LMH6601_14 Datasheet, PDF (27/37 Pages) National Semiconductor (TI) – LMH6601/LMH6601Q 250 MHz, 2.4V CMOS Operational Amplifier with Shutdown

LMH6601, LMH6601-Q1
www.ti.com
SNOSAK9E – JUNE 2006 – REVISED MARCH 2013
With the LMH6601 input bias current in the femto-amperes range, even large values of gain (R_F) do not increase
the output error term appreciably. This allows circuit operation to a lower light intensity level which is always of
special importance in these applications. Most photo-diodes have a relatively large capacitance (C_D) which would
be even larger for a photo-diode designed for higher sensitivity to light because of its larger area. Some
applications may run the photodiode with a reverse bias in order to reduce its capacitance with the disadvantage
of increased contributions from both dark current and noise current. Figure 60 shows a typical photodiode
capacitance plot vs. reverse bias for reference.
600
500 PIN-RD100
T = 23°C
400
PIN-RD100A
300
200
PIN-RD15
100
PIN-RD07
0
0.1
1
10
100
REVERSED BIAS VOLTAGE (V)
Figure 60. Typical Capacitance vs. Reverse Bias (Source: OSI Optoelectronics)
The diode capacitance (C_D) along with the input capacitance of the LMH6601 (C_A) has a bearing on the stability
of this circuit and how it is compensated. With large transimpedance gain values (R_F), the total combined
capacitance on the amplifier inverting input (C_IN = C_D + C_A) will work against R_F to create a zero in the Noise
Gain (NG) function (see Figure 61). 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 61.
OP AMP OPEN
LOOP GAIN
I-V GAIN (:)
NOISE GAIN (NG)
1 + sR_F (C_IN + C_F)
1 + sR_FC_F
1 + ^CIN
C_F
0 dB
FREQUENCY _{fz #} 1
2SR_FC_IN
1
GBWP
f_P = _2SRFCF
Figure 61. Transimpedance Amplifier Graphical Stability Analysis and Compensation
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