Specified (Continued)

LM6142/44 Application Ideas

The LM6142 brings a new level of ease of use to opamp sys-

tem design.

With greater than rail-to-rail input voltage range concern

over exceeding the common-mode voltage range is elimi-

nated.

Rail-to-rail output swing provides the maximum possible dy-

namic range at the output. This is particularly important

when operating on low supply voltages.

The high gain-bandwidth with low supply current opens new

battery powered applications, where high power consump-

tion, previously reduced battery life to unacceptable levels.

To take advantage of these features, some ideas should be

kept in mind.

ENHANCED SLEW RATE

Unlike most bipolar opamps, the unique phase reversal

prevention/speed-up circuit in the input stage causes the

slew rate to be very much a function of the input signal am-

plitude.

Figure 2 shows how excess input signal, is routed around

the input collector-base junctions, directly to the current mir-

rors.

The LM6142/44 input stage converts the input voltage

change to a current change. This current change drives the

current mirrors through the collectors of Q1–Q2, Q3–Q4

when the input levels are normal.

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

the differential input voltage rises above two diode drops.

This 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 30V to 60V/µs.

This effect is most noticeable at higher supply voltages and

lower gains where incoming signals are likely to be large.

This new input circuit also eliminates the phase reversal

seen in many opamps when they are overdriven.

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 LM6142, 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.

Noise Current vs Frequency

DS012057-45

NE vs R Source

DS012057-12

Slew Rate vs

∆ V

IN

V

DS012057-7

FIGURE 1.

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