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I am using a class A amplifier constructed around an NPN Darlington transistor (BC517) because the gain I was getting with a single NPN transistor was nowhere close to what I needed. The transistor is biased with two 100K resistor going from collector to base and base to emitter. Instead of a load, I have a 10K resistor, a 100K resistor from the collector drives the input to a comparator (this class A amp is being used as the input stage to a ADC I built). The input comes from an electret mic coupled to the base via a .1uF cap (I have given the mic a proper voltage bias with a 10K resistor). There is some small radio interference with the ADC, which I know how to deal with, but the main problem is that I am getting a massive amount of interference from a nearby AM radio station with my class A amp. The radio waves are being picked up at about the same volume as the sound from the mic. This will not do, so how can I fix it? Should I use a different kind of amplifier (it needs high gain). Is there a way to suppress interference in my existing design?

schematic

simulate this circuit – Schematic created using CircuitLab

Many thanks!

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  • \$\begingroup\$ How much gain? Could we have a circuit diagram? The usual technique for avoiding this problem is to make the entire signal path differential, so common-mode noise gets cancelled out. \$\endgroup\$
    – pjc50
    Nov 28, 2013 at 10:58
  • \$\begingroup\$ I happen to have open this app note, which is somewhat relevant: maximintegrated.com/app-notes/index.mvp/id/3573 \$\endgroup\$
    – pjc50
    Nov 28, 2013 at 11:00
  • \$\begingroup\$ Because this phenomena sounds quite odd to me, I'm wondering if it could be any aliasing effect of the ADC. I guess you have a low-pass filter before the ADC, with appropriate cut-off frequency, but please confirm. Additional question, how do yo know the interferences come from AM radio station? \$\endgroup\$
    – RawBean
    Nov 28, 2013 at 14:49
  • \$\begingroup\$ @RawBean I can hear them talking about football, so I'm sure it's from the AM station XD. \$\endgroup\$
    – Void Star
    Nov 28, 2013 at 17:50
  • \$\begingroup\$ @pjc50 I will edit my post to include a circuit diagram, and I will read that note and see if it applies to my situation. \$\endgroup\$
    – Void Star
    Nov 28, 2013 at 17:52

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I think you are picking up AM radio interference because of the relatively high resistances you are using. High resistances make it easier for a low energy signal like a radio wave to imnpress a voltage on a circuit. I would suggest 2 remedies. One is to reduce all of your resistors by a factor of 10. Why do you need to put such a large resistor (R4) in series with your output? Another remedy is to put a small bypass capacitor across R2 to filter out the AM signal. If you reduce R2 to 10k, then a capacitor of about 200 pf should be sufficient. If you reduce R2 to 10k, then use a capacitor of about 2000 pf.

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  • \$\begingroup\$ Sorry it took me so long to test this, Thanksgiving break ended and finals started. I am using a slightly different circuit now, but making resistors smaller and adding a capacitor on the base of the transistor did do wonders for smiting that bothersome AM interference. Thanks! \$\endgroup\$
    – Void Star
    Dec 14, 2013 at 6:52

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