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#1
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JFET biasing problem
Okay, like I said once before, I'm ignorant. But I am trying to fix that.
Attempting to bias 2N5486 JFET as shown in the diagram below. According to what I read, Vg should be 2.65 VDC, which it does measure to be with my Fluke. But Vs should be 5.92 or some such, and only measures 2.97, indicating I don't have the 7 or 8 mA I expected. Vgs measures -.08VDC which, to me, doesnt jive at all. Somebody please tell me, what am I missing here? Thanks, Dave |
#2
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JFET biasing problem
"Dave" wrote in message netamerica... Okay, like I said once before, I'm ignorant. But I am trying to fix that. Attempting to bias 2N5486 JFET as shown in the diagram below. According to what I read, Vg should be 2.65 VDC, which it does measure to be with my Fluke. But Vs should be 5.92 or some such, and only measures 2.97, indicating I don't have the 7 or 8 mA I expected. Vgs measures -.08VDC which, to me, doesnt jive at all. Somebody please tell me, what am I missing here? Not strictly the answer to your question - but it isn't easy to accomodate the spread of Idss/VGSoff with such a small headroom as 9V, you might find that using a single (bipolar) transistor current source instead of a source resistor works better than raising Vg above 0V. |
#3
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JFET biasing problem
"Dave" schreef in bericht netamerica... Okay, like I said once before, I'm ignorant. But I am trying to fix that. Attempting to bias 2N5486 JFET as shown in the diagram below. According to what I read, Vg should be 2.65 VDC, which it does measure to be with my Fluke. But Vs should be 5.92 or some such, and only measures 2.97, indicating I don't have the 7 or 8 mA I expected. Vgs measures -.08VDC which, to me, doesnt jive at all. Somebody please tell me, what am I missing here? Thanks, Dave You biased the FET like an NPN transistor. Using an N-channel FET for amplification requires the gate voltage kept below the source voltage. After a quick glance I'd day you'll have to remove the 3M6 resistor and increase the drain resistor to some k. petrus bitbyter |
#4
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JFET biasing problem
"petrus bitbyter" wrote in message ll.nl... "Dave" schreef in bericht netamerica... Okay, like I said once before, I'm ignorant. But I am trying to fix that. Attempting to bias 2N5486 JFET as shown in the diagram below. According to what I read, Vg should be 2.65 VDC, which it does measure to be with my Fluke. But Vs should be 5.92 or some such, and only measures 2.97, indicating I don't have the 7 or 8 mA I expected. Vgs measures -.08VDC which, to me, doesnt jive at all. Somebody please tell me, what am I missing here? Thanks, Dave You biased the FET like an NPN transistor. Using an N-channel FET for amplification requires the gate voltage kept below the source voltage. After a quick glance I'd day you'll have to remove the 3M6 resistor and increase the drain resistor to some k. Some appnotes suggest an elevated Vg as a means of managing the device spread, but its not viable with such limited headroom. |
#5
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JFET biasing problem
On Tue, 25 May 2010 11:51:04 -0500, "Dave" wrote:
Okay, like I said once before, I'm ignorant. But I am trying to fix that. Attempting to bias 2N5486 JFET as shown in the diagram below. According to what I read, Vg should be 2.65 VDC, which it does measure to be with my Fluke. But Vs should be 5.92 or some such, and only measures 2.97, indicating I don't have the 7 or 8 mA I expected. Vgs measures -.08VDC which, to me, doesnt jive at all. Somebody please tell me, what am I missing here? Thanks, Dave Jfets have absurd unit-to-unit variations. Pull the fet out and measure Idss. I bet it's about 4 mA. The spec range is 8 to 20, but with NTE one never knows... I assume everything is connected OK. Does it amplify? John |
#6
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JFET biasing problem
