Datasheet

OPA847
13
SBOS251E
www.ti.com
Finally, since C
S
and C
F
set the high-frequency noise gain,
determine C
S
using Equation 6 (solving for C
S
by using NG
2
= 24):
CNG C
S
F
=−
( )
2
1
(6)
which gives C
S
= 40.6pF.
Both of these calculated values have been reduced slightly
in Figure 5 to account for parasitics. The resulting closed-
loop bandwidth is approximately equal to Equation 7.
f Z GBP
dB
O
–3
≅ •
(7)
For the values shown in Figure 5, f
–3dB
is approximately
131MHz. This is less than that predicted by simply dividing
the GBP product by NG
1
. The compensation network controls
the bandwidth to a lower value, while providing the full slew
rate at the output and an exceptional distortion performance
due to increased loop gain at frequencies below NG
1
• Z
O
.
Using this low-gain inverting compensation, along with the
differential structure for the circuit shown on the front page of
this data sheet, gives a significant reduction in harmonic
distortion. The measured distortion at 2V
PP
output does not
rise above –95dB until frequencies > 20MHz are applied.
The Typical Characteristics show the exceptional bandwidth
control possible using this technique at low inverting gains.
Figure 6 repeats the measured results with the test circuit shown.
The compensation capacitors, C
S
and C
F
, are set by targeting
a high-frequency noise gain of 21 and using equations 4 through
6. This approach allows relatively low inverting gain applications
to use the full slew rate and low input noise of the OPA847.
LOW-NOISE FIGURE,
HIGH DYNAMIC RANGE AMPLIFIER
The low input noise voltage of the OPA847 and its very high
2-tone, 3rd-order intermodulation intercept can be used to
good advantage as a fixed-gain amplifier. While input noise
figures in the 10dB range (for a matched 50Ω input) are
easily achieved with just the OPA847, Figure 7 illustrates a
technique to reduce the noise figure even further, while
providing a broadband, high-gain HF amplifier stage using
two stages of the OPA847.
FIGURE 6. Low-Gain Inverting Performance.
1
0
–1
–2
–3
–4
–5
–6
–7
–8
–9
LOW GAIN INVERTING BANDWIDTH
Frequency (MHz)
Normalized Gain (1dB)
1 10 100 1000
G = –8
G = –4
G = –2
G = –1
V
O
= 0.2V
PP
R
F
750Ω
C
S
OPA847
V
O
V
I
C
F
R
G
0Ω Source
V
DIS
+5V
–5V
FIGURE 7. Very High Dynamic Range HF Amplifier.
OPA847
10pF
P
I
1.6pF
6.19kΩ
750Ω
1.5kΩ
200Ω
30.1Ω
420Ω
4.3dB
Noise
Figure
1:2
50Ω Source
P
O
> 55dBm
intercept
to 30MHz
OPA847
46pF
Input match
set by this
feedback path
Overall Gain = 35.6dB
P
O
P
I
+5V
–5V
+5V
–5V