Datasheet
OPA846
12
SBOS250E
www.ti.com
diode capacitance changes, the feedback capacitor must
change to maintain a stable and flat frequency response.
Using Equation 1, C
F
is adjusted to give the Butterworth
frequency responses presented in Figure 4.
FIGURE 5. Broadband, Low-Gain, Inverting Amplifier.
FIGURE 4. Transimpedance Bandwidth versus C
D
.
83
80
77
74
71
68
65
62
Frequency (MHz)
PHOTODIODE TRANSIMPEDANCE
FREQUENCY RESPONSE
Tranimpedance Gain (dB Ω)
1 10 100
C
D
= 100pF
R
F
= 10kΩ
C
F
Adjusted
20 log(10kΩ)
C
D
= 50pF
C
D
= 20pF
C
D
= 10pF
R
F
10kΩ
C
D
λ
0.01µF
10kΩ
OPA846
–V
B
I
D
V
O
=
I
D
R
F
C
F
LOW-GAIN COMPENSATION FOR IMPROVED SFDR
Where a low gain is desired, and inverting operation is
acceptable, a new external compensation technique may be
used to retain the full slew rate and noise benefits of the
OPA846, while giving increased loop gain and the associ-
ated improvement in distortion offered by the decompen-
sated architecture. This technique shapes the loop gain for
good stability, while giving an easily controlled 2nd-order
low-pass frequency response. Considering only the noise
gain (noninverting signal gain) for the circuit of Figure 5, the
low-frequency noise gain (NG
1
) is set by the resistor ratios,
while the high-frequency noise gain (NG
2
) is set by the
capacitor ratios. The capacitor values set both the transition
frequencies and the high-frequency noise gain. If this noise
gain (determined by NG
2
= 1 + C
S
/C
F
) is set to a value
greater than the recommended minimum stable gain for the
op amp and the noise gain pole (set by 1/R
F
C
F
) is placed
correctly, a very well controlled, 2nd-order, low-pass fre-
quency response results.
To choose the values for both C
S
and C
F
, two parameters and
only three equations need to be solved. The first parameter is
the target high-frequency noise gain (NG
2
), which should be
greater than the minimum stable gain for the OPA846. Here,
a target NG
2
of 10.5 is used. The second parameter is the
desired low-frequency signal gain –(R
F
/R
G
), which also sets
the low-frequency noise gain NG
1
(= 1 + R
F
/R
G
). To simplify
this discussion, target a maximally flat 2nd-order, low-pass
Butterworth frequency response (Q = 0.707). The signal gain
of –2 shown in Figure 5 sets the low-frequency noise gain to
NG
1
= 1 + R
F
/R
G
(= 3 in this example). Then, using only these
two gains and the GBP for the OPA846 (1750MHz), the key
frequency in the compensation can be determined as:
Z
GBP
NG
NG
NG
NG
NG
O
=−
−−
1
2
1
2
1
2
112
(4)
R
F
500Ω
+5V
–5V
C
S
27pF
0.01µF
167Ω
OPA846
V
O
= – V
I
R
F
R
G
Power-supply
decoupling not shown.
V
I
C
F
2.9pF
R
G
250Ω
0Ω
Source
Physically, this Z
O
(11.6MHz for these values) is set by:
1
2πRC C
F
F
S
+
(
)
and is the frequency at which the rising portion of the noise
gain would intersect the unity gain if projected back to a 0dB
gain. The actual zero in the noise gain occurs at NG
1
• Z
O
,
and the pole in the noise gain occurs at NG
2
• Z
O
. Since GBP
is expressed in Hz, multiply Z
O
by 2π, and use this to get C
F
by solving:
C
RZ NG
pF
F
F
O
==
(
)
1
2
286
2
π
.
(5)
Finally, since C
S
and C
F
set the high-frequency noise gain,
determine C
S
by using NG
2
= 10.5:
C NG C which gives C pF
S
F
S
=−
(
)
=
2
1249,.
(6)
The resulting closed-loop bandwidth is approximately equal to:
f Z GBP
dB
O
−
•
≅
3
(7)
For the values of Figure 5, f
–3dB
is approximately 142MHz.
This is less than that predicted by 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
. The capacitor values
shown in Figure 5 are calculated for NG
1
= 3 and NG
2
= 10.5
with no adjustment for parasitic components.
See Figure 6 for the measured frequency response for the
circuit of Figure 5. This shows the expected gain of –2 (6dB)
with exceptional flatness through 70MHz and a –3dB band-
width of 170MHz. Repeating the swept frequency distortion
measurement for a 2V
PP
output into a 200Ω load and
comparing to the gain of +10 data shown in the Typical
Characteristic curves illustrates the improved distortion for
this low-gain compensation circuit.
Figure 7 compares the distortion at a gain of +10 for the
circuit of Figure 1 to the distortion at a gain of –2 for the circuit
of Figure 5.