4
Application Information
Relevant Application Notes
The following Application Notes pertain to the HFA1155:
• AN9787-An Intuitive Approach to Understanding
Current Feedback Amplifiers
• AN9420-Current Feedback Amplifier Theory and
Applications
• AN9663-Converting from Voltage Feedback to Current
Feedback Amplifiers
• AN9897-Operating the HFA1155 from 5V Single
Supply
These publications may be obtained from Intersil’s web site
(www.intersil.com).
Performance Differences Between Packages
The HFA1155 is a high frequency current feedback
amplifier. As such, it is sensitive to parasitic capacitances
which influence the amplifier’s operation. The different
parasitic capacitances of different packages yield
performance differences (notably bandwidth and bandwidth
related parameters).
Because of these performance differences, designers
should evaluate and breadboard with the same package
style to be used in production.
Optimum Feedback Resistor
The enclosed frequency response graphs detail the
performance of the HFA1155 in various gains. Although the
bandwidth dependency on A
CL
isn’t as severe as that of a
voltage feedback amplifier, there is an appreciable decrease
in bandwidth at higher gains. This decrease can be minimized
by taking advantage of the current feedback amplifier’s unique
relationship between bandwidth and R
F
. All current feedback
amplifiers require a feedback resistor, even for unity gain
applications, and the R
F
, in conjunction with the internal
compensation capacitor, sets the dominant pole of the
frequency response. Thus, the amplifier’s bandwidth is
inversely proportional to R
F
. The HFA1155 is optimized for
R
F
= 604Ω, at a gain of +2. Decreasing R
F
decreases
stability, resulting in excessive peaking and overshoot (Note:
Capacitive feedback causes the same problems due to the
feedback impedance decrease at higher frequencies). At
higher gains the amplifier is more stable, so R
F
can be
decreased in a trade-off of stability for bandwidth. The table
below lists recommended R
F
values for various gains, and the
expected bandwidth.
5V Single Supply Operation
This amplifier operates at single supply voltages down to
4.5V. The dramatic supply current reduction at this operating
condition (refer also to Figure 16) makes this op amp an
even better choice for low power 5V systems. Refer to
Application Note AN9897 for further information.
Driving Capacitive Loads
Capacitive loads, such as an A/D input, or an improperly
terminated transmission line will degrade the amplifier’s
phase margin resulting in frequency response peaking and
possible oscillations. In most cases, the oscillation can be
avoided by placing a resistor (R
S
) in series with the output
prior to the capacitance.
Figure 1 details starting points for the selection of this
resistor. The points on the curve indicate the R
S
and C
L
combinations for the optimum bandwidth, stability, and
settling time, but experimental fine tuning is recommended.
Picking a point above or to the right of the curve yields an
overdamped response, while points below or left of the curve
indicate areas of underdamped performance.
R
S
and C
L
form a low pass network at the output, thus
limiting system bandwidth well below the amplifier bandwidth
of 355MHz (A
V
= +2). By decreasing R
S
as C
L
increases (as
illustrated by the curves), the maximum bandwidth is
obtained without sacrificing stability. In spite of this,
bandwidth still decreases as the load capacitance increases.
For example, at A
V
= +2, R
S
= 30Ω, C
L
= 22pF, the
bandwidth is 290MHz, but the bandwidth drops to 90MHz at
A
V
=+2, R
S
= 6Ω, C
L
= 390pF.
OPTIMUM FEEDBACK RESISTOR
A
CL
R
F
(Ω)
SOT-23
BANDWIDTH (MHz)
SOT-23
-1 576 360
+1 453, (+R
S
= 221) 365
+2 604 355
+5 475 300
+10 182 250
HFA1155
