[?] Audio amplifiers and GSM interference.

On Mon, 16 Jan 2006 21:24:11 -0800, "Walter Harley"
<[email protected]> wrote:

>"David Chapman" <[email protected]> wrote in message
>news:[email protected]...
>>
>[...]
>> When designing an audio amplifier with high immunity to such
>> interference, I can see the obvious value of adding suitable RF
>> filtering (ferrite chokes and shunt capacitors with low-Z in the UHF
>> band) to the input circuitry but wonder if a pre-amplifier using
>> differential inputs (FET or Bipolar ?) rather than single-ended would be
>> better. Transformer coupling of source to the amplifier input is another
>> possibility, of course.
>
>Here's my understanding; maybe someone more knowledgeable will correct me:
>
>Differential amplifiers reduce interference by cancelling the common mode
>signal. For that to work, the interference has to be a common mode signal.
>In general, this is achieved by balancing the impedance between the two legs
>of the signal and the interference source (which is notionally
>ground-referenced), so that the interference couples equally to both legs.
>
>If your signal is already unbalanced (meaning that the impedance of the two
>legs to ground is not the same), then a differential amplifier will not help
>cancel interference. (Interference rejection in balanced transmission has
>nothing to do with whether the "cold" leg is carrying an opposite-polarity
>voltage, as should be obvious if you consider the case where the signal is
>0V.)
>
>
>Something that has always confused me; maybe someone can explain:
>
>How can ferrites help, when input impedances are > 1k, as in much audio
>circuitry? I thought ferrite impedance was around 100 ohms or so, max. I
>wouldn't think it would be able to create enough voltage drop to make a
>difference. Is there something else going on?
>

At RF frequencies, trace impedances and IC input impedances are low.
At 1 GHz, an opamp input impedance will be ballpark 50 ohms.

And a ferrite bead is a lossy inductor, so it kills resonances, and
resonances are a major culprit here.

You can get surface-mount beads that are 600 ohms at 100 MHz, more at
higher frequencies. A bead followed by a capacitor is an excellent
lowpass filter.

John

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In message <[email protected]>
John Larkin <[email protected]> wrote:

> You can get surface-mount beads that are 600 ohms at 100 MHz, more at
> higher frequencies. A bead followed by a capacitor is an excellent
> lowpass filter.

ONLY IF the trace lengths to the capacitor are short.

Dave

Dave Higton wrote:
> In message <[email protected]>
> John Larkin <[email protected]> wrote:
>
>
>>You can get surface-mount beads that are 600 ohms at 100 MHz, more at
>>higher frequencies. A bead followed by a capacitor is an excellent
>>lowpass filter.

and it wont oscillate

>
>
> ONLY IF the trace lengths to the capacitor are short.
>
> Dave

maybe, maybe not. depends on how you do it. a "T" connection to the cap
(with a nice long stalk) is terrible, the stalk inductance is
detrimental, whereas a "V" subsumes the trace inductance into the filter

plus the cap inductance means you can only asymptotically approach some
minimum L. hence 0508 caps, interdigitated leads etc.

cheers
Terry