specified (Continued)

Application Hints

AMPLIFIER TOPOLOGY

The topology chosen for the LPC660 is unconventional

(compared to general-purpose op amps) in that the tradi-

tional unity-gain buffer output stage is not used; instead, the

output is taken directly from the output of the integrator, to al-

low rail-to-rail output swing. Since the buffer traditionally de-

livers 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, for load resistance of at least

5k

Ω. The gain while sinking is higher than most CMOS op

Inverting Large-Signal

Pulse Response

DS010547-46

Inverting Small-Signal

Pulse Response

DS010547-47

Stability vs Capacitive Load

DS010547-4

Note: Avoid resistive loads of less than 500

Ω, as they may cause

instability.

Stability vs Capacitive Load

DS010547-5

DS010547-6

FIGURE 1. LPC660 Circuit Topology (Each Amplifier)

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