LM6132/34 Application Hints

(Continued)

If the input signal exceeds the slew rate of the input stage

and the differential input voltage rises above a diode drop,

the excess signal bypasses the normal input transistors,

(Q1–Q4), and is routed in correct phase through the two ad-

ditional transistors, (Q5, Q6), directly into the current mirrors.

This rerouting of excess signal allows the slew-rate to in-

crease by a factor of 10 to 1 or more. (See

Figure 1.)

As the overdrive increases, the opamp reacts better than a

conventional opamp. Large fast pulses will raise the slew-

rate to around 25V to 30V/µs.

This effect is most noticeable at higher supply voltages and

lower gains where incoming signals are likely to be large.

This speed-up action adds stability to the system when driv-

ing large capacitive loads.

DRIVING CAPACITIVE LOADS

Capacitive loads decrease the phase margin of all opamps.

This is caused by the output resistance of the amplifier and

the load capacitance forming an R-C phase lag network.

This can lead to overshoot, ringing and oscillation. Slew rate

limiting can also cause additional lag. Most opamps with a

fixed maximum slew-rate will lag further and further behind

when driving capacitive loads even though the differential in-

put voltage raises. With the LM6132, the lag causes the slew

rate to raise. The increased slew-rate keeps the output fol-

lowing the input much better. This effectively reduces phase

lag. After the output has caught up with the input, the differ-

ential input voltage drops down and the amplifier settles

rapidly.

These features allow the LM6132 to drive capacitive loads

as large as 500 pF at unity gain and not oscillate. The scope

photos (

Figure 3 and Figure 4) above show the LM6132 driv-

ing a 500 pF load. In

Figure 3 , the lower trace is with no ca-

pacitive load and the upper trace is with a 500 pF load. Here

Slew Rate vs Differential V

IN

V

DS012349-40

FIGURE 1.

DS012349-36

FIGURE 2.

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