TLE2301_15 Datasheet, PDF (17/23 Pages) Texas Instruments – EXCALIBUR 3-STATE-OUTPUT WIDE-BANDWIDTH POWER OPERATIONAL AMPLIFIER

TLE2301
EXCALIBUR 3-STATE-OUTPUT WIDE-BANDWIDTH
POWER OPERATIONAL AMPLIFIER
SLOS131 – DECEMBER 1993
APPLICATION INFORMATION
coupling capacitor
With such a wide frequency band, the quality factor of the coupling filter needs to be low in order to avoid
unacceptably large attenuation at the band edges and to achieve a good coupling performance that is
insensitive to a wide range of loads. For a band-pass filter of this configuration, the quality factor is proportional
+ + to the reciprocal of the coupling capacitance. For low Q, the value of CC needs to be large.
Q quality factor and CC coupling capacitor
Q
∝
1
CC
Counterbalancing this need for a large value of C_C creates two more considerations. First, the capacitance
should not be so large as to allow significant 50-Hz mains current through the transformer ( I = 2 × π × f × C_C
× V). Second, the coupling capacitor is required to meet certain safety standards. The coupling capacitor is
actually an RFI-suppression capacitor that has been designed by the manufacturers to provide an adequate
level of protection when connected across the various conductors of the mains supply (consult the UL1283 or
UL1414 standards for RFI capacitors). These types of capacitors can be expensive, physically large, restricted
in capacitance value, and limited in the number of manufacturers.
As a reasonable compromise between all these factors, a coupling capacitor of 470 nF is chosen. This value
is multisourced, moderately priced, limits the mains current through the transformer to less than 36 mA rms, and
has sufficient capacitance to form the desired low-Q filter.
transformer leakage inductance
The transformer leakage inductance, inherent to the transformer, can be used to form an LC band-pass filter.
If the capacitor alone is used to couple onto the mains network, its capacitance value needs to be quite large
for it to have a reasonably low reactance at the signalling frequencies. Forming an LC filter greatly reduces the
value of capacitor required. The center frequency of the filter is not the same as the midband frequency of
65 kHz. Band-pass filters show a symmetrical shape only when plotted against the logarithm of frequency, so
+ Ǹ ^{the center frequency (fo) is given by the following formula:}
fo
flower fupper
+ ^Ǹ(40 90) kHz
+ 60 kHz
The leakage inductance of the transformer, as viewed from the winding connected to the coupling capacitor,
is derived from 2 π f_O = 1 / √ LC. The required leakage inductance of the transformer is:
+ L
1
(2πfo) ² CC
+
1
( 2 π 60 kHz )2
470 nF
+ 15 µH
Transformer Leakage Inductance
Capacitor
Figure 25. Band-Pass Coupling Filter
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