Noise

V

CC

Output

Input

10 pF

33 pF

TLV1012

Noise pick-up by a microphone in cell phones is a well known problem. A conventional JFET circuit is sensitive

for noise pick-up because of its high output impedance, which is usually around 2.2 k

Ω.

RF noise is among other noises caused by nonlinear behavior. The nonlinear behavior of the amplifier at high

frequencies, well above the usable bandwidth of the device, causes AM demodulation of high-frequency signals.

The AM modulation contained in such signals folds back into the audio band, thereby disturbing the intended

microphone signal. The GSM signal of a cell phone is such an AM-modulated signal. The modulation frequency

of 216 Hz and its harmonics can be observed in the audio band. This kind of noise is called bumblebee noise.

RF noise caused by a GSM signal can be reduced by connecting two external capacitors to ground (see

Figure 7). One capacitor reduces the noise caused by the 900-MHz carrier, and the other reduces the noise

caused by 1800/1900 MHz.

Figure 7. RF Noise Reduction

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