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Registered Member #3567
Joined: Mon Jan 03 2011, 10:49PM
Location: USA, 1960s
Posts: 260
It depends widely on the type of tube. any datasheet will show the max frequency. Most of the time it is the the tens of Megahertz. Some tubes work at around 3.2 gigahertz, like early radar tubes.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Thermionic diode valves do not have any reverse recovery time, because they have nothing to recover from - no charge carrier depletion to be made good and explained away to grieving relatives.
Diode valves do, of course, have inter-electrode capacitance, which together with the inductance due to the leads, and anode heat capacity, will determine the useful upper frequency limit of the device.
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
Proud Mary wrote ...
Thermionic diode valves do not have any reverse recovery time, because they have nothing to recover from - no charge carrier depletion to be made good and explained away to grieving relatives.
Diode valves do, of course, have inter-electrode capacitance, which together with the inductance due to the leads, and anode heat capacity, will determine the useful upper frequency limit of the device.
Really? This is just based purely on my conjecture, but I would expect that when you change the polarity of voltage on the thermionic diode then there is a certain change density of electrons accumulated in the space between electrodes, and so they will flow in the opposite direction for a bit while the field is reversed, until they are all swept out of the inter-electrode space.
Registered Member #3567
Joined: Mon Jan 03 2011, 10:49PM
Location: USA, 1960s
Posts: 260
That's what I was thinking too.
I've never seen a RRT on a tube datasheet, I just assumed that the maker used that number and other factors in the Max freq. time. Well, I learn new things every day here!
Registered Member #1667
Joined: Sat Aug 30 2008, 09:57PM
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Posts: 374
for a rectifier tube running at 1kV, 1mA, 1 cm^2 active region, I ended up with an average carrier concentration of 5*10^6 1/cm^3, which is roughly the intrinsic carrier concentration of GaAs and incredibly small (feel free to correct me if I'm wrong). Talking in terms of free electron mobility, I don't think the space charge concentration is an issue (ok, apart from Klystrons and other exotic tubes).
I'm too tired to calculate the time it'll take to deplete the space between A-K but it will be something like the transit time for an electron, like 7.5ns, allowing switching speeds of over 100 MHz. Cheap 1N4148 (trr 4ns) is not impressed
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
hboy007 wrote ...
for a rectifier tube running at 1kV, 1mA, 1 cm^2 active region, I ended up with an average carrier concentration of 5*10^6 1/cm^3, which is roughly the intrinsic carrier concentration of GaAs and incredibly small (feel free to correct me if I'm wrong). Talking in terms of free electron mobility, I don't think the space charge concentration is an issue (ok, apart from Klystrons and other exotic tubes).
I'm too tired to calculate the time it'll take to deplete the space between A-K but it will be something like the transit time for an electron, like 7.5ns, allowing switching speeds of over 100 MHz. Cheap 1N4148 (trr 4ns) is not impressed
Aren't these things simply properties of the interelectrode capacitance due to the vacuum dielectric, which I mentioned above?
Registered Member #1792
Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527
Proud Mary wrote ... Aren't these things simply properties of the interelectrode capacitance due to the vacuum dielectric, which I mentioned above?
Nope, what I hboy007 and I are talking about is that there are electrons flying between the electrodes when a diode is forward conducting. If the voltage is reversed quickly then those electrons are still there and since there is no recombination mechanism they must be swept in the reverse direction by the reverse voltage to be removed. During this time there should be a reverse current, and the time it takes to sweep those charges away would be the reverse recovery time. You can't treat the vacuum purely as a vacuum dielectric since there are in fact electrons in the dielectric, but capacitance will exist between the electrodes, and I think between the electrodes and space charge.
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vacuum diode
