LM6182 datasheet(16/24 Pages) NSC

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Typical Applications

CURRENT FEEDBACK TOPOLOGY

For a conventional voltage feedback amplifier the resulting

small-signal bandwidth is inversely proportional to the de-

sired gain to a first order approximation based on the

gain-bandwidth concept. In contrast, the current feedback

amplifier topology, such as the LM6182, transcends this limi-

tation to offer a signal bandwidth that is relatively indepen-

dent of the closed loop gain.

Figure 1A and Figure 1B illus-

trate that for closed loop gains of −1 and −5 the resulting

pulse fidelity suggests quite similiar bandwidths for both

configurations.

FEEDBACK RESISTOR SELECTION: R

Selecting the feedback resistor, R

f, is a dominant factor in

compensating the LM6182. For general applications the

LM6182 will maintain specified performance with an 820

Ω

feedback resistor. The closed-loop bandwidth of the LM6182

depends on the feedback resistance, R

f. Therefore, Rs, and

not R

f, is varied to adjust for the desired closed-loop gain as

demonstrated in

Figure 2.

Although this R

f value will provide good results for most ap-

plications, it may be advantageous to adjust this value

slightly. Consider, for instance, the effect on pulse responses

with two different configurations where both the closed-loop

gains are +2 and the feedback resistors are 820

Ω, and

1640

Ω, respectively. Figure 3A and Figure 3B illustrate the

effect

increasing

while

maintaining

the

same

closed-loop gain – the amplifier bandwidth decreases. Ac-

cordingly, larger feedback resistors can be used to slow

down the LM6182 and reduce overshoot in the time domain

response. Conversely, smaller feedback resistance values

than 820

Ω can be used to compensate for the reduction of

bandwidth at high closed-loop gains, due to 2nd order ef-

fects. For example

Figure 4A and Figure 4B illustrate reduc-

ing R

f to 500Ω to establish the desired small signal response

in an amplifier configured for a closed-loop gain of +25.

DS011926-20

1A. A

V = −1

DS011926-21

1B. A

V = −5

FIGURE 1. Variation of Closed-Loop Gain from −1 to −5

Yields Similar Responses.

DS011926-22

FIGURE 2. R

f Sets Amplifier Bandwidth and Rs is

Adjusted to Obtain the Desired Closed-Loop Gain, A

DS011926-23

3A. R

f = 820Ω

DS011926-24

3B. R

f = 1640Ω

FIGURE 3. Increase Compensation by Increasing

f,AV = +2

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