CLC446 National Semiconductor, CLC446 Datasheet - Page 8

• Manufacturer Part Number: CLC446
• Description: 400MHz/ 50mW Current-Feedback Op Amp
• Manufacturer: National Semiconductor

The CLC446 noise model in Figure 9 is used to develop
this equation for NF:
where:
To achieve a low Noise Figure while matching the source,
use a matching transformer or the Low Noise
Composite Amp With Input Matching circuit found in
the CLC446 Applications section.
Dynamic Range (distortion)
The distortion plots in the Typical Performance
Characteristics section show distortion as a function of
load resistance, frequency, and output amplitude.
Distortion places an upper limit on the CLC446’s
dynamic range.
Realized output distortion is highly dependent upon the
external circuit. Some of the common external circuit
choices that can improve distortion are:
Printed Circuit Board Layout
High Frequency op amp performance is strongly depen-
dent on proper layout, proper resistive termination and
adequate power supply decoupling. The most important
layout points to follow are:
http://www.national.com
NF 10log
R
inverting input
There is no matching resistor from the input
to ground
e
noise density terms (see the Electrical
Characteristics section)
R
gain-setting resistor
Short and equal return paths from the load to
the supplies
De-coupling capacitors of the correct value
Higher load resistance
A lower ratio of the output voltage swing to
power supply voltage
Use a ground plane
Bypass power supply pins with:
Minimize trace and lead lengths for components
between the inverting and output pins
Remove ground plane underneath the amplifier
package and 0.1" (3mm) from all input/output
pads
4 T
ni
k
S
f
ceramic capacitors of about 0.1 F placed
less than 0.1" (3mm) from the pin
tantalum capacitors of about 6.8 F for large
signal current swings or improved power
supply noise rejection; we recommend a
minimum of 2.2 F for any circuit
, i
is the feedback resistor, and R
is the source resistance at the non-
bn
e
ni
, and i
16.0 x 10
2
i R
bn s
bi
are the voltage and current
2
21
4 TR
k
J
s
4 TR
290 K
k
i
bi
T
s
R R
f
, T is in K
g
g
2
is the
4 T R R
k
f
g
8
Evaluation Board
Separate evaluation boards are available for prototyping
and measurements. Additional information is available in
the evaluation board literature.
Low Noise Composite Amp With Input Matching
The composite amp shown in Figure 10 eliminates the
need for a matching resistor to ground at the input. By
connecting two amplifiers in series, the first non-
inverting and the second inverting, an overall inverting
gain is realized. The feedback resistor (R
loop, and generates a set input resistance (R
be matched to R
matching resistor to ground at the input.
The input resistance and DC voltage gain of the
amplifier are:
Match the source resistance by setting: R
The voltage noise produced by R
V
The noise of a simple input matching resistor connected to
ground can be calculated by setting G to 0 in this equation.
Thus, this circuit reduces the thermal noise power
produced by the matching resistor by a factor of (1+G).
Rectifier Circuit
Wide bandwidth rectifier circuits have many applications.
Figure 11 shows a 200MHz wideband full-wave rectifier
circuit using a CLC446 and a CLC522 amplifier. Schottky
or PIN diodes are used for D
active half-wave rectifier whose signals are taken at the
feedback diode connection. The CLC522 takes the
difference of the two half-wave rectified signals,
producing a full-wave rectifier. The CLC522 is used at a
gain of 5 to achieve high differential bandwidth. For best
V
S
s
, is:
+
-
R
For prototyping, use flush-mount printed circuit
board pins; never use high profile DIP sockets
R
s
V
V
in
o
s
CLC446 Applications
Figure 10: Composite Amplifier
R
R
in
g1
1 G
e
R
G
Rf
f
2
+
CLC446
S
-
, where G
R
. R
in
4 TR
R
k
R
in
f
f1
R
generates less noise than a
s
s
1
R
R
and D
20
R
g2
in
1
f
f
, referred to the source
R
1 G
s
R
R
2
g1
f1
. They produce an
CLC446
+
-
R
in
f2
f
R
= R
R
) closes the
g2
in
f2
) that can
S
.
V
o