how to know ft bandwidth of mosfet?

Discussion in 'Electronics Resources' started by yourownfree, Jun 29, 2012.

  1. yourownfree

    Thread Starter Active Member

    Jul 16, 2008
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    I have a old thread here but no answer. I have yet to figure this out. I want to use a mosfet in an rf amplifier at 10 meters (yes I have a ham license) that will do about 120 watts or more per mosfet. I see irf510 units doing the job but only doing maybe 30-40 watts using two of them. How can I look at a spec sheet and figure out what the gain bandwidth is? Or for you specific people who twist the question and have no answer, It needs to be able to have at least 10 db of gain at 28 MHz . The voltage or current is of no concern to me. I need a mosfet or family of the mosfet that will do the job. I am looking for inexpensive types to use. What I need is an answer that will help me to calculate or be able to look at the specs and calculate. How much input capacitance am I looking at? I know how to calculate frequency using inductance and capacitance, but is that part of the equation in finding the proper one I need. I see it as maybe rise or fall time in the specs, lead inductance and capacitance, but there must be a more precise way of choosing. Transistors use ft bandwidth. mosfets can be figured out how?
     
  2. shortbus

    AAC Fanatic!

    Sep 30, 2009
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  3. yourownfree

    Thread Starter Active Member

    Jul 16, 2008
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    I will check out the link and study it thanks. The "ft" never threw you off. cmon if they dont know what "Ft" or "ft" is they just don't know. Actually my ECG handbook says "ft", my 1996 ARRL radio amateur handbook shows "fT" in the index and on page 8.7 but shows "FT" on page 10.26, so I doubt if that will mess too many people up any more than it already has. hard to say which one is correct. looks like a research item. Haven't had a chance to look at the link yet, and I am truly thankful for the help. I just had a thought after reading a little in the ARRL handbook. even though the current gain vs frequency, Ft , is the point it reaches the 3 db gain cutoff, the handbook did say this which I thought was interesting. (fT) using their language, is the guaranteed maximum output capacitance, the guaranteed input capacitance, and the guaranteed range of the transfer parameters applicable to a given device. Then they go into switching characteristics such as (td), rise (tr), storage time (ts) and Fall time (tf). But what struck me was, maybe I need to look at the input capacitance and output capacitance. Just my thought, that if I used the input capacitance of a device and estimated the lead inductance, I could check that value using the LC formula for finding the frequency of two combined, (not getting technical), and then find out what the frequency is of the rise time time, fall time, and storage time and see if it will allow the first calculation of the "LC" frequency to function fully within the windows of the rise, fall and storage times. Else maybe an easier way would be to just hook up said device to a signal generator and frequency counter and o'scope or power meter to find out what the 3 db bandwidth is. Not sure how the test circuit would look like but from the literature in the link and from other sources there must be a simple circuit. I do know that the input voltage will need to be set properly for each device (bias) and possibly just figure it using class B, for what they are saying a theoretical 78 percent efficiency. Not bad considering the power that can come from a mosfet. I do know that MRF-455 type mosfets are for rf but I just wonder how many cheap priced mosfets will do a good job and how high in frequency will they go. I have a few of them I guess I will have to test a couple of them for fun. First I am going to study up.
     
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