THS6002

DUAL DIFFERENTIAL LINE DRIVERS AND RECEIVERS

SLOS202D– JANUARY 1998– REVISED JULY 1999

27

POST OFFICE BOX 655303

APPLICATION INFORMATION

recommended feedback and gain resistor values

As with all current feedback amplifiers, the bandwidth of the THS6002 is an inversely proportional function of

the value of the feedback resistor. This can be seen from Figures 17 and 18. For the driver, the recommended

resistors for the optimum frequency response for a 25-

Ω load system are 680-Ω for a gain = 1 and 620-Ω for

a gain = 2 or –1. For the receivers, the recommended resistors for the optimum frequency response are

560

Ω for a gain = 1 and 390 Ω for a gain = 2 or –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 the drivers, which tend to drive low-impedance

loads. This can be seen in Figure 7, Figure 19, and Figure 20. As the load resistance increases, the output

resistance of the amplifier becomes less dominant at high frequencies. To compensate for this, the feedback

resistor should change. For 100-

Ω loads, it is recommended that the feedback resistor be changed to 820 Ω

for a gain of 1 and 560

Ω for a gain of 2 or –1. Although, for most applications, a feedback resistor value of

1 k

Ω is recommended, which is a good compromise between bandwidth and phase margin that yields a very

stable amplifier.

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 increases the distortion. 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.

offset voltage

the corresponding gains. The following schematic and formula can be used to calculate the output offset

voltage:

V

OO +

V

IO

1

)

R

F

R

G

" I

IB

)

R

S

1

)

R

F

R

G

" I

IB–

R

F

+

–

VI

+

RG

RS

RF

IIB–

VO

IIB+

Figure 53. Output Offset Voltage Model