LM359 National Semiconductor, LM359 Datasheet - Page 9

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

Application Hints
these parameters are significant in wide band amplifier de-
sign. The input stage current is approximately 3 times
I
approximations for these AC parameters are:
where C
pin (pin 3 or pin 13) to ground, A
open loop voltage gain in V/V and an ambient temperature of
25˚C is assumed (KT/q = 26 mV and
also controls the DC input bias current by the expression:
which is important for DC biasing considerations.
The total device supply current (for both amplifiers) is also a
direct function of the set currents and can be approximated
by:
with each set current programmed by individual resistors.
PROGRAMMING WITH A SINGLE RESISTOR
Operating current programming may also be accomplished
using only one resistor by letting I
programming current is now referred to as I
ated by connecting a resistor from pin 1 to pin 8 ( Figure 8 ).
I
This configuration does not affect any of the internal set cur-
rent dependent parameters differently than previously dis-
cussed except the total supply current which is now equal to:
Care must be taken when using resistors to program the set
current to prevent significantly increasing the supply voltage
above the value used to determine the set current. This
would cause an increase in total supply current due to the re-
sulting increase in set current and the maximum device
power dissipation could be exceeded. The set resistor val-
ue(s) should be adjusted for the new supply voltage.
SET(IN)
SET(IN)
FIGURE 8. Single Resistor Programming of I
= I
and by using this relationship the following first order
comp
SET(OUT)
I
supply
is the total capacitance from the compensation
= I
SET
27 x I
I
supply
SET(OUT)
37 x I
(Continued)
SET(IN)
VOL
+ 11 x I
SET
is the low frequency
equal I
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typ
SET(IN)
SET
= 150). I
SET(OUT)
and it is cre-
SET
SET(IN)
. The
9
One method to avoid this is to use an adjustable current
source which has voltage compliance to generate the set
current as shown in Figure 9 .
This circuit allows I
supply voltage range of the LM359 which also improves
power supply ripple rejection as illustrated in the Typical Per-
formance Characteristics. It should be noted, however, that
the current through the LM334 as shown will change linearly
with temperature but this can be compensated for (see
LM334 data sheet).
Pin 1 must never be shorted to ground or pin 8 never shorted
to V
catastrophic device failure.
CONSIDERATIONS FOR HIGH FREQUENCY
OPERATION
The LM359 is intended for use in relatively high frequency
applications and many factors external to the amplifier itself
must be considered. Minimization of stray capacitances and
their effect on circuit operation are the primary requirements.
The following list contains some general guidelines to help
accomplish this end:
1. Keep the leads of all external components as short as
2. Place components conducting signal current from the
3. Use reasonably low value resistances for gain setting
4. Use of a ground plane is helpful in providing a shielding
5. Use a single-point ground and single-point supply distri-
6. Avoid use of long wires (
7. Bypass the supply close to the device with a low induc-
COMPENSATION
The LM359 is internally compensated for stability with closed
loop inverting gains of 10 or more. For an inverting gain of
less than 10 and all non-inverting amplifiers (the amplifier al-
ways has 100% negative current feedback regardless of the
FIGURE 9. Current Source Programming of I
+
possible.
output of an amplifier away from that amplifier’s
non-inverting input.
and biasing.
effect between the inputs and from input to output. Avoid
using vector boards.
bution to minimize crosstalk. Always connect the two
grounds (one from each amplifier) together.
shielded wire.
tance, low value capacitor (typically a 0.01 µF ceramic)
to create a good high frequency ground. If long supply
leads are unavoidable, a small resistor (
with the bypass capacitor may be needed and using
shielded wire for the supply leads is also recommended.
without limiting the current to 2 mA or less to prevent
SET
to remain constant over the entire
>
2") but if necessary, use
z
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10 ) in series
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SET