Help with high input impedance amp.

On 06/11/2022 10:47, Martin Brown wrote:
> But I can't explain the low DC voltage at A.
> 
> 

The 10meg loading of the voltmeter might be the cause?

piglet

On 11/6/2022 2:59, 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
>    
> 

For the low input impedance I would suspect that the FET you use
turns up forward biased gate-source, can't think of another
explanation if the FET is healthy etc.

On 06/11/2022 10:08, piglet wrote:
> 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
> 
> 

Other things to try are:

1. Return R5 to ground instead of the emitter (possibly increasing its 
value too).

2. Lift one end of C3

3. Bootstrap C3 from the emitter rather than source

4. Are you sure there are no parasitic oscillations at UHF

piglet

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

You have something in the drain of T1 introducing excessive negative feedback on the gate drive. That's only way to explain the combination low input impedance and low frequency gain rolloff. If you can't do better with your layout, install a high frequency decoupling capacitor at T1 drain to ground there.

On Sunday, November 6, 2022 at 5:35:33 AM UTC-6, erichp...@hotmail.com wrote:
> On 06/11/2022 10:08, piglet wrote: 
> > 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 
> > 
> >
> Other things to try are: 
> 
> 1. Return R5 to ground instead of the emitter (possibly increasing its 
> value too). 
> 
> 2. Lift one end of C3 
> 
> 3. Bootstrap C3 from the emitter rather than source 
> 
> 4. Are you sure there are no parasitic oscillations at UHF 
> 
> piglet

 Starting with oscillation, the signal looks clean.
 What should I expect when I lift C3? 
 Voltage rolloff at 32MHz is  1 to 0.65 and when C3 is lifted I get 1 to 0.9.
 So, high frequency response improves without C3 in the circuit.
                       Mikek

On Sunday, November 6, 2022 at 7:22:24 AM UTC-6, Fred Bloggs wrote:
> 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
> You have something in the drain of T1 introducing excessive negative feedback on the gate drive. That's only way to explain the combination low input impedance and low frequency gain rolloff. If you can't do better with your layout, install a high frequency decoupling capacitor at T1 drain to ground there.

 I installed a small cap right at the drain to ground, no change.
 I'm posting a picture of the PCB,  to learn, not for harassment. :-) Tempted to shrink the  the picture, but no.
Note: I have changed semi conductors about 6 times.
https://www.dropbox.com/s/w8psnbud9b6z2se/PCB.jpg?dl=0
I have had the A, B, and C dc voltages vary over the various T1, Q2  changes, as I write I get A=2.4, B= 6.5 and C=5.8.
 My first dc measurements were A=1.48 B=8.55 and C=7.83.   I have also had A=1.0, B= 5.24, and C=4.63.  (A is altered by the 10MΩ meter impedance.)
This doesn't have much effect on Gain.
                              Mikek

BTW, I added  a series resistor and a 21V zener to lower the B+ from 24.2V. 
The BF256C is rated at 25V,  so just as a precaution I lowered it.

                           Mikek

On 06/11/2022 2:22 pm, Lamont Cranston wrote:
> On Sunday, November 6, 2022 at 7:22:24 AM UTC-6, Fred Bloggs wrote:
>> 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
>> You have something in the drain of T1 introducing excessive negative feedback on the gate drive. That's only way to explain the combination low input impedance and low frequency gain rolloff. If you can't do better with your layout, install a high frequency decoupling capacitor at T1 drain to ground there.
> 
>   I installed a small cap right at the drain to ground, no change.
>   I'm posting a picture of the PCB,  to learn, not for harassment. :-) Tempted to shrink the  the picture, but no.
> Note: I have changed semi conductors about 6 times.
> https://www.dropbox.com/s/w8psnbud9b6z2se/PCB.jpg?dl=0
> I have had the A, B, and C dc voltages vary over the various T1, Q2  changes, as I write I get A=2.4, B= 6.5 and C=5.8.
>   My first dc measurements were A=1.48 B=8.55 and C=7.83.   I have also had A=1.0, B= 5.24, and C=4.63.  (A is altered by the 10MΩ meter impedance.)
> This doesn't have much effect on Gain.
>                                Mikek

