Note 13: Avoid resistive loads of less than 500

Ω, as they may cause instability.

Application Hints

AMPLIFIER TOPOLOGY

The topology chosen for the LMC6032, shown in

Figure 1,is

unconventional (compared to general-purpose op amps) in

that the traditional unity-gain buffer output stage is not used;

instead, the output is taken directly from the output of the in-

tegrator, to allow a larger output swing. Since the buffer tra-

ditionally delivers the power to the load, while maintaining

high op amp gain and stability, and must withstand shorts to

either rail, these tasks now fall to the integrator.

As a result of these demands, the integrator is a compound

affair with an embedded gain stage that is doubly fed forward

(via C

driver. In addition, the output portion of the integrator is a

push-pull configuration for delivering heavy loads. While

sinking current the whole amplifier path consists of three

gain stages with one stage fed forward, whereas while

sourcing the path contains four gain stages with two fed

forward.

The large signal voltage gain while sourcing is comparable

to traditional bipolar op amps, even with a 600

Ω load. The

gain while sinking is higher than most CMOS op amps, due

to the additional gain stage; however, under heavy load

(600

Ω) the gain will be reduced as indicated in the Electrical

Characteristics.

COMPENSATING INPUT CAPACITANCE

The high input resistance of the LMC6032 op amps allows

the use of large feedback and source resistor values without

losing gain accuracy due to loading. However, the circuit will

be especially sensitive to its layout when these large-value

resistors are used.

Every amplifier has some capacitance between each input

and AC ground, and also some differential capacitance be-

tween the inputs. When the feedback network around an

amplifier is resistive, this input capacitance (along with any

additional capacitance due to circuit board traces, the

socket, etc.) and the feedback resistors create a pole in the

feedback path. In the following General Operational Amplifier

Circuit,

Figure 2, the frequency of this pole is

where C

cluding amplifier input capacitance and any stray capaci-

tance from the IC socket (if one is used), circuit board traces,

etc., and R

formula, as well as all formulae derived below, apply to in-

verting and non-inverting op-amp configurations.

When the feedback resistors are smaller than a few k

Ω, the

frequency of the feedback pole will be quite high, since C

generally less than 10 pF. If the frequency of the feedback

pole is much higher than the “ideal” closed-loop bandwidth

(the nominal closed-loop bandwidth in the absence of C

the pole will have a negligible effect on stability, as it will add

only a small amount of phase shift.

However, if the feedback pole is less than approximately 6 to

10 times the “ideal” −3 dB frequency, a feedback capacitor,

C

ing input of the op amp. This condition can also be stated in

terms of the amplifier’s low-frequency noise gain: To main-

tain stability, a feedback capacitor will probably be needed if

Stability vs

Capacitive Load

DS011135-32

Stability vs

Capacitive Load

DS011135-33

DS011135-3

FIGURE 1. LMC6032 Circuit Topology (Each Amplifier)

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