clc425 National Semiconductor Corporation, clc425 Datasheet - Page 7

• Manufacturer Part Number: clc425
• Description: Ultra Low Noise Wideband Op Amp
• Manufacturer: National Semiconductor Corporation

Inverting Gains Less Than 10V/V
The lag compensation of Figure 9 will achieve stability
for lower gains. Placing the network between the two
input terminals does not affect the closed-loop nor noise
gain, but is best used for the invering configuration
because of its affect on the non-inverting input imped-
ance.
Single-Supply Operation
The CLC425 can be operated with single power supply
as shown iin Figure 10. Both the input and output are
capacitively coupled to set the dc operating point.
Low Noise Transimpedance Amplifier
The circuit of Figure 11 implements a low-noise transim-
pedance amplifier commonly used with photo-diodes.
The transimpedance gain is set by R
frequency response is shown in Figure 12 and shows
the influence C
provides the total input current noise density (i
tion for the basic transimpedance configuration and is
plotted against feedback resistance (R
contributing noise sources in Figure 13. This plot indi-
cates the expected total equivalent input current noise
density (i
equivalent output voltage noise density (e
R
V
s =
ac
V
75
Figure 8: External Shunt Compensation
ni
in
Figure 9: External Lag Compensation
39pF
) for a given feedback resistance (R
Figure 10: Single Supply Operation
C
C
in
R
R
R
g
V
R
f
cc
g =
R
C
has over gain flatness. Equation 4
124
R
C
75
R
V
R
g
2
cc
in
b
CLC425
CLC425
R
f
R
V
f
cc
CLC425
C
R
f =
I
cc
f =
=5mA
10pF
124
C
V
V
out
out
R
out
R
f
=
. The simulated
out
V
2
cc
f
) showing all
75
no
+ A
R
) is i
f
). The total
L
R
v
V
L
ni
ac
) equa-
ni
75
R
f
.
7
Figure 13: Current Noise Density vs. Feedback Resistance
Very Low Noise Figure Amplifier
The circuit of Figure 14 implements a very low Noise
Figure amplifier using a step-up transformer combined
with a CLC425 and a CLC404. The circuit is configured
with a gain of 35.6dB. The circuit achieves measured
Noise Figures of less than 2.5dB in the 10-40MHz
region. 3
cies less than 40MHz and gain flatness of 0.5dB is measured
in the 1-50MHz pass bands. Application Note OA-14 provides
greater detail on these low Noise Figure techniques.
Figure 12: Transimpedance Amplifier Frequency Response
Figure 11: Transimpedance Amplifier Configuration
Equation 4: Total Equivalent Input Refered Current
rd
order intercepts exceed +30dBm for frequen-
+V
i
cc
ni
R
b
i
n
2
CLC425
R
e
n
f
C
R
f
f
2
A
v
4
R
kT
f
http://www.national.com
I
in
R
f