On Sunday, November 6, 2022 at 11:48:31 AM UTC-6, John Larkin wrote:> At 30 MHz, capacitance to the backside copper won't matter. At a > couple GHz, trace impedances might matter some. > > It might benefit from vias from top to bottom ground. I like 2-56 > screws, which are good ground lugs too.If the ground plane is causing some of the stray capacitance do I even want a ground plane? Mikek
Help with high input impedance amp.
Started by ●November 5, 2022
Reply by ●November 6, 20222022-11-06
Reply by ●November 6, 20222022-11-06
On 06/11/2022 17:48, John Larkin wrote:> On Sun, 6 Nov 2022 16:56:02 +0000, piglet <erichpwagner@hotmail.com> > wrote: > >> On 06/11/2022 15:03, Fred Bloggs wrote: >>> On Sunday, November 6, 2022 at 9:22:33 AM UTC-5, 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 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 >>>>> 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 10M? meter impedance.) >>>> This doesn't have much effect on Gain. >>>> Mikek >>> >>> Okay I was afraid of that. It's intrinsic to your FET/ construction, assuming your signal source is 50R or even remotely close. Looking at your circuit board, it looks like you may have significant coupling capacitance shunts to ac ground all over the place. The most damaging ones will be at high impedance nodes. That will do it. >> >> Good point. If the pcb is double sided with vast ground plane then the >> strays to ground might explain all. Perhaps Lamont/Mikek can try peeling >> off a section of groundplane under the gate nodes and measure if there >> is an improvement. >> >> piglet >> > > At 30 MHz, capacitance to the backside copper won't matter. At a > couple GHz, trace impedances might matter some. > > It might benefit from vias from top to bottom ground. I like 2-56 > screws, which are good ground lugs too. >Yes but OP wants the normally incompatible mix of high impedence and high frequency. Even 0.15pF is going to get Zin down to 30k at 30MHz and even a 0.1 inch by 0.1 inch pad over a ground plane is going to exceed that. piglet
Reply by ●November 6, 20222022-11-06
On 06/11/2022 17:31, Lamont Cranston wrote:> On Sunday, November 6, 2022 at 10:56:09 AM UTC-6, erichp...@hotmail.com wrote: >> On 06/11/2022 15:03, Fred Bloggs wrote: > >>> Okay I was afraid of that. It's intrinsic to your FET/ construction, assuming your signal source is 50R or even remotely close. Looking at your circuit board, it looks like you may have significant coupling capacitance shunts to ac ground all over the place. The most damaging ones will be at high impedance nodes. That will do it. >> Good point. If the pcb is double sided with vast ground plane then the >> strays to ground might explain all. Perhaps Lamont/Mikek can try peeling >> off a section of groundplane under the gate nodes and measure if there >> is an improvement. >> >> piglet > > The goal here is a high input impedance circuit flat from 50kHz to 30MHz. > I'm willing to make another board, should I just remove all the copper and have a positive and ground rail on either side. > I can superglue islands for the connection points. I punch and cut islands from Rogers 5880 1/16" teflon PCB and glue > it to the pcb. Or I can Dremel another board. > Should I use leaded components and space them off the board or just remove all the groundplane and stick with the surface mount resistors? > That's all I have, not smd caps or semiconductors. > > Also, is this a better circuit to try, this one is tried and proven, I think it is out of Linear's databook or some other company. > https://www.dropbox.com/s/6n78s84gd9kaouu/High%20impedance%20input%20-%20Copy.jpg?dl=0 > With the limitation of the parts I have BF256C, J310, 2N3819 2N5485 >> MPSH10, 2N3866, 2n4401, BC549. > Mikek > >Just lift the gate node a few mm off the board as a first try. All the other nodes are Lo-Z and can stay as they are. Could be easy to adapt your existing unit into that other design, major substantive difference is the bootstrapped drain. Try to have the gate up in the air. piglet
Reply by ●November 6, 20222022-11-06
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 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?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
Reply by ●November 6, 20222022-11-06
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 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?A gate-resistor greater than 10 MOhm on sFET is stupid.
Reply by ●November 6, 20222022-11-06
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 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?Bootstrapping only increases the impedance of the voltage devider.
Reply by ●November 6, 20222022-11-06
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
Reply by ●November 6, 20222022-11-06
On Sun, 6 Nov 2022 10:10:16 -0800 (PST), Lamont Cranston <amdx62@gmail.com> wrote:>On Sunday, November 6, 2022 at 11:48:31 AM UTC-6, John Larkin wrote: > >> At 30 MHz, capacitance to the backside copper won't matter. At a >> couple GHz, trace impedances might matter some. >> >> It might benefit from vias from top to bottom ground. I like 2-56 >> screws, which are good ground lugs too. > > If the ground plane is causing some of the stray capacitance do I even want a ground plane? > MikekFigure 15 pF per square inch for 0.062 thick FR4, side to side. That's tiny for, say, a 100x100 mil pad.
Reply by ●November 6, 20222022-11-06
Am 06.11.2022 um 20:47 schrieb Lamont Cranston:> 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 wrongThe 2N3644 is an PNP-type. So it is wrong.
Reply by ●November 6, 20222022-11-06
On Sun, 6 Nov 2022 18:55:02 +0000, piglet <erichpwagner@hotmail.com> wrote:>On 06/11/2022 17:48, John Larkin wrote: >> On Sun, 6 Nov 2022 16:56:02 +0000, piglet <erichpwagner@hotmail.com> >> wrote: >> >>> On 06/11/2022 15:03, Fred Bloggs wrote: >>>> On Sunday, November 6, 2022 at 9:22:33 AM UTC-5, 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 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 >>>>>> 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 10M? meter impedance.) >>>>> This doesn't have much effect on Gain. >>>>> Mikek >>>> >>>> Okay I was afraid of that. It's intrinsic to your FET/ construction, assuming your signal source is 50R or even remotely close. Looking at your circuit board, it looks like you may have significant coupling capacitance shunts to ac ground all over the place. The most damaging ones will be at high impedance nodes. That will do it. >>> >>> Good point. If the pcb is double sided with vast ground plane then the >>> strays to ground might explain all. Perhaps Lamont/Mikek can try peeling >>> off a section of groundplane under the gate nodes and measure if there >>> is an improvement. >>> >>> piglet >>> >> >> At 30 MHz, capacitance to the backside copper won't matter. At a >> couple GHz, trace impedances might matter some. >> >> It might benefit from vias from top to bottom ground. I like 2-56 >> screws, which are good ground lugs too. >> > >Yes but OP wants the normally incompatible mix of high impedence and >high frequency. Even 0.15pF is going to get Zin down to 30k at 30MHz and >even a 0.1 inch by 0.1 inch pad over a ground plane is going to exceed that. > >piglet >High impedances basically don't exist at high frequencies, certainly not with TO92 jfets. His input connector might be a couple of pF.
