THS6042 Datasheet, PDF (26/33 Pages) Texas Instruments – 350 mA, ±12 V ADSL CPE LINE DRIVERS

THS6042, THS6043
350 mA, ±12 V ADSL CPE LINE DRIVERS
SLOS264G – MARCH 2000 – REVISED DECEMBER 2001
APPLICATION INFORMATION
recommended feedback and gain resistor values
As with all current feedback amplifiers, the bandwidth of the THS6042/3 is an inversely proportional function
of the value of the feedback resistor. This can be seen from Figures 1 to 6. The recommended resistors for the
optimum frequency response are shown in Table 1. These should be used as a starting point and once optimum
values are found, 1% tolerance resistors should be used to maintain frequency response characteristics.
Because there is a finite amount of output resistance of the operational amplifier, load resistance can play a
major part in frequency response. This is especially true with these drivers, which tend to drive low-impedance
loads. This can be seen in Figures 1–6. As the load resistance increases, the output resistance of the amplifier
becomes less dominant at high frequencies. To compensate for this, the feedback resistor may need to be
changed. For most applications, a feedback resistor value of 750 Ω is recommended, which is a good
compromise between bandwidth and phase margin that yields a very stable amplifier.
Table 1. Recommended Feedback (R_f) Values for Optimum Frequency Response
GAIN
1
2, –1
4
8
RL = 25 Ω
VCC = ±6 V VCC = ±12 V
680 Ω
560 Ω
470 Ω
430 Ω
270 Ω
240 Ω
200 Ω
200 Ω
RL = 100 Ω
VCC = ±6 V VCC = ±12 V
620 Ω
510 Ω
430 Ω
390 Ω
270 Ω
240 Ω
200 Ω
200 Ω
Consistent with current feedback amplifiers, increasing the gain is best accomplished by changing the gain
resistor, not the feedback resistor. This is because the bandwidth of the amplifier is dominated by the feedback
resistor value and internal dominant-pole capacitor. The ability to control the amplifier gain independently of the
bandwidth constitutes a major advantage of current feedback amplifiers over conventional voltage feedback
amplifiers. Therefore, once a frequency response is found suitable to a particular application, adjust the value
of the gain resistor to increase or decrease the overall amplifier gain.
Finally, it is important to realize the effects of the feedback resistance on distortion. Increasing the resistance
decreases the loop gain and may increase the distortion. Decreasing the feedback resistance too low may
increase the bandwidth, but an increase in the load on the output may cause distortion to increase instead of
decreasing. It is also important to know that decreasing load impedance increases total harmonic distortion
(THD). Typically, the third order harmonic distortion increases more than the second order harmonic distortion.
This is illustrated in Figure 10 to 12 and Figures 16 to 18.
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