Mosfet switching times ?
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From: Joerg on 11 Jul 2010 16:50 ian field wrote: > "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message > news:7k3k36l23atp4jd8n9vdimuvlcaupn7lm3(a)4ax.com... >> On Sun, 11 Jul 2010 17:09:24 +0100, "ian field" >> <gangprobing.alien(a)ntlworld.com> wrote: >> >>> "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in >>> message >>> news:0ibh365789vlqn1hqm31m8clbobvv1v2kp(a)4ax.com... >>>> On Sat, 10 Jul 2010 13:22:46 -0400, Hammy <spam(a)spam.com> wrote: >>>> >>>>> On Sat, 10 Jul 2010 09:28:14 -0700, John Larkin >>>>> <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>>>> >>>>> [snip] >>>>>> Most mosfets will switch much faster than their datasheets suggest, if >>>>>> you just drive them hard. The Fairchild BSS123 datasheet cites a >>>>>> typical turn-on rise time of 9 ns, and 17 for turnoff. >>>>> I never paid much attention too data sheet switching times until >>>>> recently. Mainly from the emphasis some people here and in other >>>>> forums place on them. So I was beginning to think I may be missing >>>>> something, but I guess not. >>>>> >>>>> I've always just used Qg and my drivers sink/source ability to >>>>> estimate times. >>>> I generally assume that mosfet silicon is infinitely fast, and that >>>> only capacitances and wirebond inductances get in the way. Seems to >>>> work so far. >>> >>> Recently I was asking in various groups what frequency people thought I >>> might shove through a 2N7000 in grounded gate - some people thought the >>> limit might be around 100MHz. >>> >>> Do you have an estimate on this? >>> >> Do you mean as a switch, or as an RF sinewave amplifier? >> >> If you drive the gate hard, grounded-source, you can turn a 2N7002 >> on/off, with 50 volt drain swing, in about a nanosecond. I'd imagine >> you could get useful grounded-gate power gain at 250 MHz at least, >> especially if things were tuned+matched for the operating frequency. >> The 2N7000 version will have a little more lead inductance, but that >> can be tuned out. > > My intention was to re-radiate DAB (VHF high band) from a communal aerial > socket in one corner to the radio in the diagonally opposite corner. > Aerial socket, now that must be a truly Bri'ish expression :-) > Originally the 7000 was in cascode with a 2N3819 - it almost worked. In the > end I settled for a BF998 driving a BFQ162A. > Don't know what a BFQ162 is but yes, why torture yourself with low frequency devices when you can get a hotrod RF transistor with tens of gigeehoitzes for under a buck? Or less than a quid in rightpondian. Some of the bigger MMIC, the ones with a grounded tab that can be soldered onto copperclad for heatsink, should also work. Of course, re-radiation can ruffle some feathers with the authorities. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: John Larkin on 11 Jul 2010 17:29 On Sun, 11 Jul 2010 13:50:23 -0700, Joerg <invalid(a)invalid.invalid> wrote: >ian field wrote: >> "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message >> news:7k3k36l23atp4jd8n9vdimuvlcaupn7lm3(a)4ax.com... >>> On Sun, 11 Jul 2010 17:09:24 +0100, "ian field" >>> <gangprobing.alien(a)ntlworld.com> wrote: >>> >>>> "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in >>>> message >>>> news:0ibh365789vlqn1hqm31m8clbobvv1v2kp(a)4ax.com... >>>>> On Sat, 10 Jul 2010 13:22:46 -0400, Hammy <spam(a)spam.com> wrote: >>>>> >>>>>> On Sat, 10 Jul 2010 09:28:14 -0700, John Larkin >>>>>> <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>>>>> >>>>>> [snip] >>>>>>> Most mosfets will switch much faster than their datasheets suggest, if >>>>>>> you just drive them hard. The Fairchild BSS123 datasheet cites a >>>>>>> typical turn-on rise time of 9 ns, and 17 for turnoff. >>>>>> I never paid much attention too data sheet switching times until >>>>>> recently. Mainly from the emphasis some people here and in other >>>>>> forums place on them. So I was beginning to think I may be missing >>>>>> something, but I guess not. >>>>>> >>>>>> I've always just used Qg and my drivers sink/source ability to >>>>>> estimate times. >>>>> I generally assume that mosfet silicon is infinitely fast, and that >>>>> only capacitances and wirebond inductances get in the way. Seems to >>>>> work so far. >>>> >>>> Recently I was asking in various groups what frequency people thought I >>>> might shove through a 2N7000 in grounded gate - some people thought the >>>> limit might be around 100MHz. >>>> >>>> Do