开云体育

Not quite that simple.? Here is the complete eqn for calculating the Linvill Stability Factor, K, or in this presentation, C:

Dave - W?LEV

Virus-free.

On Sat, Feb 8, 2025 at 5:08?PM James Redding WA9VEZ via <wa9vez=[email protected]> wrote:

While you are in the S parameters (wish they would expressly state the reference impedance), the first numbers of the S parameter pairs is the magnitude. Second is the angle.

If you divide the |S21| by |S12| that is the "K" factor which must be larger than unity for stability. Less than unity is "unstable".

For example an NXP BUF520 shows a K of 6.8 at 6 ghz. at 8V/20 ma and around 4000 at 40 mhz.

Jim/VEZ

?

On Friday, February 7, 2025 at 05:16:32 PM EST, W0LEV via <davearea51a=[email protected]> wrote:

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

On Fri, Feb 7, 2025 at 9:37?PM Mike M via <groups=[email protected]> wrote:

From the data sheets here are the absolute Max Voltages. As you can see the 2SC3357 has lower VCBO and VCEO:

?

????? 2SC5551??? 2SC3357

VCBO??? 40???????? 20

VCEO??? 30???????? 12

VEBO? ?? 2??? ? ??? 3

?

Measuring on an LZ1AQ amp with 2SC3357 operating at 12V, here are the max voltages over all four transistors:

?

???? Measured?? Margin

VCBO?? 5.5?????? 14.5

VCEO?? 6.2??????? 5.8

VEBO?? 0.743??? ? 2.257

?

?

So for the LZ1AQ design the 2SC3357 operates with plenty of margin.

--

===================================================================
Mike M

--

Dave - W?LEV

--

Dave - W?LEV