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Discussion in 'Amplifiers' started by 9C1Driver, Feb 25, 2017.
A lot of input capacitance.
great info guys.
i think you are on the right track.
here is an interesting write up using IRFP260n's.
another great article explaining the use of high power mosfets as amplifiers.
if nothing else, just read the introduction.
Interesting on the 260n. Almost twice the gate capacitance as the 250n. Interesting to build an amplifier using 2 of each. See what happens. You know on the LDMOS amps, they have been able to parallel several. I wonder if same could be done with these.
these were the two most interesting parts to me. first one deals with operating the device at a lower voltage, and the second has to do with input impedance.
1. "When forward biased with a constant gate voltage, the quiescent drain current will rise as the temperature of the die increases. Operating at the typical drain voltage for these parts, about one third of the rated BVdss, the power dissipation due to the increasing Idq results in “hot spotting” and subsequent thermal runaway. This is an unstable system. The dissipation increases so rapidly that the outside surface of the case does not follow the internal junction temperature. As a result, a bias compensation scheme that uses temperature sensing cannot keep up with the Vth shift and the device is destroyed. The power dissipation within the die is a direct function of the operating voltage. By lowering the operating voltage the thermal loop gain can be reduced to a point where the gate threshold shift can be compensated for. Thermal stability can be achieved by sensing the case temperature. Linear operation thus becomes practical at 100V and below. While this is less than 25% of the rated BVdss and results in less gain, a very rugged and useful linear amplifier results."
2. " In an ideal MOSFET, the input impedance is a pure capacitor. It has no real part. At 50 MHz the ARF448 has an input impedance of just 0.2 + j 0.5. This indicates that the impedance of the bonding wire inductance is slightly larger than the gate capacitance and that there is very little gate loss resistance. It also suggests that careful attention to the design of the input matching network will be necessary to obtain the design goals."
i have lots more reading to do before i really understand any of this.
So feeding these devices with 13.8v (instead of 30v) is too little? Or is this how RF Limited kept these devices from going into runaway? RFL kept these devices at/around 75w recommended output. Bet ya it isn't the same device they used.
I'm sure the 448 they mentioned cost more than a dollar or 2.
I never owned anything with a ERF2030 so I just assumed since it was too high wattage that it was just a TO220 case.
Total gate charge for the IRF520 is max 25 nC (nanoCoulombs).
For FQP13N10 it's 16 nC.
For the IRF250 it's 123 nC.
Does this mean that it's 5 times slower to charge and discharge the gate than a IRF520?
Or 7-1/2 times slower than the FQP13N10?
I suppose this could explain how the FQP can be a bit zippier than the IRF520, but not by a lot. Stores only 2/3 the charge on its gate than the IRF520.
Or you can think of the 520 as storing half-again more charge in the gate than the FQP. But a difference of one-and-a-half to one, or two-thirds is nowhere near a five-to-one or 7-plus to one comparison.
But I still don't know how to translate the MOSFET's "gate charge" spec into a max frequency or likely power gain number.
Does make the 250 look like a long shot candidate. Would probably be great for your pirate AM-broadcast transmitter at 1300 on the dial. But at 20 (or so) times that frequency? Not so sure.
Every power MOSFET designed as a switch will exhibit a drop in gain as the RF frequency increases. The 7530 only gets you around 4-to-one or 5-to-one power gain at 27 MHz.. Time to hook up a 7530 and see if I can measure the gate capacitance. Would give me something to compare with the specs they normally list for a switch-designed MOSFET.
As far as I understand most mosfets are rated as switches. There rise and fall times make them work in an RF environment. Even at 13.8 volts you can still bias them into saturation. I volted some IFR520's to 30 volts I really didn't see any significant amount of gain. They are also not as susceptible to thermal runaway like bipolar's.
I'm having the same problem. Can't find 7530s anywhere.
Irl540pbf has anyone looked into these?
Nope; that too has a TO-220 case.
Not a TO-247.
did anyone find this??