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LMH6601_09 Datasheet, PDF (24/28 Pages) National Semiconductor (TI) – 250 MHz, 2.4V CMOS Operational Amplifier with Shutdown
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The diode capacitance (CD) along with the input capacitance
of the LMH6601 (CA) has a bearing on the stability of this cir-
cuit and how it is compensated. With large transimpedance
gain values (RF), the total combined capacitance on the am-
plifier inverting input (CIN = CD + CA) will work against RF to
create a zero in the Noise Gain (NG) function (see Figure
14). If left untreated, at higher frequencies where NG equals
the open loop transfer function there will be excess phase shift
around the loop (approaching 180°) and therefore, the circuit
could be unstable. This is illustrated in Figure 14.
Figure 14 shows that placing a capacitor, CF, with the proper
value, across RF will create a pole in the NG function at fP.
For optimum performance, this capacitor is usually picked so
that NG is equal to the op amp's open loop gain at fP. This will
cause a “flattening” of the NG slope beyond the point of in-
tercept of the two plots (open loop gain and NG) and will
results in a Phase Margin (PM) of 45° assuming fP and fZ are
at least a decade apart. This is because at the point of inter-
cept, the NG pole at fP will have a 45° phase lead contribution
which leaves 45° of PM. For reference, Figure 14 also shows
the transimpedance gain (I-V (Ω))
Here is the theoretical expression for the optimum CF value
and the expected −3 dB bandwidth:
(9)
(10)
Table 3, below, lists the results, along with the assumptions
and conditions, of testing the LMH6601 with various photodi-
odes having different capacitances (CD) at a transimpedance
gain (RF) of 10 kΩ.
20136460
FIGURE 14. Transimpedance Amplifier Graphical
Stability Analysis and Compensation
TABLE 3. Transimpedance Amplifier Figure 12 Compensation and Performance Results
CD
CIN
CF_Calculated
CF used
−3 dB BW
−3 dB BW
Step Response
(pF)
(pF)
(pF)
(pF)
Calculated (MHz) Measured (MHz) Overshoot (%)
10
12
1.1
1
14
15
6
50
52
2.3
3
7
7.0
4
500
502
7.2
8
2
2.5
9
CA = 2 pF
GBWP = 155 MHz
VS = 5V
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