LM6182AMN datasheet(17/24 Pages) NSC

17 / 24 page

Typical Applications (Continued)

The extent of the amplifier’s dependence on R

f is displayed

Figure 5 for one particular closed-loop gain.

CAPACITIVE FEEDBACK

Current feedback amplifiers rely on feedback impedance for

proper compensation. Even in unity gain current feedback

amplifiers require a feedback resistor. LM6182 performance

is specified for a feedback resistance of 820

Ω. Decreasing

the feedback impedance below 820

Ω extends the amplifier’s

bandwidth leading to possible instability. Capacitive feed-

back should therefore not be used because the impedance

of a capacitor decreases with increasing frequency.

For voltage feedback amplifiers it is quite common to place a

small lead compensation capacitor in parallel with feedback

resistance, R

f. This compensation serves to reduce the am-

plifier’s peaking. One application of the lead compensation

capacitor is to counteract the effects of stray capacitance

from the inverting input to ground in circuit board layouts.

The LM6182 current feedback amplifier does not require this

lead compensation capacitor and has an even simpler, more

elegant solution.

To limit the bandwidth and peaking of the LM6182 current

feedback amplifier, do not use a capacitor across R

f as in

Figure 7. This actually has the opposite effect and extends

the bandwidth of the amplifier leading to possible instability.

Instead, simply increase the value of the feedback resistor

as shown in

Figure 3.

Non-inverting applications can also reduce peaking and limit

bandwidth by adding an RC circuit as illustrated in

Figure 8.

DS011926-25

4A. R

f = 820Ω

DS011926-26

4B. R

f = 500Ω

FIGURE 4. , 4B. Reducing R

f to Increase Bandwidth for

Large Closed-Loop Gains, A

V = +25

DS011926-27

FIGURE 5. −3 dB Bandwidth Is Determined By

Selecting R

DS011926-28

FIGURE 6. Current Feedback Amplifiers are Unstable

with Capacitive Feedback

DS011926-29

FIGURE 7. Compensation Capacitors Are Not Used

with the LM6182, Instead Simply Increase R

f to

Compensate

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