1

2 R C

p

F

F

=

GBP

4 R C

p

F

D

+

49 pF

0.3 pF

OPA657

+5 V

-5 V

0.55 pF

50

Oscilloscope

with 50

Inputs

200 k



Supply Decoupling not

shown

OPA657

www.ti.com

SBOS197F – DECEMBER 2001 – REVISED AUGUST 2015

9.2 Typical Application

The high GBP and low input voltage and current noise for the OPA657 make it an ideal wideband

transimpedance amplifier for moderate to high transimpedance gains. Unity-gain stability in the operational

amplifier is not required for application as a transimpedance amplifier.

Figure 34. Wideband, High-Sensitivity, Transimpedance Amplifier Diagram

9.2.1 Design Requirements

Design a high-bandwidth, high-gain transimpedance amplifier with the design requirements shown in Table 1.

Table 1. Design Requirements

TARGET BANDWIDTH (MHz)

TRANSIMPEDANCE GAIN (k

Ω)

PHOTODIODE CAPACITANCE (pF)

1

200

49

9.2.2 Detailed Design Procedure

Designs that require high bandwidth from a large area detector with relatively high transimpedance gain benefit

from the low input voltage noise of the OPA657. This input voltage noise is peaked up over frequency by the

diode source capacitance, and can, in many cases, become the limiting factor to input sensitivity. The key

circuit with the parameters as described in Table 1. With these three variables set (and including the parasitic

control the frequency response. To achieve a maximally-flat second-order Butterworth frequency response, the

feedback pole should be set to:

(7)

Adding the common-mode and differential mode input capacitance (0.7 + 4.5) pF and the trace PCB capacitance

of approximately 0.3 pF to the 49-pF diode source capacitance of Figure 34, and targeting a 200-k

Ω

transimpedance gain using the 1600-MHz GBP for the OPA657 requires a feedback pole set to 3.5 MHz. This

requires a total feedback capacitance of 0.2 pF. Such low capacitance values are difficult to achieve due to

parasitics from the PCB and the surface mount components.

Copyright © 2001–2015, Texas Instruments Incorporated

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