Thanks for the PCB photo - you said you determine input impedance by 
adding series resistors, I cannot see them in the photo, where do you 
place them?

piglet

On 06/11/2022 2:22 pm, Lamont Cranston wrote:
> On Sunday, November 6, 2022 at 7:22:24 AM UTC-6, Fred Bloggs wrote:
>> 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
>> You have something in the drain of T1 introducing excessive negative feedback on the gate drive. That's only way to explain the combination low input impedance and low frequency gain rolloff. If you can't do better with your layout, install a high frequency decoupling capacitor at T1 drain to ground there.
> 
>   I installed a small cap right at the drain to ground, no change.
>   I'm posting a picture of the PCB,  to learn, not for harassment. :-) Tempted to shrink the  the picture, but no.
> Note: I have changed semi conductors about 6 times.
> https://www.dropbox.com/s/w8psnbud9b6z2se/PCB.jpg?dl=0
> I have had the A, B, and C dc voltages vary over the various T1, Q2  changes, as I write I get A=2.4, B= 6.5 and C=5.8.
>   My first dc measurements were A=1.48 B=8.55 and C=7.83.   I have also had A=1.0, B= 5.24, and C=4.63.  (A is altered by the 10MΩ meter impedance.)
> This doesn't have much effect on Gain.
>                                Mikek

BTW you really shouldn't need that huge tantalum bead input capacitor, 
with a hi-z input in the tens kilohoms even 1nF would still be good down 
to 10-20kHz. Tants have leakage and with the high DC impedance a few 
hundred nA cap leakage will be significant. I don't see how cap leakage 
could explain the phenomenon you see but it is a complication you can do 
without?

piglet

On 06/11/2022 2:50 pm, piglet wrote:
> On 06/11/2022 2:22 pm, Lamont Cranston wrote:
>> On Sunday, November 6, 2022 at 7:22:24 AM UTC-6, Fred Bloggs wrote:
>>> 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
>>> You have something in the drain of T1 introducing excessive negative 
>>> feedback on the gate drive. That's only way to explain the 
>>> combination low input impedance and low frequency gain rolloff. If 
>>> you can't do better with your layout, install a high frequency 
>>> decoupling capacitor at T1 drain to ground there.
>>
>>   I installed a small cap right at the drain to ground, no change.
>>   I'm posting a picture of the PCB,  to learn, not for harassment. :-) 
>> Tempted to shrink the  the picture, but no.
>> Note: I have changed semi conductors about 6 times.
>> https://www.dropbox.com/s/w8psnbud9b6z2se/PCB.jpg?dl=0
>> I have had the A, B, and C dc voltages vary over the various T1, Q2  
>> changes, as I write I get A=2.4, B= 6.5 and C=5.8.
>>   My first dc measurements were A=1.48 B=8.55 and C=7.83.   I have 
>> also had A=1.0, B= 5.24, and C=4.63.  (A is altered by the 10MΩ meter 
>> impedance.)
>> This doesn't have much effect on Gain.
>>                                Mikek
> 
> BTW you really shouldn't need that huge tantalum bead input capacitor, 
> with a hi-z input in the tens kilohoms even 1nF would still be good down 
> to 10-20kHz. Tants have leakage and with the high DC impedance a few 
> hundred nA cap leakage will be significant. I don't see how cap leakage 
> could explain the phenomenon you see but it is a complication you can do 
> without?
> 
> piglet
> 

Speaking of caps - how big is the output cap? The schematic you posted 
showed it as 0.1nF (which is 100pF) and seems way too small for a low-Z 
output. I'd suggest 0.1uF (100nF) as better and that looks like what you 
did in the PCB photo?

piglet