HA5023883 Datasheet, PDF (16/19 Pages) Intersil Corporation – Dual 125MHz Video Current Feedback Amplifier

HA5023
DESIGN INFORMATION (Continued)
The information contained in this section has been developed through characterization by Intersil Corporation and is for use as application
and design information only. No guarantee is implied.
Application Information
Optimum Feedback Resistor
The plots of inverting and non-inverting frequency response,
see Figure 1 and Figure 2 in the typical performance section,
illustrate the performance of the HA5023 in various closed
loop gain configurations. Although the bandwidth depen-
dency on closed loop gain isn’t as severe as that of a voltage
feedback amplifier, there can be an appreciable decrease in
bandwidth at higher gains. This decrease may 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 R_F, in conjunction with the inter-
nal compensation capacitor, sets the dominant pole of the
frequency response. Thus, the amplifier’s bandwidth is
inversely proportional to R_F. The HA5023 design is opti-
mized for a 1000Ω R_F at a gain of +1. Decreasing R_F in a
unity gain application decreases stability, resulting in exces-
sive peaking and overshoot. At higher gains the amplifier is
more stable, so R_F can be decreased in a trade-off of stabil-
ity for bandwidth.
The table below lists recommended R_F values for various
gains, and the expected bandwidth.
GAIN
(A_CL)
-1
+1
+2
+5
+10
-10
R_F (Ω)
750
1000
681
1000
383
750
BANDWIDTH
(MHz)
100
125
95
52
65
22
PC Board Layout
The frequency response of this amplifier depends greatly on
the amount of care taken in designing the PC board. The
use of low inductance components such as chip resistors
and chip capacitors is strongly recommended. If leaded
components are used the leads must be kept short espe-
cially for the power supply decoupling components and
those components connected to the inverting input.
Attention must be given to decoupling the power supplies. A
large value (10µF) tantalum or electrolytic capacitor in paral-
lel with a small value (0.1µF) chip capacitor works well in
most cases.
A ground plane is strongly recommended to control noise.
Care must also be taken to minimize the capacitance to
ground seen by the amplifier’s inverting input (-IN). The
larger this capacitance, the worse the gain peaking, resulting
in pulse overshoot and possible instability. It is recom-
mended that the ground plane be removed under traces
connected to -IN, and that connections to -IN be kept as
short as possible to minimize the capacitance from this node
to ground.
Driving Capacitive Loads
Capacitive loads will degrade the amplifier’s phase margin
resulting in frequency response peaking and possible oscilla-
tions. In most cases the oscillation can be avoided by placing
an isolation resistor (R) in series with the output as shown in
Figure 34.
V_IN
+_-
R
R_T
R_I
R_F
V_OUT
C_L
FIGURE 34. PLACEMENT OF THE OUTPUT ISOLATION
RESISTOR, R
The selection criteria for the isolation resistor is highly
dependent on the load, but 27Ω has been determined to be
a good starting value.
Power Dissipation Considerations
Due to the high supply current inherent in dual amplifiers, care
must be taken to insure that the maximum junction tempera-
ture (T_J, see Absolute Maximum Ratings) is not exceeded.
Figure 35 shows the maximum ambient temperature versus
supply voltage for the available package styles. It is recom-
mended that thermal calculations, which take into account
output power, be performed by the designer.
165
155
145
CERDIP
135
125
5
7
9
11
13
15
SUPPLY VOLTAGE (V)
FIGURE 35. MAXIMUM OPERATING AMBIENT TEMPERATURE
vs SUPPLY VOLTAGE
16