–3–

AN-733

at the positive input terminal of the amplifier, denoted as

V+ = BL+. This continues until the capacitor voltage

bistable multivibrator switches to the other stable state

The capacitor then begins to discharge, and its voltage,

V–, decreases exponentially toward L–. This continues

the bistable multivibrator switches to the positive output

state, and the cycle repeats itself.

It is important to note that the frequency of the square

components being used. Any variation in L+ will cause

V+ to vary in proportion, ensuring the same transition

time and the same oscillation frequency. The maximum

operating frequency is determined by the amplifier

speed, which can be increased significantly by using

faster devices.

The lowest operating frequency depends on the practical

upper limits set by R7 and C9.

Using the name convention outlined on the PRA OPAMP

evaluation board, the circuit should be connected

as follows:

B = R4/(R4 + R9); feedback factor (noninverting input)

T = 2R7  C9  ln((1 + B)/(1 – B)); period of oscillation

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Figure 6. Series Resistor Compensation

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Figure 7. Capacitive Load Drive Without Resistor

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Figure 8. Capacitive Load Drive with Resistor

EXTERNAL COMPENSATION TECHNIQUES

Series Resistor Compensation

The use of external compensation networks may be

required to optimize certain applications. Figure 6 is a

typical representation of a series resistor compensation

for stabilizing an op amp driving capacitive load. The sta-

bilizing effect of the series resistor isolates the op amp

output and the feedback network from the capacitive

load. The required amount of series resistance depends

on the part used, but values of 5  to 50  are usually

sufficient to prevent local resonance. The disadvantages

of this technique are a reduction in gain accuracy and

extra distortion when driving nonlinear loads.

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Figure 10. Capacitive Load Drive Without Snubber

REV. A