THS3061 Datasheet, PDF (13/25 Pages) Texas Instruments – LOW DISTORTION HIGH SLEW RATE CURRENT FREEBACK AMPLIFIERS

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APPLICATION INFORMATION
THS3061
THS3062
SLOS394A – JULY 2002 – OCTOBER 2002
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
and PNP transistors possessing f_Ts of several GHz. This configuration implements an exceptionally
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
1
2, –1
5
10
RF for VCC = ±15 V
750 Ω
560 Ω
357 Ω
200 Ω
RF for VCC = ±5 V
750 Ω
560 Ω
383 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.
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