THS3061

THS3062

SLOS394A – JULY 2002 – OCTOBER 2002

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APPLICATION INFORMATION

INTRODUCTION

The THS306x is a high-speed, operational amplifier configured in a current-feedback architecture. The device is built

using Texas Instruments BiCOM–I process, a 30-V, dielectrically isolated, complementary bipolar process with NPN

high-performance amplifier that has a wide bandwidth, high slew rate, fast settling time, and low distortion.

RECOMMENDED FEEDBACK AND GAIN RESISTOR VALUES

As with all current-feedback amplifiers, the bandwidth of the THS306x is an inversely proportional function of the

value of the feedback resistor. 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. For most applications, a feedback resistor value of 750

Ω is

recommended

a good compromise between bandwidth and phase margin that yields a very stable amplifier.

Table 1. Recommended Resistor Values for Optimum Frequency Response

GAIN

RF for VCC = ±15 V

RF for VCC = ±5 V

1

750

Ω

750

Ω

2, –1

560

Ω

560

Ω

5

357

Ω

383

Ω

10

200

Ω

200

Ω

As shown in Table 1, to maintain the highest bandwidth with an increasing gain, the feedback resistor is reduced. The

advantage of dropping the feedback resistor (and the gain resistor) is the noise of the system is also reduced

compared to no reduction of these resistor values, see noise calculations section. Thus, keeping the bandwidth as

high as possible maintains very good distortion performance of the amplifier by keeping the excess loop gain as high

as possible.

Care must be taken to not drop these values too low. The amplifier’s output must drive the feedback resistance (and

gain resistance) and may place a burden on the amplifier. The end result is that distortion may actually increase due

to the low impedance load presented to the amplifier. Careful management of the amplifier bandwidth and the

associated loading effects needs to be examined by the designer for optimum performance.

The THS3061/62 amplifiers exhibit very good distortion performance and bandwidth with the capability of utilizing

up to +15 V power supplies. Their excellent current drive capability of up to +145 mA driving into a 50-

Ω load allows

for many versatile applications. One application is driving a twisted pair line (i.e. telephone line). Figure 48 shows

a simple circuit for driving a twisted pair differentially.