Help with high input impedance amp....

[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 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

 

[Ricky]

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.

- Get 1,000 miles of free Supercharging
- Tesla referral code - https://ts.la/richard11209

 

[John Larkin]

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?

 

[piglet]

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

 

[Martin Brown]

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

 

[Lamont Cranston]

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

 

[Lamont Cranston]

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

 

[piglet]

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

 

[Lamont Cranston]

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

 

[Lamont Cranston]

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

 

[piglet]

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

 

[Dimiter_Popoff]

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.

 

[piglet]

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

 

[Fred Bloggs]

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.

 

[Lamont Cranston]

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

 

[Lamont Cranston]

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

 

[Lamont Cranston]

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

 

[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

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

 

[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

 

[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