you have an estimate on this? >>>> >>> Do you mean as a switch, or as an RF sinewave amplifier? >>> >>> If you drive the gate hard, grounded-source, you can turn a 2N7002 >>> on/off, with 50 volt drain swing, in about a nanosecond. I'd imagine >>> you could get useful grounded-gate power gain at 250 MHz at least, >>> especially if things were tuned+matched for the operating frequency. >>> The 2N7000 version will have a little more lead inductance, but that >>> can be tuned out. >> >> My intention was to re-radiate DAB (VHF high band) from a communal aerial >> socket in one corner to the radio in the diagonally opposite corner. >> > >Aerial socket, now that must be a truly Bri'ish expression :-) He said "communal." Must be a Russky. John
From: Joerg on 11 Jul 2010 17:44 John Larkin wrote: > On Sun, 11 Jul 2010 13:50:23 -0700, Joerg <invalid(a)invalid.invalid> > wrote: > >> ian field wrote: >>> "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message >>> news:7k3k36l23atp4jd8n9vdimuvlcaupn7lm3(a)4ax.com... >>>> On Sun, 11 Jul 2010 17:09:24 +0100, "ian field" >>>> <gangprobing.alien(a)ntlworld.com> wrote: >>>> >>>>> "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in >>>>> message >>>>> news:0ibh365789vlqn1hqm31m8clbobvv1v2kp(a)4ax.com... >>>>>> On Sat, 10 Jul 2010 13:22:46 -0400, Hammy <spam(a)spam.com> wrote: >>>>>> >>>>>>> On Sat, 10 Jul 2010 09:28:14 -0700, John Larkin >>>>>>> <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>>>>>> >>>>>>> [snip] >>>>>>>> Most mosfets will switch much faster than their datasheets suggest, if >>>>>>>> you just drive them hard. The Fairchild BSS123 datasheet cites a >>>>>>>> typical turn-on rise time of 9 ns, and 17 for turnoff. >>>>>>> I never paid much attention too data sheet switching times until >>>>>>> recently. Mainly from the emphasis some people here and in other >>>>>>> forums place on them. So I was beginning to think I may be missing >>>>>>> something, but I guess not. >>>>>>> >>>>>>> I've always just used Qg and my drivers sink/source ability to >>>>>>> estimate times. >>>>>> I generally assume that mosfet silicon is infinitely fast, and that >>>>>> only capacitances and wirebond inductances get in the way. Seems to >>>>>> work so far. >>>>> Recently I was asking in various groups what frequency people thought I >>>>> might shove through a 2N7000 in grounded gate - some people thought the >>>>> limit might be around 100MHz. >>>>> >>>>> Do you have an estimate on this? >>>>> >>>> Do you mean as a switch, or as an RF sinewave amplifier? >>>> >>>> If you drive the gate hard, grounded-source, you can turn a 2N7002 >>>> on/off, with 50 volt drain swing, in about a nanosecond. I'd imagine >>>> you could get useful grounded-gate power gain at 250 MHz at least, >>>> especially if things were tuned+matched for the operating frequency. >>>> The 2N7000 version will have a little more lead inductance, but that >>>> can be tuned out. >>> My intention was to re-radiate DAB (VHF high band) from a communal aerial >>> socket in one corner to the radio in the diagonally opposite corner. >>> >> Aerial socket, now that must be a truly Bri'ish expression :-) > > He said "communal." Must be a Russky. > Hey, I just went to communion and I'm not a Russky :-) -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: ian field on 12 Jul 2010 07:56 "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:4rdk36t0kcj08iqm3msfch7deh7budlgv3(a)4ax.com... > On Sun, 11 Jul 2010 13:50:23 -0700, Joerg <invalid(a)invalid.invalid> > wrote: > >>ian field wrote: >>> "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in >>> message >>> news:7k3k36l23atp4jd8n9vdimuvlcaupn7lm3(a)4ax.com... >>>> On Sun, 11 Jul 2010 17:09:24 +0100, "ian field" >>>> <gangprobing.alien(a)ntlworld.com> wrote: >>>> >>>>> "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in >>>>> message >>>>> news:0ibh365789vlqn1hqm31m8clbobvv1v2kp(a)4ax.com... >>>>>> On Sat, 10 Jul 2010 13:22:46 -0400, Hammy <spam(a)spam.com> wrote: >>>>>> >>>>>>> On Sat, 10 Jul 2010 09:28:14 -0700, John Larkin >>>>>>> <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>>>>>> >>>>>>> [snip] >>>>>>>> Most mosfets will switch much faster than their datasheets suggest, >>>>>>>> if >>>>>>>> you just drive them hard. The Fairchild BSS123 datasheet cites a >>>>>>>> typical turn-on rise time of 9 ns, and 17 for turnoff. >>>>>>> I never paid much attention too data sheet switching times until >>>>>>> recently. Mainly from the emphasis some people here and