THS4601 Datasheet, PDF (18/27 Pages) Texas Instruments – WIDEBAND, FET-INPUT OPERATIONAL AMPLIFIER

THS4601
SLOS388B – OCTOBER 2001 – REVISED JUNE 2002
APPLICATION INFORMATION
alternative transimpedance configurations (continued)
The third configuration uses a capacitive T-network to achieve fine control of the compensation capacitance.
The capacitor C_F3 can be used to tune the total effective feedback capacitance to a very fine degree. This circuit
behaves the same as the basic transimpedance configuration, with the effective C_F given by equation 4.
ǒ Ǔ ¹
C_FEQ
+
1
C_F1
1
)
C_F3
C_F2
(4)
CF3
CF1
CF2
RF
_
λ
+
RL
–VBias
NOTE: A capacitive T-network enables fine control of the effective
feedback capacitance using relatively large capacitor values.
Figure 35. Alternative Transimpedance Configuration #3
summary of key decisions in transimpedance design
The following is a quick, simplified process for basic transimpedance circuit design. This process gives a quick
start to the design process, though it does ignore some aspects that may be critical to the circuit.
Step 1: Determine the capacitance of the source.
Step 2:
Step 3:
Calculate the total source capacitance, including the amplifier input capacitance, C_ICM and C_IDIFF .
Determine the magnitude of the possible current output from the source, including the minimum
signal current anticipated and maximum signal current anticipated.
Step 4:
Choose a feedback resistor value such that the input current levels create the desired output signal
voltages, and ensure that the output voltages can accommodate the dynamic range of the input
signal.
Step 5: Calculate the optimum feedback capacitance using equation 1.
Step 6: Calculate the bandwidth given the resulting component values.
Step 7: Evaluate the circuit to see if all design goals are satisfied.
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