"John Larkin" wrote in message ... On Tue, 25 May 2010 11:51:04 -0500, "Dave" wrote: Okay, like I said once before, I'm ignorant. But I am trying to fix that. Attempting to bias 2N5486 JFET as shown in the diagram below. According to what I read, Vg should be 2.65 VDC, which it does measure to be with my Fluke. But Vs should be 5.92 or some such, and only measures 2.97, indicating I don't have the 7 or 8 mA I expected. Vgs measures -.08VDC which, to me, doesnt jive at all. Somebody please tell me, what am I missing here? Thanks, Dave Jfets have absurd unit-to-unit variations. Pull the fet out and measure Idss. I bet it's about 4 mA. The spec range is 8 to 20, but with NTE one never knows... I assume everything is connected OK. Does it amplify? John Hey John, thanks for the reply. Sorry it took me so long to get back onto this. Believe it or not I have been trying to eliminate variables and possibilities since I posted previously. You are right on the money, Idss is roughly 4mA. So I tried to work with that limitation and bias the device accordingly. Please see attached file... On my workbench, input to the device is is a 500mV P-P 10kHz square wave, but the device output is like 50mV. I have checked and double checked the configuration and all connections, replacing the device when I ran out of things to check. No change. I am posting the entire schematic, just for complete measure, cause I am obviously missing something but have no idea what it is. Output from the entire circuit is 500mV P-P 10kHz, so I figure that the rest of the circuit is at least trying to overcome the deficiencies of the first stage of pseudo-amplification. In case anyone is wondering, the 5M trimpot is intended to eliminate the possibility of poorly chosen values for R2, in the potential divider biasing setup. This way I can adjust it for maximum output, if it can be called that... Could my problem be the large values I am using for the potential divider on the NTE451? I read that they must be quite large to avoid other problems (slips my mind at the moment) but am now wondering. *Any* ideas are eagerly sought. Vg measures 3.4V, and Vgs measures approx .2V, but changing these values has no positive effect. Beginning to suspect that Ian was right, and there just isn't enough "space" for using a JFET. Wondering if I need to make the shift to 12VDC. Out of other ideas... Dave |
#7
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JFET biasing problem
On Fri, 28 May 2010 22:17:52 -0500, "Dave" wrote:
"John Larkin" wrote in message .. . On Tue, 25 May 2010 11:51:04 -0500, "Dave" wrote: Okay, like I said once before, I'm ignorant. But I am trying to fix that. Attempting to bias 2N5486 JFET as shown in the diagram below. According to what I read, Vg should be 2.65 VDC, which it does measure to be with my Fluke. But Vs should be 5.92 or some such, and only measures 2.97, indicating I don't have the 7 or 8 mA I expected. Vgs measures -.08VDC which, to me, doesnt jive at all. Somebody please tell me, what am I missing here? Thanks, Dave Jfets have absurd unit-to-unit variations. Pull the fet out and measure Idss. I bet it's about 4 mA. The spec range is 8 to 20, but with NTE one never knows... I assume everything is connected OK. Does it amplify? John Hey John, thanks for the reply. Sorry it took me so long to get back onto this. Believe it or not I have been trying to eliminate variables and possibilities since I posted previously. You are right on the money, Idss is roughly 4mA. So I tried to work with that limitation and bias the device accordingly. Please see attached file... On my workbench, input to the device is is a 500mV P-P 10kHz square wave, but the device output is like 50mV. I have checked and double checked the configuration and all connections, replacing the device when I ran out of things to check. No change. I am posting the entire schematic, just for complete measure, cause I am obviously missing something but have no idea what it is. Output from the entire circuit is 500mV P-P 10kHz, so I figure that the rest of the circuit is at least trying to overcome the deficiencies of the first stage of pseudo-amplification. In case anyone is wondering, the 5M trimpot is intended to eliminate the possibility of poorly chosen values for R2, in the potential divider biasing setup. This way I can adjust it for maximum output, if it can be called that... Could my problem be the large values I am using for the potential divider on the NTE451? I read that they must be quite large to avoid other problems (slips my mind at the moment) but am now wondering. *Any* ideas are eagerly sought. Vg measures 3.4V, and Vgs measures approx .2V, but changing these values has no positive effect. Beginning to suspect that Ian was right, and there just isn't enough "space" for using a JFET. Wondering if I need to make the shift to 12VDC. Out of other ideas... Dave Your second stage, the bipolar thing, is running at around 100 mA. Assuming a beta of 100, its base input impedance is ballpark 30 ohms. That's raelly loading down the fet stage. If the transconductance (Gm) of the fet were 0.01 siemens, the unloaded gain would be Gm * Rl = 0.01 * 1K = 10. Load it with 30 ohms and the gain is 0.01 * 30 = 0.3. Something like that. If that's an antenna on the left, the 100 ohms and the 1K pot are going to gobble up all the signal right at the front end. John |
