el5191cs Intersil Corporation, el5191cs Datasheet - Page 15

• Manufacturer Part Number: el5191cs
• Description: 1ghz Current Feedback Amplifier With Enable
• Manufacturer: Intersil Corporation

Applications Information
Product Description
The EL5191 is a current-feedback operational amplifier that
offers a wide -3dB bandwidth of 1GHz and a low supply
current of 9mA per amplifier. The EL5191 works with supply
voltages ranging from a single 5V to 10V and they are also
capable of swinging to within 1V of either supply on the
output. Because of their current-feedback topology, the
EL5191 does not have the normal gain-bandwidth product
associated with voltage-feedback operational amplifiers.
Instead, its -3dB bandwidth to remain relatively constant as
closed-loop gain is increased. This combination of high
bandwidth and low power, together with aggressive pricing
make the EL5191 the ideal choice for many low-power/high-
bandwidth applications such as portable, handheld, or
battery-powered equipment.
For varying bandwidth needs, consider the EL5192 with
600MHz on a 6mA supply current or the EL5193 with 300MHz
on a 4mA supply current. Versions include single, dual, and
triple amp packages with 5 Ld SOT-23, 16 Ld QSOP, and 8 Ld
or 16 Ld SOIC outlines.
Power Supply Bypassing and Printed Circuit
Board Layout
As with any high frequency device, good printed circuit
board layout is necessary for optimum performance. Low
impedance ground plane construction is essential. Surface
mount components are recommended, but if leaded
components are used, lead lengths should be as short as
possible. The power supply pins must be well bypassed to
reduce the risk of oscillation. The combination of a 4.7µF
tantalum capacitor in parallel with a 0.01µF capacitor has
been shown to work well when placed at each supply pin.
For good AC performance, parasitic capacitance should be
kept to a minimum, especially at the inverting input. (See
“Capacitance at the Inverting Input” on page 15.) Even when
ground plane construction is used, it should be removed
from the area near the inverting input to minimize any stray
capacitance at that node. Carbon or Metal-Film resistors are
acceptable with the Metal-Film resistors giving slightly less
peaking and bandwidth because of additional series
inductance. Use of sockets, particularly for the SOIC
package, should be avoided if possible. Sockets add
parasitic inductance and capacitance which will result in
additional peaking and overshoot.
Disable/Power-Down
The EL5191A amplifier can be disabled placing its output in
a high impedance state. When disabled, the amplifier supply
current is reduced to < 150µA. The EL5191A is disabled
when its CE pin is pulled up to within 1V of the positive
supply. Similarly, the amplifier is enabled by floating or
pulling its CE pin to at least 3V below the positive supply. For
±5V supply, this means that an EL5191A amplifier will be
15
EL5191, EL5191A
enabled when CE is 2V or less, and disabled when CE is
above 4V. Although the logic levels are not standard TTL,
this choice of logic voltages allows the EL5191A to be
enabled by tying CE to ground, even in 5V single supply
applications. The CE pin can be driven from CMOS outputs.
Capacitance at the Inverting Input
Any manufacturer’s high-speed voltage- or current-feedback
amplifier can be affected by stray capacitance at the
inverting input. For inverting gains, this parasitic capacitance
has little effect because the inverting input is a virtual
ground. But for non-inverting gains, this capacitance (in
conjunction with the feedback and gain resistors) creates a
pole in the feedback path of the amplifier. This pole, if low
enough in frequency, has the same destabilizing effect as a
zero in the forward open-loop response. The use of large
value feedback and gain resistors exacerbates the problem
by further lowering the pole frequency (increasing the
possibility of oscillation.)
The EL5191 has been optimized with a 250Ω feedback
resistor. With the high bandwidth of these amplifiers, these
resistor values might cause stability problems when
combined with parasitic capacitance, thus ground plane is
not recommended around the inverting input pin of the
amplifier.
Feedback Resistor Values
The EL5191 has been designed and specified at a gain of +2
with R
gives 600MHz of -3dB bandwidth at A
peaking. With A
bandwidth with 0.6dB of peaking. Since the EL5191 is a
current-feedback amplifier, it is also possible to change the
value of R
Frequency Response for Various R
peaking can be easily modified by varying the value of the
feedback resistor.
Because the EL5191 is a current-feedback amplifier, its
gain-bandwidth product is not a constant for different closed-
loop gains. This feature actually allows the EL5191 to
maintain about the same -3dB bandwidth. As gain is
increased, bandwidth decreases slightly while stability
increases. Since the loop stability is improving with higher
closed-loop gains, it becomes possible to reduce the value
of R
resulting in only a slight loss of bandwidth with increased
closed-loop gain.
Supply Voltage Range and Single-Supply
Operation
The EL5191 has been designed to operate with supply
voltages having a span of greater than 5V and less than 10V.
In practical terms, this means that the EL5191 will operate
on dual supplies ranging from ±2.5V to ±5V. With single-
supply, the EL5191 will operate from 5V to 10V.
F
below the specified 250Ω and still retain stability,
F
approximately 250Ω. This value of feedback resistor
F
to get more bandwidth. As seen in the curve of
V
= -2, that same R
F
F
and R
gives 450MHz of
V
= 2 with about 2dB of
G
, bandwidth and
May 16, 2007
FN7180.3