Help with high input impedance amp.

On Sun, 6 Nov 2022 20:13:14 +0100, Leo Baumann <ib@leobaumann.de>
wrote:

>Am 06.11.2022 um 01:59 schrieb Lamont Cranston:
>>    Years ago on this group, someone worked this circuit out for me and I have finally built it. It has two problems. The input impedance is about 30k? not 500M?, and it rolls of way to early.
>>   I may have created the problems, I don't have the FET and transistor recommended, 2N4416 and MMBTH10. I used a BF256C and a MPSH10 through hole parts. Although I doubt that created the low input impedance.
>> https://www.dropbox.com/s/kdwhcxmbk53ugqm/Dagmar%27s%20Fast%20high%20Imp%20amp%20with%20voltage%20labelsand%20RF%20voltages.jpg?dl=0
>>    Anyone care to tell me why such low input impedance and how to make the frequency flat to 30MHz?
>
>If you assume that the input impedance is high at 10 MHz, then you are 
>very wrong. - Have a look at this with LTSpice.
>
>regards

The data sheets of that fet don't seem to specify capacitances. Or
much of anything.

Am 06.11.2022 um 21:16 schrieb John Larkin:
>> If you assume that the input impedance is high at 10 MHz, then you are
>> very wrong. - Have a look at this with LTSpice.
>>
>> regards
> The data sheets of that fet don't seem to specify capacitances. Or
> much of anything.
> 

The real part of the impedanz at 10 MHz is about 1.6 kOhm
The imag part of the impedanz at 10 MHz is about 20 kOhm

On Sunday, November 6, 2022 at 2:16:22 PM UTC-6, John Larkin wrote:

> > 
> >regards
> The data sheets of that fet don't seem to specify capacitances. Or 
> much of anything.

 I ran into a couple that didn't even show pin out!
And this that is right, but it's wrong.
https://datasheetspdf.com/datasheet/BF256C.html
           Mikek

On 06/11/2022 19:47, Lamont Cranston wrote:
> Well this is interesting, the tried and proven schematic I showed before,
> https://www.dropbox.com/s/6n78s84gd9kaouu/High%20impedance%20input%20-%20Copy.jpg?dl=0
>   the transistor, a 2N3644 drawn as a NPN is actually a PNP.
>   So, is the part # wrong or is the transistor drawn wrong?  Ft is a little low at 100MHz, maybe wrong #.
>                      Mikek

Cool! Guess that shows that after 50 years getting copied from databook 
to databook nobody yet tried to build it. As JL has said before a lot of 
rubbish is printed.

piglet

On Sunday, November 6, 2022 at 2:27:05 PM UTC-6, Lamont Cranston wrote:

 Here's part of a high input impedance circuit I built a long time ago.
The input has a 17 times attenuation and it followed by an amp with a gain of 17.
 Because there is no data on the BF256C, and this fellow says there is a 17 x attenuation,
 that would make it 5.1pf of gate capacitance.  (0.3pf input capacitor)
What would you figure the input impedance of this is at 30MHz. It is built hanging in the air.
 https://www.dropbox.com/s/5oud5mbdu45o73i/Kleijer%27s%20input%20circuit.jpg?dl=0
 I'll get it out to test. I'm sure the 17 x amp is not going to make it 30MHz. But the first FET, maybe.
 Mikek

Am 06.11.2022 um 21:54 schrieb Lamont Cranston:
> Because there is no data on the BF256C, and this fellow says there is a 17 x attenuation,
>   that would make it 5.1pf of gate capacitance.

The gate-capacitance of a sFET BF256C is about 0.8 pF.

On Sun, 6 Nov 2022 21:25:49 +0100, Leo Baumann <ib@leobaumann.de>
wrote:

>Am 06.11.2022 um 21:16 schrieb John Larkin:
>>> If you assume that the input impedance is high at 10 MHz, then you are
>>> very wrong. - Have a look at this with LTSpice.
>>>
>>> regards
>> The data sheets of that fet don't seem to specify capacitances. Or
>> much of anything.
>> 
>
>The real part of the impedanz at 10 MHz is about 1.6 kOhm
>The imag part of the impedanz at 10 MHz is about 20 kOhm
>
>

Possibly a lot higher z when used as a follower.

Am 06.11.2022 um 22:01 schrieb John Larkin:
>> The real part of the impedanz at 10 MHz is about 1.6 kOhm
>> The imag part of the impedanz at 10 MHz is about 20 kOhm
>>
>>
> Possibly a lot higher z when used as a follower.

I watched into the Zin in LTSpice of one of my follower.

On Sunday, November 6, 2022 at 2:59:07 PM UTC-6, Leo Baumann wrote:
> Am 06.11.2022 um 21:54 schrieb Lamont Cranston: 
> > Because there is no data on the BF256C, and this fellow says there is a 17 x attenuation, 
> > that would make it 5.1pf of gate capacitance.
> The gate-capacitance of a sFET BF256C is about 0.8 pF.

 Do you have a reference for that?

 I get 15.4 attenuation ratio, input to the source of the fet @1MHz.
https://www.dropbox.com/s/5oud5mbdu45o73i/Kleijer%27s%20input%20circuit.jpg?dl=0 
The fet source goes to a 100nF cap and about 500Ω to ground.
                         Mikek

Am 06.11.2022 um 22:48 schrieb Lamont Cranston:
>> The gate-capacitance of a sFET BF256C is about 0.8 pF.
>   Do you have a reference for that?

As I wrote above the imag part of the input impedanz is about 20 kOhm 
and the real part is about 1.6 kOhm at 10 MHz on a BC256C as follower.

I have checked that in one of my LTSpice simulations.

That means Cin=1/(20 kOhm*2*PI*10 MHz) = 0.79 pF.

BTW the Cis of a BF556C is 1.7 pF.