Datasheet

OPA843
11
SBOS268C
www.ti.com
the 3rd-harmonic much lower. 2-tone 3rd-order intermodulation
terms will be much lower than most other solutions using the
circuit shown on the front page. The differential typical charac-
teristic curves also show that a 4V
PP
output will have
> 80dBc SFDR through 20MHz using this differential approach.
WIDE DYNAMIC RANGE “IF” AMPLIFIER
The OPA843 offers an attractive alternative to standard fixed-
gain IF amplifier stages. Narrowband systems will benefit from
the exceptionally high 2-tone 3rd-order intermodulation inter-
cept, as shown in the Typical Characteristics. Op amps with
high open-loop gain, like the OPA843, provide an intercept
that decreases with frequency along with the loop gain. The
OPA843’s 3rd-order intercept shows a decreasing intercept
with frequency. The OPA843’s intercept is > 30dBm up to
50MHz but improves to > 50dBm as the operating frequency
is reduced below 10MHz. Broadband systems will also benefit
from the very low even-order harmonics and intermodulation
components produced by the OPA843. Compared to standard
fixed-gain IF amplifiers, the OPA843 operating at IF’s below
50MHz provides much higher intercepts for its quiescent
power dissipation (200mW), superior gain accuracy, higher
reverse isolation, and lower I/O return loss. The noise figure
for the OPA843 will be higher than alternative fixed-gain
stages. If the application comes late in the amplifier chain with
significant gain in prior stages, this higher noise figure may be
acceptable. Figure 3 shows an example of a noninverting
configuration for the OPA843 used as an IF amplifier.
1dB through 50MHz. For narrowband IF’s in the 44MHz
region, this configuration of the OPA843 will show a 3rd-order
intercept of 33dBm while dissipating only 200mW (23dBm)
power from ±5V supplies.
PHOTODIODE TRANSIMPEDANCE AMPLIFIER
High Gain Bandwidth Product (GBP) and low input voltage
and current noise make the OPA843 an ideal wideband
transimpedance amplifier for low to moderate gains. Note
that unity-gain stability is not required for transimpedance
applications. Figure 4 shows an example photodiode ampli-
fier circuit. The key parameters of this design are the esti-
mated diode capacitance (C
D
) at the applied DC reverse bias
voltage (–V
B
), the desired transimpedance gain (R
F
), and the
GBP for the OPA843 (800MHz). With these three variables
set (and adding the OPA843’s parasitic input capacitance to
the value of C
D
to get C
S
), the feedback capacitor value (C
F
)
is selected to provide stability for the transimpedance fre-
quency response.
The input signal and the gain resistor are AC-coupled through
the 0.01µF blocking capacitors. This holds the DC input and
output operating point at ground independent of source im-
pedance and gain setting. The R
G
value in Figure 3 (144Ω),
sets the gain to the matched load at 12dB. Using standard 1%
tolerance resistors for R
F
and R
G
will hold the gain to a ±0.2dB
tolerance. This example will give a –3dB bandwidth of ap-
proximately 100MHz while maintaining gain flatness within
To achieve a maximally flat 2nd-order Butterworth frequency
response, the feedback pole should be set to:
1
24ππRC
GBP
RC
FF F
S
CCC
S
DI
=
=+
(1)
Adding the OPA843’s common-mode and differential mode
input capacitances C
I
= (1.0 + 1.2)pF to the 20pF diode
source capacitance of Figure 4, and targeting a 10kΩ tran-
simpedance gain using the 800MHz GBP for the OPA843,
the required feedback pole frequency is 16.9MHz. This will
require a total feedback capacitance of 0.94pF. Typical
surface-mount resistors have a parasitic capacitance of
0.2pF, leaving the required 0.75pF value shown in Figure 4
to get the required feedback pole.
This will set the –3dB bandwidth according to:
F
GBP
RC
Hz
dB
F
S
−
≅
3
2π
(2)
The example of Figure 4 will give approximately 24MHz
–3dB bandwidth using the 0.75pF feedback compensation.
OPA843
+5V
+5V
R
S
50Ω
V
O
P
I
P
0
0.01µF
R
G
144Ω
1kΩ
52.3Ω
R
F
1kΩ
50Ω Source
50Ω Load
0.01µF
Power-supply
decoupling not shown.
Gain =
P
I
P
O
= 20log
1
2
1 +
R
F
R
G














dB = 12dB with valuesshown
FIGURE 3. High Dynamic Range IF Amplifier.
R
F
10kΩ
Power-supply decoupling
not shown.
λ
OPA843
+5V
–5V
–V
B
C
F
0.75pF
I
D
V
O
= I
D
R
F
C
D
20pF
0.01µF10kΩ
FIGURE 3. High Dynamic Range IF Amplifier.