Author Topic: Help NFB amp  (Read 1036 times)

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Offline Qmavam

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Help NFB amp
« on: April 27, 2017, 12:26:02 am »
  Hi guys,
 I am trying a different amp because the last one rolled off 1.3 db at 10Mz.
I built the amp on this page. I only put the first 2 transistors in and not the output buffer.
This amp also rolls of 1.1db at 10 MHz. Referenced to 1MHz.
http://chemandy.com/technical-articles/building-hf-amplifier/building-hf-amplifier-project-article1.htm
 This amp is supposed to go out to 30MHz, I don't know if it could be my layout, my 2N3866 transistors,
or if I just need to adjust the feedback,
I have attached a picture of my layout.
 Note, I have PCB pads under the solder joints, except those that go to ground.
                                                  Mikek
 

Offline w2aew

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Re: Help NFB amp
« Reply #1 on: April 27, 2017, 01:42:30 am »
If you didn't include the output buffer, it is likely that the output stage of this circuit isn't up to the task of whatever you connected to it to test it (output impedance too high).  It might be just fine if you include the output buffer, to preserve the BW of the previous stage.
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Offline Qmavam

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Re: Help NFB amp
« Reply #2 on: April 27, 2017, 02:34:55 am »
If you didn't include the output buffer, it is likely that the output stage of this circuit isn't up to the task of whatever you connected to it to test it (output impedance too high).  It might be just fine if you include the output buffer, to preserve the BW of the previous stage.

 I'm just monitoring it with my scope, no loading.
 

Offline Audioguru

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Re: Help NFB amp
« Reply #3 on: April 27, 2017, 02:21:46 pm »
The input cable of your 'scope has a capacitance to ground that cuts high frequencies when the output impedance of the circuit is higher than with the low output impedance buffer.
The parts on your pcb are too far apart that adds stray capacitance.
Stray capacitance around the parts also cut high frequencies.
 

Offline Qmavam

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Re: Help NFB amp
« Reply #4 on: April 27, 2017, 04:15:47 pm »
The input cable of your 'scope has a capacitance to ground that cuts high frequencies when the output impedance of the circuit is higher than with the low output impedance buffer.

  Ya, but, it a 300ohm... ya, but nothing! 15pf at 10 MHz is 1,061?s.
2.95 volts into a 330?--1061? voltage divider is 2.3 volts out.
 I didn't post it here, but I have 2.95 volts output at 1MHz and 2.3 volts
output a 10MHz. That's a pretty close correlation!
 Hmm, maybe my other amp was OK.
Well how the heck am I going to be able to measure this thing.
 I guess I just need to stick the Diode Detector on it and see
 how linear my output is with frequency.

Quote

The parts on your pcb are too far apart that adds stray capacitance.
Stray capacitance around the parts also cut high frequencies.
I could probably tighten it up some, but if I'm going to do that, I might as well go to SMD.
 Back to the bench.
Thanks very much for the advice, I was aware of this, but I'm so used to working at 1MHz, that
I didn't even consider that could be the problem!
                                Thanks,  Mikek
« Last Edit: April 27, 2017, 10:40:57 pm by Qmavam »
 


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