THS3096PWPR datasheet(19/34 Pages) TI

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−VS

49.9

Ω

715

Ω

Ferrite Bead

µF

178

Ω

100

Ω LOAD

−

Ω

n Lines

VO(1)

VO(n)

75-

Ω Transmission Line

909

Ω

909

Ω

−15 V

15 V

Driving Capacitive Loads

100

CL − Capacitive Load − pF

Gain = 5,

VS = ±15 V

−

178

Ω

715

Ω

RISO

−VS

49.9

Ω

715

Ω

5.11

Ω

µF

178

Ω

100

Ω LOAD

RISO

−VS

49.9

Ω

5.11

Ω

µF

178

Ω

27 pF

715

Ω

715

Ω

100

Ω LOAD

RIN

THS3092

THS3096

SLOS428A – DECEMBER 2003 – REVISED FEBRUARY 2004

Figure 60.

Figure 57. Video Distribution Amplifier

Application

Placing a small series resistor, RISO, between the

amplifier’s output and the capacitive load, as shown

in Figure 59, is an easy way of isolating the load

capacitance.

Applications such as FET line drivers can be highly

capacitive

and

cause

stability

problems

for

Using a ferrite chip in place of RISO, as shown in

high-speed amplifiers.

Figure 60, is another approach of isolating the output

of the amplifier. The ferrite's impedance characteristic

Figure 58 through Figure 63 show recommended

versus frequency is useful to maintain the low fre-

methods for driving capacitive loads. The basic idea

quency load independence of the amplifier while

is to use a resistor or ferrite chip to isolate the phase

isolating the phase shift caused by the capacitance at

shift at high frequency caused by the capacitive load

high frequency. Use a ferrite with similar impedance

from the amplifier’s feedback path. See Figure 58 for

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

recommended resistor values versus capacitive load.

at dc.

Figure 61 shows another method used to maintain

the low frequency load independence of the amplifier

while isolating the phase shift caused by the capaci-

tance at high frequency. At low frequency, feedback

is mainly from the load side of RISO. At high fre-

quency, the feedback is mainly via the 27-pF capaci-

tor. 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 62 gives similar

results with reduced dc offset and low frequency

noise. (See the ADDITIONAL REFERENCE MA-

TERIAL

section

for

Expanding

the

usability

Figure 58. Recommended RISO vs Capacitive Load

current-feedback amplifiers.)

Figure 59.

Figure 61.

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