? Thanks! --- End quote --- The major difference is that every bipolar transistor can be driven and used in a switching application with less than 1 volt of compliance. Vbe is order of magnitude more predictable making design for variation much easier. Meanwhile FETs are graded into separate part numbers for different Vgs(th) the same way that some bipolar transistors are graded for hfe except Vgs(th) is more critical than hfe. The result is standard power MOSFETs, low threshold MOSFETs for 5 volt gate drive, lower threshold MOSFETs for 3.3 volt gate drive, even lower threshold MOSFETs for 1.8 volt gate drive, etc. And this extends to linear circuits where variation in Vgs(th) of a single type is much worse than Vbe of bipolar transistors. Try using parallel MOSFETs in a linear application with the same value of ballast resistor which bipolar transistors would require. --- End quote --- Got it. I see now how it can be easier: pick a BJT and you hardly go wrong. Pick a MOSFET and you don't know what you picked. Cheers |
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