Suggest low noise transistor - Page 1
Forum section: electronics / projects

author: fonograph
author_rank: Frequent Contributor
timestamp: October 25, 2017, 06:39:27 pm
content: I dont know what type of transistor should I use.Mesfet,Jfet,galium nitride HEMT,HBT.... there is so many transistor types and I have no idea why I should use one over another.It will be used as OFF/ON switch only,no linearity or biasing needed.I need low noise.Voltage emitter > collector is 10 to 20 V,curret will be 100 miliampere RMS,but probably alot higher peak currents for very short duration.Switching frequency will be 3MHz but I would like if rise and fall times are less than 10% of the pulse square,generaly the faster the better but I dont need microwave capability.High gain is nice but I dont need it.Size,weight,and price doesnt matter.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 25, 2017, 07:52:46 pm
content: As a switch, one should use FETs of some kind. Due to the high current it's likely silicon MOSFETs or maybe GaN JFETs.In a switching use it is essentially the on resistance that causes noise, just like with any other resistor. 100 mA at 3 MHz Switching is already quite fast, so quite a significant gate current. Getting a low gate current might be more important that low resistance / noise.

author: Benta
author_rank: Super Contributor
timestamp: October 25, 2017, 07:53:26 pm
content: Why on earth would you need "low noise" for a switching transistor? Just curious, as it's not a specification you'll find anywhere.

author: fonograph
author_rank: Frequent Contributor
timestamp: October 25, 2017, 10:45:41 pm
content: Quote from: Kleinstein on October 25, 2017, 07:52:46 pmIs there any benefit when using bjt or jfet for switching instead of mosfet?

author: David Hess
author_rank: Super Contributor
timestamp: October 26, 2017, 03:58:49 am
content: I am confused about the low noise requirement. Low added noise? Low charge injection?A bipolar transistor would be difficult to use at that frequency but possible. A power MOSFET would be a natural choice but will require significant peak gate current for fast switching. GaN and SiC MOSFETs are suitable for fast switching but are intended for much higher voltages.I would say a standard silicon power MOSFET with a big gate driver to get switching times below 100 nanoseconds.If low charge injection is a requirement, then a two transistor cascode switch would be more suitable.

author: David Hess
author_rank: Super Contributor
timestamp: October 26, 2017, 12:30:27 pm
content: So there are low voltage GaN power MOSFETs now. Those look like they would be perfect except for packaging and price.

author: fonograph
author_rank: Frequent Contributor
timestamp: October 26, 2017, 08:49:58 pm
content: Quote from: David Hess on October 26, 2017, 12:30:27 pmYou mean those 100 V rated or did you find lower voltage ones? I really need only 20 V,100 is overkill

author: rs20
author_rank: Super Contributor
timestamp: October 26, 2017, 09:04:58 pm
content: Quote from: fonograph on October 26, 2017, 08:49:58 pmThere's no such thing as overkill, just bad compromises. If the suggested part has the lowest resistance and simplest gate drive requirements, then it is the best option. Rejecting on the basis that "100V is overkill" is totally absurd.

author: fonograph
author_rank: Frequent Contributor
timestamp: October 27, 2017, 03:07:47 pm
content: That is very true.I am not rejecting it,I am just wondering if there is some cool new gan transistor with lower breakdown voltage.Becose for if they lower the breakdown voltage,they can lower the gate capacitance and rds on.

author: David Hess
author_rank: Super Contributor
timestamp: October 27, 2017, 05:55:13 pm
content: Quote from: fonograph on October 27, 2017, 03:07:47 pmI was referring to the ones blueskull mentioned. But they come only in passivated die form and the prices are high compared to a Si MOSFET solution even with a more powerful driver or 2 transistor cascode.http://epc-co.com/epc/I did a quick search and these are the lowest voltage GaN or SiC MOSFETs that I am aware of. There are some other 60 volts parts but they are very high current.As far as the trade offs, MOSFET die size is roughly proportional to the voltage squared so using a higher voltage part considerably compromises die size (price) and capacitance. This is why minority carrier devices like bipolar transistors and insulated gate bipolar transistors are preferred at high voltages.