LM359 datasheet(10/24 Pages) NSC

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Application Hints (Continued)

gain in the non-inverting configuration) some external fre-

quency compensation is required because the stray capaci-

tance to ground from the (−) input and the feedback resistor

add additional lagging phase within the feedback loop. The

value of the input capacitance will typically be in the range of

6 pF to 10 pF for a reasonably constructed circuit board.

When using a feedback resistance of 30 k

Ω or less, the best

method of compensation, without sacrificing slew rate, is to

add a lead capacitor in parallel with the feedback resistor

with a value on the order of 1 pF to 5 pF as shown in

Figure

10 .

Another method of compensation is to increase the effective

value of the internal compensation capacitor by adding ca-

pacitance from the COMP pin of an amplifier to ground. An

external 20 pF capacitor will generally compensate for all

gain settings but will also reduce the gain bandwidth product

and the slew rate. These same results can also be obtained

by reducing I

SET(IN) if the full capabilities of the amplifier are

not required. This method is termed over-compensation.

Another area of concern from a stability standpoint is that of

capacitive loading. The amplifier will generally drive capaci-

tive loads up to 100 pF without oscillation problems. Any

larger C loads can be isolated from the output as shown in

Figure 11. Over-compensation of the amplifier can also be

used if the corresponding reduction of the GBW product can

be afforded.

In most applications using the LM359, the input signal will be

AC coupled so as not to affect the DC biasing of the ampli-

fier. This gives rise to another subtlety of high frequency cir-

cuits which is the effective series inductance (ESL) of the

coupling capacitor which creates an increase in the imped-

ance of the capacitor at high frequencies and can cause an

unexpected gain reduction. Low ESL capacitors like solid

tantalum for large values of C and ceramic for smaller values

are recommended. A parallel combination of the two types is

even better for gain accuracy over a wide frequency range.

AMPLIFIER DESIGN EXAMPLES

The ability of the LM359 to provide gain at frequencies

higher than most monolithic amplifiers can provide makes it

most useful as a basic broadband amplification stage. The

design of standard inverting and non-inverting amplifiers,

though different than standard op amp design due to the cur-

rent differencing inputs, also entail subtle design differences

between the two types of amplifiers. These differences will

be best illustrated by design examples. For these examples

a practical video amplifier with a passband of 8 Hz to 10 MHz

and a gain of 20 dB will be used. It will be assumed that the

input will come from a 75

Ω source and proper signal termi-

nation will be considered. The supply voltage is 12 V

DC and

single resistor programming of the operating current, I

SET,

will be used for simplicity.

AN INVERTING VIDEO AMPLIFIER

Basic circuit configuration:

Determine the required I

SET

from the characteristic

curves for gain bandwidth product.

GBW

MIN= 10x10MHz = 100 MHz

For a flat response to 10 MHz a closed loop response to two

octaves above 10 MHz (40 MHz) will be sufficient.

Actual GBW = 10x40MHz = 400 MHz

SET required = 0.5 mA

DS007788-15

Cf = 1 pF to 5 pF for stability

FIGURE 10. Best Method of Compensation

DS007788-16

FIGURE 11. Isolating Large Capacitive Loads

DS007788-17

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