TLE2301

EXCALIBUR 3-STATE-OUTPUT WIDE-BANDWIDTH

POWER OPERATIONAL AMPLIFIER

SLOS131 – DECEMBER 1993

17

POST OFFICE BOX 655303

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

Q

+ quality factor and C

C +

coupling capacitor

Q

∝ 1

C

C

should not be so large as to allow significant 50-Hz mains current through the transformer ( I = 2

× 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

fo +

f

lower

fupper

+ 60 kHz

+ (40

90 ) kHz

The leakage inductance of the transformer, as viewed from the winding connected to the coupling capacitor,

is derived from 2

+ 15 µH

L

+

1

(2

πfo)2 C

C

+

1

(2

π

60 kHz )2

470 nF

Transformer Leakage Inductance

Capacitor

Figure 25. Band-Pass Coupling Filter