Transimpedance amplifier circuits

Low inverting, input current noise (2pA/

) makes the

CLC420 ideal for high-sensitivity transimpedance amplifier

circuits for applications such as pin-diode optical receivers,

and detectors in receiver IFs. However, feedback resistors

4k

Ω or greater are required if feedback resistor noise current

is going to be less than the input current noise contribution of

the op-amp.

With feedback resistors this large, shunt capacitance on the

inverting input of the op-amp (from the pin-diode, etc.) will

unacceptably degrade phase margin causing frequency re-

sponse peaking or oscillations a small valued capacitor

shunting the feedback resistor solves this problem (Note:

This approach does not work for a current-feedback op-amp

configured for transimpedance applications). To determine

the value of this capacitor, refer to the “Transimpedance BW

vs. R

For example, let’s assume an optical transimpedance re-

ceiver is being developed. Total capacitance from the invert-

ing input to ground, including the photodiode and strays is

5pF. A 5k

Ω feedback resistor value has been determined to

provide best dynamic range based on the response of the

photodiode and the range of incident optical powers, etc.

From the “Transimpedance BW vs. R

C

that C

0.5dB frequency response peaking) and a −3dB bandwidth

of approximately 27MHz.

Printed circuit layout

As with any high frequency device, a good PCB layout will

enhance performance. Ground plane construction and good

power supply bypassing close to the package are critical to

achieving full performance. The amplifier is sensitive to stray

capacitance to ground at the output and inverting input:

Node connections should be small with minimal coupling to

the ground plane.

Parasitic or load capacitance directly on the output (pin 6)

will introduce additional phase shift in the loop degrading the

loop phase margin and leading to frequency response peak-

ing. A small series resistor before this capacitance, if

present, effectively decouples this effect. The graphs on the

preceding page, “ Settling Time vs. C

quired resistor value and resulting performance vs. capaci-

tance.

Evaluation PC boards (part no. 730013 for through-hole and

CLC730027 for SOIC) are available for the CLC420.

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