Help with high input impedance amp....

[piglet]

On 09/11/2022 11:02 am, Lamont Cranston wrote:

In the real world performance will be dominated by physical layout
strays.

There is the rub, proper layout, on iteration 3, I followed a couple of the ideas given here,
removing the ground plane and getting the parts raised above the board, although
without the ground plane I don\'t think raising the parts mattered. I suspect removing the ground plane
from the enter circuit was,

not as important as for the high impedance FET area?

I used long leads to keep the parts physically separated, but I know that leads to inductive strays,
so...
l \'ll be reading up on how to mitigate pcb strays before my next build.
If anyone has a favorite site on the subject of stray mitigation, please post.

Is there any advantage using smd components?

I thought there was, but the last build did pretty well with leaded components.

Is it worth using a pcb software program, so I could make thinner tracks to help minimize strays?

If you build it please let us know.
piglet

I did order the transistors and FETs last night, so there is a possibility,
but other projects first.

I can think of only one way to test the input impedance, that is with my Q meter.
Set up a LC at resonance and then add the high input impedance circuit across the
tuning capacitor. Then read out the change of the tuning capacitor and how
much the Q drops, then do the math on the Q change.

With the input current so low, is there another way?
Thanks, Mikek

If 30MHz is your max frequency then lead length inductance of THT
components is unlikely to be a big problem. Your goal of Hi-Z means low
stray C and capacitance is proportional to area divided by distance. THT
parts will get you distance between nodes whereas SMD parts will reduce
node area. Usually SMD wins but with care you could get THT to work.
Don\'t forget the dielectric constant of pcb substrate is 4-5 times that
of air (and lossy).

The BF256 is similar to 2SK192 and none of the parts in the new circuit
are special.

Didn\'t you say before you measured input impedance by inserting variable
high resistances between input jack and FET gate and finding values that
gave 3dB drop? I recall the figure 30kohm from an early post?

The Q meter method should work too and since that appears to be your end
application that method might be the gold standard even though tedious?

piglet

 

[Lamont Cranston]

On Wednesday, November 9, 2022 at 8:58:44 AM UTC-6, erichp...@hotmail.com wrote:

30MHz is your max frequency then lead length inductance of THT
components is unlikely to be a big problem.

Don\'t forget the dielectric constant of pcb substrate is 4-5 times that
of air (and lossy).

Yes, and that is part of removing the ground plane, although it had a slight level peak at 29MHz and
was up a little at 30MHz. That calmed down when I add the underside ground plane.

The thread (on an IO Group) that got me started, was asking about building a Q meter with a range of
1.5MHz to 150Mhz. I don\'t care about that, I don\'t know that I will ever go above 10MHz, but increasing
the frequency response is fun, interesting and for me educational.

The BF256 is similar to 2SK192 and none of the parts in the new circuit
are special.

Not, that I understand a lot about FET characteristics, but the 2SK192A has a minimally lower Crss and there is no spec for Ciss on the BF256C
but the 2sk192A spec is 3.5pf, so I\'m thinking it might also be a bit lower than the 256C. The Forward Transfer Admittance is higher on the 2sk192A
and ! can\'t compare the power gain because they are measured at widely different frequencies.
So, for those reasons I chose to go with the specified FET. It\'s either right or it\'s wrong and an expense I didn\'t need!

Didn\'t you say before you measured input impedance by inserting variable
high resistances between input jack and FET gate and finding values that
gave 3dB drop? I recall the figure 30kohm from an early post?

I did, and I think that was at 1MHz. Although later I tested again and with the 30KΩ the output was lower,
I don\'t know if my generator was set to a higher frequency, but, I let that go until I have another amp
to compare it to. I also don\'t know what strays I\'m adding with my input series resistor.
I like the Q meter, because it has a + and - 3pf tuning cap graduated in 10ths of a pf.
A 0.2pf change on a high Q inductor is clearly seen, so I figure its a good way to find the input capacitance.
With a with a high Q inducto,r 1250Q at 1MHz, a 50 point of Q drop is 45MΩ additional load resistance, I
should easily see any drops. I\'ll need to setup my Q meter and verify that. Seems high, but that\'s what I calculate,
although that does not include the losses in the gate capacitance and strays..
Thanks, Mikek

 

[Lamont Cranston]

On Wednesday, November 9, 2022 at 12:50:54 PM UTC-6, Lamont Cranston wrote:

On Wednesday, November 9, 2022 at 8:58:44 AM UTC-6, erichp...@hotmail.com wrote:

30MHz is your max frequency then lead length inductance of THT
components is unlikely to be a big problem.
Don\'t forget the dielectric constant of pcb substrate is 4-5 times that
of air (and lossy).
Yes, and that is part of removing the ground plane, although it had a slight level peak at 29MHz and
was up a little at 30MHz. That calmed down when I add the underside ground plane.

The thread (on an IO Group) that got me started, was asking about building a Q meter with a range of
1.5MHz to 150Mhz. I don\'t care about that, I don\'t know that I will ever go above 10MHz, but increasing
the frequency response is fun, interesting and for me educational.

The BF256 is similar to 2SK192 and none of the parts in the new circuit
are special.
Not, that I understand a lot about FET characteristics, but the 2SK192A has a minimally lower Crss and there is no spec for Ciss on the BF256C
but the 2sk192A spec is 3.5pf, so I\'m thinking it might also be a bit lower than the 256C. The Forward Transfer Admittance is higher on the 2sk192A
and ! can\'t compare the power gain because they are measured at widely different frequencies.
So, for those reasons I chose to go with the specified FET. It\'s either right or it\'s wrong and an expense I didn\'t need!
Didn\'t you say before you measured input impedance by inserting variable
high resistances between input jack and FET gate and finding values that
gave 3dB drop? I recall the figure 30kohm from an early post?
I did, and I think that was at 1MHz. Although later I tested again and with the 30KΩ the output was lower,
I don\'t know if my generator was set to a higher frequency, but, I let that go until I have another amp
to compare it to. I also don\'t know what strays I\'m adding with my input series resistor.
I like the Q meter, because it has a + and - 3pf tuning cap graduated in 10ths of a pf.
A 0.2pf change on a high Q inductor is clearly seen, so I figure its a good way to find the input capacitance.
With a with a high Q inductor 1250Q at 1MHz, a 50 point of Q drop is 45MΩ additional load resistance, I
should easily see any drops. I\'ll need to setup my Q meter and verify that. Seems high, but that\'s what I calculate,
although that does not include the losses in the gate capacitance and strays..
Thanks, Mikek