Telit EVK2 User Guide
1vv0300704 Rev.13- 2012/06/25
Reproduction forbidden without Telit Communications S.p.A. written authorization - All Rights Reserved
page
30 of 149
7.3.2.
Benefits and disadvantages
Differential amplifiers are desirable to use, especially in audio applications where signal
levels are very low such as those from microphones.
Classically, the benefits obtained from differential amplification are:
Increase of Common Mode Rejection Ratio (CMRR)
Differential inputs enable cancellation of any noise common on both inputs. Noise
generated at the input of the amplifier has a greater effect than noise generated at
the output, because any noise on the input is multiplied by the gain of the amplifier.
Increase Signal to Noise Ratio (SNR)
The inputs to the amplifier are especially sensitive to noise because they are
typically not driven by a very low impedance source.
High Rejection in Electromagnetic Interference (EMI)
Noise immunity is very important in wireless phones because the RF signal is sent
in bursts such that the frequency between bursts is in the audio band. RF
rectification is such a problem that many manufacturers shield the audio portion of
the phone.
Double Useful signals level
The signal levels from microphone and the voltage swing to the load are doubled.
Then the AF power to the load it is 4 times the single-ended Afpower at the same
voltage supply.
No output blocking capacitor is needed
Even if the differential outputs are biased at half-supply; no DC voltage exists
across the load. You do not need the big, expensive and heavy blocking capacitors
(
generally from 33 μF to 1000 μF
), lowering the cost and saving PCB space
There is no frequency limiting effect due to the high pass filter network created with
the speaker impedance and the coupling capacitance.
Less shielding is required from amplifier to load
Mainly we have only one disadvantage using differential amplification: the routing of one
more signal line could be more difficult and the additional trace requires more board space.
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