Help with high input impedance amp.

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

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

 

Years ago, I built this Kleijer High input impedance amp.
http://www.crystal-radio.eu/fetamp/enfetamp.htm
The input impedance test:
  I put an inductor on my Q meter and resonated it at 1MHz.
Tuning cap 151.1pf, Xc = 1,053Ω, Q = 1068
Rp of the LC is ,Rp = Q x Xc, so, 1,068 x 1,053 =1,124,604Ω =Rp.
Then I put the Kleijer amp input in parallel with the tuning capacitor.
I had to reduce the Q meter tuning capacitor by 1.1pf to get back to resonance.
The new Q with the Kleijer amp attached dropped to 1,062. So Rp= 1062 x 1053 = 1,118,286Ω
 To get 1,118,286 Ω I need to parallel the original (unloaded) 1,124,604 Ω with 200,000,000 Ω.
i.e. 1,124,604Ω // 200,000,000Ω  = 1,118,286Ω
 So, I think the input R of the amp is 200MΩ.
Does this sound like a correct method?
Does the input capacitance matter, (as a load) if I'm resonating it out.
                       Mikek
 P.S. Kleijer uses an air input cap, I used a tiny dot of roger 5880 pcb as my capacitor,
it would be interesting to make mine an air cap and see if there is a difference.

On Thursday, November 10, 2022 at 5:59:56 AM UTC-6, Lamont Cranston wrote:
> Years ago, I built this Kleijer High input impedance amp. 
> http://www.crystal-radio.eu/fetamp/enfetamp.htm 
> The input impedance test: 
> I put an inductor on my Q meter and resonated it at 1MHz. 
> Tuning cap 151.1pf, Xc = 1,053Ω, Q = 1068 
> Rp of the LC is ,Rp = Q x Xc, so, 1,068 x 1,053 =1,124,604Ω =Rp. 
> Then I put the Kleijer amp input in parallel with the tuning capacitor. 
> I had to reduce the Q meter tuning capacitor by 1.1pf to get back to resonance. 
> The new Q with the Kleijer amp attached dropped to 1,062. So Rp= 1062 x 1053 = 1,118,286Ω 
> To get 1,118,286 Ω I need to parallel the original (unloaded) 1,124,604 Ω with 200,000,000 Ω. 
> i.e. 1,124,604Ω // 200,000,000Ω = 1,118,286Ω 
> So, I think the input R of the amp is 200MΩ. 
> Does this sound like a correct method? 
> Does the input capacitance matter, (as a load) if I'm resonating it out. 
> Mikek 
> P.S. Kleijer uses an air input cap, I used a tiny dot of roger 5880 pcb as my capacitor, 
> it would be interesting to make mine an air cap and see if there is a difference.

Seems to be a problem with the amp from the data book.
I did the same experimant shown above but with the databook amp.
Instead of it loading the LC, the Q more than doubles, instead of 1,000 it's 2500!
What's the fix for that? :-)
                           Mikek

On 10/11/2022 5:53 pm, Lamont Cranston wrote:
> On Thursday, November 10, 2022 at 5:59:56 AM UTC-6, Lamont Cranston wrote:
>> Years ago, I built this Kleijer High input impedance amp.
>> http://www.crystal-radio.eu/fetamp/enfetamp.htm
>> The input impedance test:
>> I put an inductor on my Q meter and resonated it at 1MHz.
>> Tuning cap 151.1pf, Xc = 1,053Ω, Q = 1068
>> Rp of the LC is ,Rp = Q x Xc, so, 1,068 x 1,053 =1,124,604Ω =Rp.
>> Then I put the Kleijer amp input in parallel with the tuning capacitor.
>> I had to reduce the Q meter tuning capacitor by 1.1pf to get back to resonance.
>> The new Q with the Kleijer amp attached dropped to 1,062. So Rp= 1062 x 1053 = 1,118,286Ω
>> To get 1,118,286 Ω I need to parallel the original (unloaded) 1,124,604 Ω with 200,000,000 Ω.
>> i.e. 1,124,604Ω // 200,000,000Ω = 1,118,286Ω
>> So, I think the input R of the amp is 200MΩ.
>> Does this sound like a correct method?
>> Does the input capacitance matter, (as a load) if I'm resonating it out.
>> Mikek
>> P.S. Kleijer uses an air input cap, I used a tiny dot of roger 5880 pcb as my capacitor,
>> it would be interesting to make mine an air cap and see if there is a difference.
> 
> Seems to be a problem with the amp from the data book.
> I did the same experimant shown above but with the databook amp.
> Instead of it loading the LC, the Q more than doubles, instead of 1,000 it's 2500!
> What's the fix for that? :-)
>                             Mikek

Perhaps some of all that bootstrapping is getting back into the LC tuned 
circuit - you have recreated the old-time Q-multiplier. Or the input 
levels are so high the buffer is overdriven?

Padding down the input with a capacitive attentuator like Kleijer did 
could be a good way to reduce interaction. Also I guess at resonance the 
volatge is pretty high so overdriving is a real risk.

piglet

On Thursday, November 10, 2022 at 3:11:28 PM UTC-6, erichp...@hotmail.com wrote:

> > Seems to be a problem with the amp from the data book. 
> > I did the same experiment shown above but with the databook amp. 
> > Instead of it loading the LC, the Q more than doubles, instead of 1,000 it's 2500! 
> > What's the fix for that? :-) 
> > Mikek
> Perhaps some of all that bootstrapping is getting back into the LC tuned 
> circuit - you have recreated the old-time Q-multiplier. Or the input 
> levels are so high the buffer is overdriven? 
> 
> Padding down the input with a capacitive attentuator like Kleijer did 
> could be a good way to reduce interaction. Also I guess at resonance the 
> voltage is pretty high so overdriving is a real risk. 
> 
> piglet

 I'll try a tiny input capacitor. If I create a 10x capacitive divider, I'll need a 10X amp to get back to
a gain of almost 1.
   I put a series 0.3pf cap in series with the input. Same as the Kleijer amp.
Turns out that is a 10x was close it's 9.8 to 1, the output drops by a factor 9.8.
The input capacitance is 0.45pf and the input resistance measures 180MΩ.
This compares to the 200MΩ of the Kleijer amp, a little surprising sense the databook amp
has bootstrapping and the Kleijer amp doesn't.
 I need to get some or make some smaller inductors so I can do these tests at 1, 10, 20, and 30MHz,
to see if or how much the input impedance drops.
 Thanks, Mikek