DRIVING CAPACITIVE LOADS

Capacitive output loading applications will benefit from the

use of a series output resistor as shown in Figure 5. The

charts "Suggested R

value for selecting a series output resistor for mitigating

capacitive loads. The values suggested in the charts are

selected for .5 dB or less of peaking in the frequency re-

sponse. This gives a good compromise between settling

time and bandwidth. For applications where maximum fre-

quency response is needed and some peaking is tolerable,

the value of R

mended values.

There will be amplitude lost in the series resistor unless the

gain is adjusted to compensate; this effect is most noticeable

with heavy loads (R

Ω).

An alternative approach is to place R

loop as shown in Figure 6. This will preserve gain accuracy,

but will still limit maximum output voltage swing.

INVERTING INPUT PARASITIC CAPACITANCE

Parasitic capacitance is any capacitance in a circuit that was

not intentionally added. It is produced through electrical

interaction between conductors and can be reduced but

never entirely eliminated. Most parasitic capacitances that

cause problems are related to board layout or lack of termi-

nation on transmission lines. Please see the section on

Layout Considerations for hints on reducing problems due to

parasitic capacitances on board traces. Transmission lines

should be terminated in their characteristic impedance at

both ends.

High speed amplifiers are sensitive to capacitance between

the inverting input and ground or power supplies. This shows

up as gain peaking at high frequency. The capacitor raises

device gain at high frequencies by making R

G

appear

smaller. Capacitive output loading will exaggerate this effect.

One possible remedy for this effect is to slightly increase the

value of the feedback (and gain set) resistor. This will tend to

offset the high frequency gain peaking while leaving other

parameters relatively unchanged. If the device has a capaci-

tive load as well as inverting input capacitance, using a

series output resistor as described in the section on "Driving

Capacitive Loads" will help.

When higher currents are required than a single amplifier

can provide, the circuit of Figure 7 can be used. Although the

example circuit was intended for the LMH6725 quad op amp,

higher thermal efficiency can be obtained by using four

separate SOIC op amps. Careful attention to a few key

components will optimize performance from this circuit. The

first thing to note is that the buffers need slightly higher value

feedback resistors than if the amplifiers were individually

configured. As well, R

compensation to further improve stability. The composite

amplifier has approximately twice the phase delay of a single

circuit. The larger values of R

high frequency attenuation provided by C

that the circuit does not oscillate.

Resistors R

current distribution between the amplifiers. Since they are

inside the feedback loop they have no effect on the gain of

the circuit. The circuit shown in Figure 7 has a gain of 5. The

frequency response of this circuit is shown in Figure 8.

20078934

FIGURE 5. Decoupling Capacitive Loads

20078935

FIGURE 6. Series Output Resistor inside feedback loop

20078942

FIGURE 7. High Output Current Composite Amplifier

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