9.2.4 Wide-Band Transimpedance Amplifier

Wide bandwidth, low input bias current, and low input voltage and current noise make the OPAx620 devices

excellent choices for wideband photo-diode transimpedance amplifier applications, particularly in low-voltage

single-supply designs. Low-voltage noise is critical because photo-diode capacitance causes the effective noise

gain of the circuit to increase at high frequencies.

9.2.4.1 Detailed Design Procedure

The key elements of a transimpedance amplifier design include the expected diode capacitance, CD (including

parasitic input common-mode and differential-mode capacitance of 2 + 2pF for the OPAx620), the desired

transimpedance gain (RF), and the gain-bandwidth product (GBW) of the OPAx620 (100MHz typical). Once

these three variables are determined, the feedback capacitor value (CF) is selected to control the frequency

response. The feedback capacitance calculation should include stray capacitance, which is typically 0.2pF for

surface-mount resistors

OPAx620

(prevents gain peaking)

I

C

D

10M

VOUT

V+

RF

CF < 1pF

V–

Figure 9-5. Transimpedance Amplifier

To achieve a maximally flat, second-order, Butter-worth frequency response, the feedback pole must be set as

shown in Equation 1

1

2 × π × RF × CF  =  GBW

4 × π × RF × CD

(5)

Typical surface-mount resistors have a parasitic capacitance of approximately 0.2pF. Deduct that parasitic

capacitance from the calculated feedback capacitance value. Equation 2 calculates the bandwidth:

f−3dB =  GBW

2× π × RF × CD

(6)

For even higher transimpedance bandwidth, use the high speed FET input OPA355 (200MHz GBW) or the

OPA657 (1600MHz GBW)

9.3 Power Supply Recommendations

The OPAx620 family of devices is specified for operation from 2.7V to 5.5V (±1.35V to ±2.75V); many

specifications apply from –40°C to +125°C.

Place 0.1µF bypass capacitors close to the power-supply pins to reduce errors coupling in from noisy or

high-impedance power supplies. See also Section 9.4.1.

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OPA620, OPA2620

SLOSEG2A – APRIL 2026 – REVISED JUNE 2026

Copyright © 2026 Texas Instruments Incorporated

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