Re: LZ1AQ amp With 2SC3357
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All of these BFU5xx series from NXP seem to have a good high ft.? However, if you consult the gain vs. frequency, all of them pretty much die out above 1.8 GHz.? NXP does not rate gain or noise figure above 1.8 GHz.? With an ft of 9 GHz on the BFU550, I'd expect FAR BETTER GAIN above 1.8 GHz, but it just isn't there.?
Yes, I realize ft is technically the maximum frequency of oscillation or where the silicon can overcome losses to oscillate.? But, why?? Here are some very telling plots from the NXP data sheet for the BFU550:
MSG = Maximum Stable gain. |s21|^2 = 10 log [s21]^2
Nothing useful above 1.8 GHz.? NXP rates the "Transition Frequency" measured at 25 mA and 900 MHz at 11 GHz.? OK.? This isn't what I learned long ago and far away in my EE classes.
Well, I looked it up online.? The transition frequency is the frequency at which the gain drops to unity.? Therefore that would be the same as the maximum frequency of oscillation.? So........how do they measure 11 GHz when operating at 900 MHz??? Seems the max frequency of oscillation would be far easier to obtain.
Dave - W?LEV
From the data sheets here are the absolute Max Voltages. As you can see the 2SC3357 has lower VCBO and VCEO:
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????? 2SC5551??? 2SC3357
VCBO??? 40???????? 20
VCEO??? 30???????? 12
VEBO? ?? 2??? ? ??? 3
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Measuring on an LZ1AQ amp with 2SC3357 operating at 12V, here are the max voltages over all four transistors:
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???? Measured?? Margin
VCBO?? 5.5?????? 14.5
VCEO?? 6.2??????? 5.8
VEBO?? 0.743??? ? 2.257
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So for the LZ1AQ design the 2SC3357 operates with plenty of margin.
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Mike M
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