Help with high input impedance amp.

  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 30kΩ not 500MΩ, 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?
                                 Mikek
  

On Saturday, November 5, 2022 at 8:59:49 PM UTC-4, Lamont Cranston wrote:
> 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 30kΩ not 500MΩ, 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? 
> Mikek

I'm no expert at this sort of stuff, but it does appear to be a high impedance input to me.   The voltage follower with a bit of positive feedback should boost the impedance significantly.  The 100pF cap would provide more feedback at higher frequencies, raising the input impedance further.   

It will be interesting to see what others have to say. 

-- 

Rick C.

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On Sat, 5 Nov 2022 17:59:45 -0700 (PDT), Lamont Cranston
<amdx62@gmail.com> wrote:

>  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?
>                                 Mikek
>  

Is the fet connected right?

On 06/11/2022 00:59, Lamont Cranston wrote:
>    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 30kΩ not 500MΩ, 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?
>                                   Mikek
>    
> 

My first thought is gate-drain capacity. Can you add a resistor and 
capacitor and bootstrap the drain like that circuit we discussed back in 
late September?

piglet

On 06/11/2022 04:22, John Larkin wrote:
> On Sat, 5 Nov 2022 17:59:45 -0700 (PDT), Lamont Cranston
> <amdx62@gmail.com> wrote:
> 
>>   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?
>>                                  Mikek
>>   
> 
> Is the fet connected right?

And/or is it still a working FET?

If the gate isolation has been damaged then current will leak from it.

Even under the most pessimistic assumptions about wrong resistor values 
its input impedance should be at least R3 ~ 20M (or whatever value is there)

OP may want to try out the much simpler idealised common drain circuit 
to see if his FET is still a working FET. Rin 33k says that it isn't.

https://en.wikipedia.org/wiki/Common_drain

My instinct is that the circuit as built cannot be the same as the 
circuit diagram since the T1 pins are connected to nodes with voltages

S	24v
G	3.8v (via R3= 20M) measured 1.5v but should be ~4.5v
D       8.55v

I reckon path AB Gate to Drain now has a ~30k resistance.
But I can't explain the low DC voltage at A.


-- 
Regards,
Martin Brown

On Saturday, November 5, 2022 at 8:39:30 PM UTC-5, Ricky wrote:

> I'm no expert at this sort of stuff, but it does appear to be a high impedance input to me. The voltage follower with a bit of positive feedback should boost the impedance significantly. The 100pF cap would provide more feedback at higher frequencies, raising the input impedance further. 
> 
> It will be interesting to see what others have to say. 

> Rick C. 

My method to check input impedance:  I install a resistor in series between the 50Ω source and the input capacitor, when the output drops by 1/2, I have found my input impedance. I used a 1206 smd resistor.
 Is this wrong? 
    Mikek

On Saturday, November 5, 2022 at 11:22:27 PM UTC-5, John Larkin wrote:

> >
> Is the fet connected right?

 I always write the pin out on the bag, so, I verified I wrote it correctly from a data sheet (3). I then rechecked the mounting on the pcb.
I have it right according to the data sheets. Although my cheap Chinese Tester reads out the Drain and Source reversed from the datasheet.
            Mikek

On 06/11/2022 11:10, Lamont Cranston wrote:
> On Saturday, November 5, 2022 at 11:22:27 PM UTC-5, John Larkin wrote:
> 
>>>
>> Is the fet connected right?
> 
>   I always write the pin out on the bag, so, I verified I wrote it correctly from a data sheet (3). I then rechecked the mounting on the pcb.
> I have it right according to the data sheets. Although my cheap Chinese Tester reads out the Drain and Source reversed from the datasheet.
>              Mikek

Your chinese tester is not at fault - in most small jfets drain and 
source are interchangeable and symmetric.

piglet

On Sunday, November 6, 2022 at 4:47:24 AM UTC-6, Martin Brown wrote:

> >> 
> > 
> > Is the fet connected right?
> And/or is it still a working FET? 
> 
> If the gate isolation has been damaged then current will leak from it. 
> 
> Even under the most pessimistic assumptions about wrong resistor values 
> its input impedance should be at least R3 ~ 20M (or whatever value is there) 
> 
> OP may want to try out the much simpler idealised common drain circuit 
> to see if his FET is still a working FET. Rin 33k says that it isn't. 
> 
> https://en.wikipedia.org/wiki/Common_drain 
> 
> My instinct is that the circuit as built cannot be the same as the 
> circuit diagram since the T1 pins are connected to nodes with voltages 
> 
> S 24v 
> G 3.8v (via R3= 20M) measured 1.5v but should be ~4.5v 

If you do the calculations, you will find that adding a 10MΩ input meter on the gate you will measure 1.5V, 
because of the loading caused my the meter.

> D 8.55v 
> 
> I reckon path AB Gate to Drain now has a ~30k resistance. 
> But I can't explain the low DC voltage at A. 

I did, just above.

> Regards, 
> Martin Brown

I'll try changing the FET, again.
       Mikek

On Sunday, November 6, 2022 at 5:19:55 AM UTC-6, erichp...@hotmail.com wrote:
> On 06/11/2022 11:10, Lamont Cranston wrote: 
> > On Saturday, November 5, 2022 at 11:22:27 PM UTC-5, John Larkin wrote: 
> > 
> >>> 
> >> Is the fet connected right? 
> > 
> > I always write the pin out on the bag, so, I verified I wrote it correctly from a data sheet (3). I then rechecked the mounting on the pcb. 
> > I have it right according to the data sheets. Although my cheap Chinese Tester reads out the Drain and Source reversed from the datasheet. 
> > Mikek
> Your chinese tester is not at fault - in most small jfets drain and 
> source are interchangeable and symmetric. 
> 
> piglet

 I figured that.
              Mikek