LM359 National Semiconductor, LM359 Datasheet - Page 10

• Manufacturer Part Number: LM359
• Description: Dual/ High Speed/ Programmable/ Current Mode Norton Amplifiers
• Manufacturer: National Semiconductor

www.national.com
Application Hints
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 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 .
C
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
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-
f
= 1 pF to 5 pF for stability
FIGURE 11. Isolating Large Capacitive Loads
FIGURE 10. Best Method of Compensation
SET(IN)
if the full capabilities of the amplifier are
(Continued)
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10
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
nation will be considered. The supply voltage is 12 V
single resistor programming of the operating current, I
will be used for simplicity.
AN INVERTING VIDEO AMPLIFIER
1. Basic circuit configuration:
2. Determine the required I
For a flat response to 10 MHz a closed loop response to two
octaves above 10 MHz (40 MHz) will be sufficient.
Actual GBW = 10 x 40 MHz = 400 MHz
I
SET
curves for gain bandwidth product.
required = 0.5 mA
GBW
MIN
= 10 x 10 MHz = 100 MHz
source and proper signal termi-
SET
from the characteristic
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DC
SET
and
,