EMRFD Message Archive 14387

Message Date From Subject
14387 2017-10-13 23:10:05 mosaicmerc Broadband HF Xformer from Push Pull pA acting like a tank circuit?
Hi Alll:
I have a couple IRF530s in a Push pull arrangement driving a centre tapped 3:7 ratio type 43 binocular core into a 50 ohm load.

I am seeing a lot of 'ripple' on the 3Mhz that mostly dies away by 14Mhz or so.
Placing a 10nF cap across the xformer primary seems to bypass a lot of the ripple improving the low HF purity, but when I up the frequency it also bypasses the amplification the higher frequencies.

So I am looking for advice on how these harmonics come into being please. They are not present  on the gates of the IRF530s if I DC bias the gates off, but they show up once the FETs are biased on at Vth.

I have 220 ohms of negative feedback with a 10nF cap in the FB loop of the IRF 530s.

14389 2017-10-15 10:28:18 kb1gmx Re: Broadband HF Xformer from Push Pull pA acting like a tank circui

Maybe less capacitance would be in order 10 NF is a lot of capacitance.  Try something 
between 330 and 1000 pf.

Also the waveform may be do to incorrect bias (high or low), inadequate bypassing,
and/or device oscillation.  Also lead length is a major thing especially the Source lead.
Construction is everything at higher power and high RF currents.  
When working with IRF5xx fets I often cut the drain lead off at the case and use the tab
as electrical connection so the output circuit is further from the input, especially when 
mounted to a heatsink.  To connect to the Drain tab use a solder lug under the shoulder 
washer so the screw does not short the output to the heatsink which should also be 
ground.  Of course a mica or aluminum oxide insulator is needed under the device
for the same reason.

FYI the input imeadance of these devices at DC is infinite but at RF it is very low so 
if the input circuit is not correct that can make matters worse as the feedback 
(drain to gate) capacitance of the IRF530 will cause low gain and possible oscillation.

If the device is oscillating or showing parasitic oscillation try 1 to 4 ohms 1/4W 
non-inductive resistor in series with the gate lead near as possible to the case.

The output circuit is likely not a tank as 10000pf and the rather high inductance would 
resonate at a very low frequency (below 1mhz).

Also at the impedances of the circuit try using -61 ferrite as its a bit less lossy and 
useable over a wider bandwidth.

14390 2017-10-15 10:48:12 kb1gmx Re: Broadband HF Xformer from Push Pull pA acting like a tank circui
Also you do not need a driver with the FT817 as 1W (certinly lass then 2W) is enough to 
drive IRF530s to full capability on 40M and 20M.

Additionally for 12V nominal None of the IRF5xx parts are optimum for power out over maybe 15-20W.
To get more power you nearly have to saturate them and that makes the very inefficient.  Also at higher voltages the IMD is far better.  Part of this is the gate to drain output capacitance which drops with 
increasing drain voltage.

For decent power I usually run ttem at 24 to 28V, for example my IRF510 P-P amp does 55W on 40M with 1.4W, on 10M 37W with the same drive.  At 6M 8 of them in a 4x4 P-P amp will do 200W with a 30V