SM73302_15 Datasheet, PDF (22/31 Pages) Texas Instruments – SM73302 88 MHz, Precision, Low Noise, 1.8V CMOS Input, Decompensated Operational Amplifier

SM73302
SNOSB93A – AUGUST 2011 – REVISED APRIL 2013
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2.5V
D_PHOTO
C_F
2.5V
R_F
-
C_D
C_CM
+
V_OUT
-2.5V
Figure 59. Transimpedance Amplifier
Figure 59 is the complete schematic for a transimpedance amplifier. Only the supply bypass capacitors are not
shown. C_D represents the photodiode capacitance which is given on its datasheet. C_CM is the input common
mode capacitance of the op amp and, for the SM73302 it is shown in Figure 43. In Figure 59 the inverting input
pin of the SM73302 is kept at virtual ground. Even though the diode is connected to the 2.5V line, a power
supply line is AC ground, thus C_D is connected to ground.
Figure 60 shows the schematic needed to derive F, the feedback factor, for a transimpedance amplifier. In this
figure C_D + C_CM = C_IN. Therefore it is critical that the designer knows the diode capacitance and the op amp input
capacitance. The photodiode is close to an ideal current source once its capacitance is included in the model.
What kind of circuit is this? Without C_F there is only an input capacitor and a feedback resistor. This circuit is a
differentiator! Remember, differentiator circuits are inherently unstable and must be compensated. In this case C_F
compensates the circuit.
C_F
R_F
I_DIODE
V_A
-
C_IN
+
V_OUT
Figure 60. Transimpedance Feedback Model
Using feedback theory, F = V_A/V_OUT, this becomes a voltage divider giving the following equation:
F = ^{1 + sCFRF}
1 + sR_F (C_F + C_IN)
(12)
The noise gain is 1/F. Because this is a differentiator circuit, a zero must be inserted. The location of the zero is
given by:
¶´z ⁼
1
1 + sR_F (C_F + C_IN)
(13)
C_F has been added for stability. The addition of this part adds a pole to the circuit. The pole is located at:
¶´p ⁼
1
1 + sC_FR_F
(14)
To attain maximum bandwidth and still have good stability the pole is to be located on the open loop gain curve
which is A. If additional compensation is required one can always increase the value of C_F, but this will also
reduce the bandwidth of the circuit. Therefore A = 1/F, or AF = 1. For A the equation is:
A
=
Z
GBW
Z
=
´¶_GBW
´¶
(15)
22
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