in other >>>>>>> forums place on them. So I was beginning to think I may be missing >>>>>>> something, but I guess not. >>>>>>> >>>>>>> I've always just used Qg and my drivers sink/source ability to >>>>>>> estimate times. >>>>>> I generally assume that mosfet silicon is infinitely fast, and that >>>>>> only capacitances and wirebond inductances get in the way. Seems to >>>>>> work so far. >>>>> >>>>> Recently I was asking in various groups what frequency people thought >>>>> I >>>>> might shove through a 2N7000 in grounded gate - some people thought >>>>> the >>>>> limit might be around 100MHz. >>>>> >>>>> Do you have an estimate on this? >>>>> >>>> Do you mean as a switch, or as an RF sinewave amplifier? >>>> >>>> If you drive the gate hard, grounded-source, you can turn a 2N7002 >>>> on/off, with 50 volt drain swing, in about a nanosecond. I'd imagine >>>> you could get useful grounded-gate power gain at 250 MHz at least, >>>> especially if things were tuned+matched for the operating frequency. >>>> The 2N7000 version will have a little more lead inductance, but that >>>> can be tuned out. >>> >>> My intention was to re-radiate DAB (VHF high band) from a communal >>> aerial >>> socket in one corner to the radio in the diagonally opposite corner. >>> >> >>Aerial socket, now that must be a truly Bri'ish expression :-) > > He said "communal." Must be a Russky. The block of flats has a master aerial with a bunch of co-ax cables running along the wall to each flat.
From: ian field on 12 Jul 2010 08:01 "John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:gcbk36p33odvna1oeki3j2at2npt8uukcq(a)4ax.com... > On Sun, 11 Jul 2010 21:43:58 +0100, "ian field" > <gangprobing.alien(a)ntlworld.com> wrote: > >> >>"John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in >>message >>news:7k3k36l23atp4jd8n9vdimuvlcaupn7lm3(a)4ax.com... >>> On Sun, 11 Jul 2010 17:09:24 +0100, "ian field" >>> <gangprobing.alien(a)ntlworld.com> wrote: >>> >>>> >>>>"John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in >>>>message >>>>news:0ibh365789vlqn1hqm31m8clbobvv1v2kp(a)4ax.com... >>>>> On Sat, 10 Jul 2010 13:22:46 -0400, Hammy <spam(a)spam.com> wrote: >>>>> >>>>>>On Sat, 10 Jul 2010 09:28:14 -0700, John Larkin >>>>>><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>>>>> >>>>>>[snip] >>>>>>> >>>>>>>Most mosfets will switch much faster than their datasheets suggest, >>>>>>>if >>>>>>>you just drive them hard. The Fairchild BSS123 datasheet cites a >>>>>>>typical turn-on rise time of 9 ns, and 17 for turnoff. >>>>>> >>>>>>I never paid much attention too data sheet switching times until >>>>>>recently. Mainly from the emphasis some people here and in other >>>>>>forums place on them. So I was beginning to think I may be missing >>>>>>something, but I guess not. >>>>>> >>>>>>I've always just used Qg and my drivers sink/source ability to >>>>>>estimate times. >>>>> >>>>> I generally assume that mosfet silicon is infinitely fast, and that >>>>> only capacitances and wirebond inductances get in the way. Seems to >>>>> work so far. >>>> >>>> >>>>Recently I was asking in various groups what frequency people thought I >>>>might shove through a 2N7000 in grounded gate - some people thought the >>>>limit might be around 100MHz. >>>> >>>>Do you have an estimate on this? >>>> >>> >>> Do you mean as a switch, or as an RF sinewave amplifier? >>> >>> If you drive the gate hard, grounded-source, you can turn a 2N7002 >>> on/off, with 50 volt drain swing, in about a nanosecond. I'd imagine >>> you could get useful grounded-gate power gain at 250 MHz at least, >>> especially if things were tuned+matched for the operating frequency. >>> The 2N7000 version will have a little more lead inductance, but that >>> can be tuned out. >> >>My intention was to re-radiate DAB (VHF high band) from a communal aerial >>socket in one corner to the radio in the diagonally opposite corner. >> >>Originally the 7000 was in cascode with a 2N3819 - it almost worked. In >>the >>end I settled for a BF998 driving a BFQ162A. >> > > Those little darlington MMICs are great for stuff like this, absurdly > easy to use. Like the MiniCircuits ERA series. Couldn't see much mention in the OP's posts about the application - if its a direct off line switcher, MOSFETs can be cascoded for a speed advantage. The bottom MOSFET should be a high current, low voltage ultra fast device, this makes the upper MOSFET less critical in a number of criteria.
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