#8
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JFET biasing problem
"flipper" wrote in message ... On Fri, 28 May 2010 21:47:33 -0700, John Larkin wrote: On Fri, 28 May 2010 22:17:52 -0500, "Dave" wrote: "John Larkin" wrote in message ... On Tue, 25 May 2010 11:51:04 -0500, "Dave" wrote: Okay, like I said once before, I'm ignorant. But I am trying to fix that. Attempting to bias 2N5486 JFET as shown in the diagram below. According to what I read, Vg should be 2.65 VDC, which it does measure to be with my Fluke. But Vs should be 5.92 or some such, and only measures 2.97, indicating I don't have the 7 or 8 mA I expected. Vgs measures -.08VDC which, to me, doesnt jive at all. Somebody please tell me, what am I missing here? Thanks, Dave Jfets have absurd unit-to-unit variations. Pull the fet out and measure Idss. I bet it's about 4 mA. The spec range is 8 to 20, but with NTE one never knows... I assume everything is connected OK. Does it amplify? John Hey John, thanks for the reply. Sorry it took me so long to get back onto this. Believe it or not I have been trying to eliminate variables and possibilities since I posted previously. You are right on the money, Idss is roughly 4mA. So I tried to work with that limitation and bias the device accordingly. Please see attached file... On my workbench, input to the device is is a 500mV P-P 10kHz square wave, but the device output is like 50mV. I have checked and double checked the configuration and all connections, replacing the device when I ran out of things to check. No change. I am posting the entire schematic, just for complete measure, cause I am obviously missing something but have no idea what it is. Output from the entire circuit is 500mV P-P 10kHz, so I figure that the rest of the circuit is at least trying to overcome the deficiencies of the first stage of pseudo-amplification. In case anyone is wondering, the 5M trimpot is intended to eliminate the possibility of poorly chosen values for R2, in the potential divider biasing setup. This way I can adjust it for maximum output, if it can be called that... Could my problem be the large values I am using for the potential divider on the NTE451? I read that they must be quite large to avoid other problems (slips my mind at the moment) but am now wondering. *Any* ideas are eagerly sought. Vg measures 3.4V, and Vgs measures approx .2V, but changing these values has no positive effect. Beginning to suspect that Ian was right, and there just isn't enough "space" for using a JFET. Wondering if I need to make the shift to 12VDC. Out of other ideas... Dave Your second stage, the bipolar thing, is running at around 100 mA. Assuming a beta of 100, its base input impedance is ballpark 30 ohms. That's raelly loading down the fet stage. If the transconductance (Gm) of the fet were 0.01 siemens, the unloaded gain would be Gm * Rl = 0.01 * 1K = 10. Load it with 30 ohms and the gain is 0.01 * 30 = 0.3. Something like that. If that's an antenna on the left, the 100 ohms and the 1K pot are going to gobble up all the signal right at the front end. John Plus. the source bypass rolloff is about 13kHz and the inter stage coupling is an order of magnitude worse. Q1 may not even be conducting, or not much, because Vgs tolerance for the 2N5486 extends to -6V. Wow. God. Thank you John, and thank you Flipper. I knew if I put the entire schematic out there, I would find out things I had no idea I was doing wrong. I knew that I didn't really know what I was doing, but I had no idea it was this bad. weak grin So anyways, the things you point out describe exactly what I am seeing. Sooo, let me try to replace the varactor with a real capacitor (getting rid of the 100 ohm resistor and the 1K pot). I would try to eliminate the attempt at tuning the input altogether, but am afraid of swamping the input with 60 Hz hum. This thing is supposed to be the part of a Fox/Hound setup that pulls a 10kHz signal out of a breaker box to indicate which breaker has the Fox plugged into it (remember that?) Not even sure where to start with the rest. Should I put another NTE451 in as the second stage, and use that to drive the final two stages as shown? (Making four transistors, rather than three.) Let me also back off on driving the NTE85 (audio preamp, can't remember what it subs for) so hard, to make it easier for the FETs to do their thing. Thanks so much for the feedback and help. It's been nearly 30 years since I was in school, and I think I've forgotten everything I knew at one time. Take it easy... Dave |
#9
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JFET biasing problem
On Sat, 29 May 2010 08:09:46 -0500, "Dave" wrote:
