THS3115IPWPRG4 datasheet(13/35 Pages) TI1

13 / 35 page

49.9 W

100-

Load

-V

5.11

ISO

49.9 W

100-

Load

-V

5.11 W

27pF

750 W

49.9 W

100-

Load

-V

Ferrite

Bead

49.9 W

100-

Load

-V

5.11 W

27pF

Ferrite

Bead

THS3112

THS3115

www.ti.com

SLOS385C – SEPTEMBER 2001 – REVISED SEPTEMBER 2010

Placing a small series resistor, RISO, between the

Figure 46 shows another method used to maintain

amplifier output and the capacitive load, as shown in

the low-frequency load independence of the amplifier

Figure 44, is an easy way of isolating the load

while isolating the phase shift caused by the

capacitance.

capacitance at high frequency. At low frequency,

feedback is mainly from the load side of RISO. At high

frequency, the feedback is mainly via the 27-pF

capacitor. The resistor RIN in series with the negative

input is used to stabilize the amplifier and should be

equal to the recommended value of RF at unity gain.

Replacing RIN with a ferrite of similar impedance at

about 100 MHz as shown in Figure 47 gives similar

results with reduced dc offset and low frequency

noise.

Figure 44. Resistor to Isolate Capacitive Load

Using a ferrite chip in place of RISO, as Figure 45

shows, is another approach of isolating the output of

the amplifier. The ferrite impedance characteristic

versus frequency is useful to maintain the low

frequency load independence of the amplifier while

isolating the phase shift caused by the capacitance at

high frequency. Use a ferrite with similar impedance

to RISO, 20 Ω to 50 Ω, at 100 MHz and low

Figure 46. Feedback Technique with Input

impedance at dc.

Resistor for Capacitive Load

Figure 45. Ferrite Bead to Isolate Capacitive Load

Figure 47. Feedback Technique with Input Ferrite

Bead for Capacitive Load

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