2 setting the speaker driver gain, Figure 2. speaker driver distortion, An168 – Cirrus Logic AN168 User Manual
Page 6
AN168
6
AN168REV2
The Appendix lists five example speaker driver
circuits that are suitable for full-duplex hands-free
systems.
1.3.2
Setting the Speaker Driver Gain
The speaker’s RATED power and the power driven
into the speaker are RMS powers. The RMS power
is given by the product of the RMS current and the
RMS voltage, or the square of the RMS voltage
over the speaker resistance.
The maximum speaker driver gain is determined by
the square root of the product of the RMS power
and the speaker driver resistance:
where P = RMS power delivered to speaker, V =
RMS voltage across speaker terminals, I = RMS
current through speaker, and R = resistance of
speaker in Ohms.
where Vin = full-scale voltage at the AO pin of the
CS6422, which is 1 Vrms, or 0 dBV.
Additionally, the gain can be expressed in dB using
the following relationship:
The following example shows how to derive the
gain required to drive 2 Watts of RMS power into
a 4
Ω speaker. Keep in mind that the RATED
power for this speaker should be 4 Watts or greater,
and the MAX power should be 8 Watts or greater.
Many speaker drivers suitable for hands-free
full-duplex design have fixed gains of 20 dB or
more, or are not stable for gains less than 20 dB.
Adding 20 dB of gain to the full-scale output of the
CS6422 (=1 Vrms, =0 dBV, =2.8 Vpp) results in a
huge signal at the speaker terminals (=10 Vrms,
=20 dBV, =28 Vpp). Because of this, the speaker
driver gain is implemented in two stages, an
attenuator stage followed by a gain stage. The
attenuator can be implemented using a simple
resistor voltage divider network as shown in
Figure 3.
VCC
GND
Speaker Driver
Rx Out
Vout
Figure 2. Speaker Driver Distortion
P
V
I
×
V
2
R
------
=
=
V
P
R
×
=
Gain
V
Vin
---------
=
Gain dB
(
)
20
Gain
(
)
log
×
=
P
I
V
×
V
2
R
------
=
=
V
P
R
Ч
2
4
Ч
8
2.828
=
=
=
=
Gain
V
Vin
---------
2.828
1
-------------
2.828
=
=
=
Gain dB
(
)
20
2.828
(
)
log
×
9dB
=
=