EL2170C EL2270C EL2470C
70 MHz 1 mA Current Mode Feedback Amplifiers
Applications Information
Product Description
The EL2170C EL2270C EL2470C are current-
feedback operational amplifiers that offer a wide
b
3 dB bandwidth of 70 MHz and a low supply
current of 1 mA per amplifier All of these prod-
ucts also feature high output current drive The
EL2170C can output 100 mA while the EL2270C
and the EL2470C can output 55 mA per amplifi-
er The EL2170C EL2270C EL2470C work with
supply voltages ranging from a single 3V to
g
6V and they are also capable of swinging to
within 1V of either supply on the input and the
output Because of their current-feedback topolo-
gy the EL2170C EL2270C EL2470C do not
have the normal gain-bandwidth product associ-
ated with voltage-feedback operational amplifi-
ers This allows their
b
3 dB bandwidth to re-
main relatively constant as closed-loop gain is in-
creased This combination of high bandwidth and
low power together with aggressive pricing make
the EL2170C EL2270C EL2470C the ideal
choice for many low-power high-bandwidth ap-
plications such as portable computing HDSL
and video processing
For applications where board space is extremely
critical the EL2170C is available in the tiny 5-
Lead SOT23 package which has a footprint 28%
the size of an 8-Lead SOIC The EL2170C
EL2270C EL2470C are each also available in in-
dustry standard pinouts in PDIP and SOIC pack-
ages
For Single and Dual applications with disable
consider the EL2176C (8-Pin Single) and
EL2276C (14-Pin Dual) If higher speed is re-
quired refer to the EL2180C EL2186C family
which provides Singles Duals Triples and
Quads with 250 MHz of bandwidth while con-
suming 3 mA of supply current per amplifier
Power Supply Bypassing and Printed
Circuit Board Layout
As with any high-frequency device good printed
circuit board layout is necessary for optimum
performance Ground plane construction is high-
ly recommended 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 7
mF
tantalum capacitor in
parallel with a 0 1
mF
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 the Capacitance at the In-
verting Input section) Ground plane construc-
tion should be used but it should be removed
from the area near the inverting input to mini-
mize 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 their additional series
inductance Use of sockets particularly for the
SO package should be avoided if possible Sock-
ets add parasitic inductance and capacitance
which will result in some additional peaking and
overshoot
Capacitance at the Inverting Input
Any manufacturer’s high-speed voltage- or cur-
rent-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 amplifi-
er 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 feed-
back and gain resistors further exacerbates the
problem by further lowering the pole frequency
10
ELANTEC [ ELANTEC SEMICONDUCTOR ]
