THS3095D datasheet(20/31 Pages) TI

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Driving Capacitive Loads

100

CL − Capacitive Load − pF

Gain = 5,

RL = 100 Ω,

VS = ±15 V

−VS

49.9

Ω

1 k

Ω

5.11

Ω

µF

249

Ω

100-

Ω LOAD

RISO

−VS

49.9

Ω

5.11

Ω

µF

249

Ω

27 pF

1 k

Ω

1 k

Ω

100-

Ω LOAD

RIN

−VS

49.9

Ω

1 k

Ω

Ferrite Bead

µF

249

Ω

100-

Ω LOAD

THS3091

THS3095

SLOS423C – SEPTEMBER 2003 – REVISED AUGUST 2004

Placing a small series resistor, RISO, between the

amplifier’s output and the capacitive load, as shown

Applications such as FET line drivers can be highly

in Figure 64, is an easy way of isolating the load

capacitive

and

cause

stability

problems

for

capacitance.

high-speed amplifiers.

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

Figure 63 through Figure 68 show recommended

Figure 65, is another approach of isolating the output

methods for driving capacitive loads. The basic idea

of the amplifier. The ferrite's impedance characteristic

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

versus

frequency

useful

maintain

the

shift at high frequency caused by the capacitive load

low-frequency load independence of the amplifier

from the amplifier’s feedback path. See Figure 63 for

while isolating the phase shift caused by the capaci-

recommended resistor values versus capacitive load.

tance at high frequency. Use a ferrite with similar

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

impedance at dc.

Figure 66 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

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 67 gives similar

results with reduced dc offset and low-frequency

Figure 63. Recommended RISO vs Capacitive Load

noise. (See the ADDITIONAL REFERENCE MA-

TERIAL section for expanding the usability of cur-

rent-feedback amplifiers.)

Figure 64.

Figure 66.

Figure 65.

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