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From: John Larkin on 11 Jul 2010 14:50
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.

Try this:

9 volt battery, 470 ohm resistor, LED, 2N7000. You can pull the gate
up to 9 volts, or down to ground - just use your fingers - to turn the
LED on or off. And then let it float in either state for hours. Or
briefly touch the drain to the gate to turn the LED partly on, then
float the gate again. The LED brightness will very slowly creep up or
down. The gate leakage is not too many electrons per second.

In that last state, take a pencil, or some insulated/metal thing like
a small screwdriver or a trimpot tool, and pump charge into or out of
the gate in small steps by alternately touching +9 or ground, then the
gate. Some of us are easily amused.

John


From: Joerg on 11 Jul 2010 16:14
John Larkin wrote:
> 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.
>

I have done it on a design. It was with the BSS123 but it's quite
similar. It all depends on the gain you want and the gate-drain
capacitance is your enemy there. I guess the 2N7002 would be a smidgen
better. AFAIR my amp was around 30dB and linear all the way up to 40Mhz
but we stopped there, didn't need more.

[...]

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
From: ian field on 11 Jul 2010 16:43

"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.


From: ian field on 11 Jul 2010 16:46

"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.
>
> Try this:
>
> 9 volt battery, 470 ohm resistor, LED, 2N7000. You can pull the gate
> up to 9 volts, or down to ground - just use your fingers - to turn the
> LED on or off. And then let it float in either state for hours. Or
> briefly touch the drain to the gate to turn the LED partly on, then
> float the gate again. The LED brightness will very slowly creep up or
> down. The gate leakage is not too many electrons per second.
>
> In that last state, take a pencil, or some insulated/metal thing like
> a small screwdriver or a trimpot tool, and pump charge into or out of
> the gate in small steps by alternately touching +9 or ground, then the
> gate. Some of us are easily amused.

Evidently.


From: John Larkin on 11 Jul 2010 16:47
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.

John

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