"flipper" wrote in message .. . On Fri, 28 May 2010 21:47:33 -0700, John Larkin wrote: On Fri, 28 May 2010 22:17:52 -0500, "Dave" wrote: "John Larkin" wrote in message m... On Tue, 25 May 2010 11:51:04 -0500, "Dave" wrote: Okay, like I said once before, I'm ignorant. But I am trying to fix that. Attempting to bias 2N5486 JFET as shown in the diagram below. According to what I read, Vg should be 2.65 VDC, which it does measure to be with my Fluke. But Vs should be 5.92 or some such, and only measures 2.97, indicating I don't have the 7 or 8 mA I expected. Vgs measures -.08VDC which, to me, doesnt jive at all. Somebody please tell me, what am I missing here? Thanks, Dave Jfets have absurd unit-to-unit variations. Pull the fet out and measure Idss. I bet it's about 4 mA. The spec range is 8 to 20, but with NTE one never knows... I assume everything is connected OK. Does it amplify? John Hey John, thanks for the reply. Sorry it took me so long to get back onto this. Believe it or not I have been trying to eliminate variables and possibilities since I posted previously. You are right on the money, Idss is roughly 4mA. So I tried to work with that limitation and bias the device accordingly. Please see attached file... On my workbench, input to the device is is a 500mV P-P 10kHz square wave, but the device output is like 50mV. I have checked and double checked the configuration and all connections, replacing the device when I ran out of things to check. No change. I am posting the entire schematic, just for complete measure, cause I am obviously missing something but have no idea what it is. Output from the entire circuit is 500mV P-P 10kHz, so I figure that the rest of the circuit is at least trying to overcome the deficiencies of the first stage of pseudo-amplification. In case anyone is wondering, the 5M trimpot is intended to eliminate the possibility of poorly chosen values for R2, in the potential divider biasing setup. This way I can adjust it for maximum output, if it can be called that... Could my problem be the large values I am using for the potential divider on the NTE451? I read that they must be quite large to avoid other problems (slips my mind at the moment) but am now wondering. *Any* ideas are eagerly sought. Vg measures 3.4V, and Vgs measures approx .2V, but changing these values has no positive effect. Beginning to suspect that Ian was right, and there just isn't enough "space" for using a JFET. Wondering if I need to make the shift to 12VDC. Out of other ideas... Dave Your second stage, the bipolar thing, is running at around 100 mA. Assuming a beta of 100, its base input impedance is ballpark 30 ohms. That's raelly loading down the fet stage. If the transconductance (Gm) of the fet were 0.01 siemens, the unloaded gain would be Gm * Rl = 0.01 * 1K = 10. Load it with 30 ohms and the gain is 0.01 * 30 = 0.3. Something like that. If that's an antenna on the left, the 100 ohms and the 1K pot are going to gobble up all the signal right at the front end. John Plus. the source bypass rolloff is about 13kHz and the inter stage coupling is an order of magnitude worse. Q1 may not even be conducting, or not much, because Vgs tolerance for the 2N5486 extends to -6V. Wow. God. Thank you John, and thank you Flipper. I knew if I put the entire schematic out there, I would find out things I had no idea I was doing wrong. I knew that I didn't really know what I was doing, but I had no idea it was this bad. weak grin So anyways, the things you point out describe exactly what I am seeing. Sooo, let me try to replace the varactor with a real capacitor (getting rid of the 100 ohm resistor and the 1K pot). I would try to eliminate the attempt at tuning the input altogether, but am afraid of swamping the input with 60 Hz hum. This thing is supposed to be the part of a Fox/Hound setup that pulls a 10kHz signal out of a breaker box to indicate which breaker has the Fox plugged into it (remember that?) Not even sure where to start with the rest. Should I put another NTE451 in as the second stage, and use that to drive the final two stages as shown? (Making four transistors, rather than three.) Let me also back off on driving the NTE85 (audio preamp, can't remember what it subs for) so hard, to make it easier for the FETs to do their thing. Thanks so much for the feedback and help. It's been nearly 30 years since I was in school, and I think I've forgotten everything I knew at one time. Take it easy... Dave A 10 KHz resonant LC would be a good front end. Then a fet-input opamp maybe. That could drive a headphone directly, or maybe one more gain stage for a speaker. If you want to go with discretes, consider Tuned LC Jfet follower audio-preamp type bipolar amp stages, low current at first. John |
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