2
\$\begingroup\$

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!

\$\endgroup\$
  • \$\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 '13 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 '13 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 '13 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 '13 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 '13 at 17:52
4
\$\begingroup\$

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.

\$\endgroup\$
  • \$\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 '13 at 6:52

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.