THS6092SLASH3 datasheet(26/46 Pages) TI1

26 / 46 page

THS6042, THS6043

350 mA, ±12 V ADSL CPE LINE DRIVERS

SLOS264G − MARCH 2000 − REVISED DECEMBER 2001

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

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 (Rf) Values for Optimum Frequency Response

GAIN

RL = 25 Ω

RL = 100 Ω

GAIN

VCC = ±6 V

VCC = ±12 V

VCC = ±6 V

VCC = ±12 V

680

Ω

560

Ω

620

Ω

510

Ω

2, −1

470

Ω

430

Ω

430

Ω

390

Ω

270

Ω

240

Ω

270

Ω

240

Ω

200

Ω

200

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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