Transimpedance amplifier (TIA)

01

Set gain, bandwidth and stability

Texas Instruments' analog engineer's circuit note on the transimpedance amplifier sets the gain resistor from the output and input ranges, R1 = (5 V − 0 V) ÷ 50 µA = 100 kΩ, and the feedback capacitor for the bandwidth, C1 ≤ 1 ÷ (2π × 100 kΩ × 10 kHz) ≈ 159 pF, using 150 pF. It then checks stability: the op amp's gain-bandwidth must exceed (Ci + C1) ÷ (2π × R1 × C1²), about 11 kHz here with 6 pF of total input capacitance (source plus differential and common-mode amplifier capacitance). [1]

The note recommends JFET or CMOS input op amps with low bias current to reduce DC errors, operating within the linear output swing, and adding a bias voltage to the non-inverting input to set the output for zero input current; its example uses the OPA170, with 8 pA bias current and 1.2 MHz unity-gain bandwidth. [1]

For fast optical systems, TI's OPA855 data sheet describes a bipolar-input amplifier with 8 GHz gain-bandwidth, stable at gains of 7 V/V or more, with 0.6 pF common-mode and 0.2 pF differential input capacitance, which enables high closed-loop bandwidths at transimpedance gains up to tens of kilohms in lidar, OTDR and time-of-flight receivers. [2]

Give the supplier the sensor and its capacitance and dark current, the input current range, required output range, bandwidth and rise time, noise and DC accuracy targets, supply voltages, output (single-ended or differential, ADC drive), temperature range, package or module form, connector and quantities.

02

Check bias current, noise and supply

The OPA855 lists input-referred current noise of 2.5 pA/√Hz at 1 MHz, input bias current of −12 µA typical (−18.5 to −5 µA) at 25 °C with −0.08 µA/°C drift, a 3.3–5.25 V supply, 17.8 mA quiescent current, 3 V peak-to-peak output swing and −40 to +125 °C operation, in an 8-pin WSON package or as bare die. [2]

Editorial calculation: 12 µA of bias current through a 10 kΩ transimpedance gain gives about 0.12 V of output offset, acceptable for pulsed lidar with AC coupling but not for DC light measurement; the OPA170's 8 pA through 100 kΩ gives under 1 µV. Choose the input technology for the signal: low-bias JFET/CMOS for slow DC currents, high-speed bipolar for fast pulses. [1] [2]

At the slow, sensitive end, FEMTO's DLPCA-200 variable-gain current amplifier (data sheet R19, 2019) shows the gain–bandwidth–noise trade directly. Its transimpedance switches from 10³ to 10¹¹ V/A with ±1% accuracy and bandwidth independent of detector capacitance up to 1 nF. In low-noise mode the −3 dB bandwidth falls from 500 kHz at 10³–10⁴ V/A to 200 kHz at 10⁶, 7 kHz at 10⁸ and 1.1 kHz at 10⁹ V/A, while input noise current density falls from 20 pA/√Hz to 130 fA/√Hz at 10⁶ and 4.3 fA/√Hz at 10⁹ V/A. Input bias current is 1 pA typical (3 pA maximum), output ±10 V into at least 100 kΩ, and an adjustable ±10 V detector bias is provided. [3]

Editorial calculation: at the DLPCA-200's 10⁸ V/A setting, 1 nA of photocurrent gives 0.1 V of output, and the maximum linear input of 0.1 µA gives the full 10 V; a signal that must be resolved at kHz rates and also span three or more decades of current may need gain switching or a log amplifier. [3]

Editorial calculation: 17.8 mA at 5 V is about 89 mW per channel for the OPA855; multi-channel receivers multiply this. [2]

03

Match the feedback resistor to the photodiode's shunt resistance

Hamamatsu's technical note on Si photodiodes describes the basic photodiode-to-op-amp connection, in which the output is Vout = −Isc × Rf. If the feedback resistance is made greater than the photodiode's shunt resistance Rsh, the op amp's equivalent input voltage noise and input offset voltage are multiplied by (1 + Rf/Rsh) and appear at the output, and bias current error also grows, "making it not practical to use an infinitely large feedback resistance". A feedback capacitance Cf prevents instability when there is input capacitance Ct, and Cf × Rf sets a low-pass time constant. Its example circuit uses Rf = 10 MΩ, Cf = 10 pF, Ct = 100 pF and Rsh = 100 MΩ. [4]

The bias current figures explain the op amp choice on this page. TI's circuit note uses a CMOS op amp with 8 pA bias in its 100 kΩ example, while the bipolar OPA855 has −12 µA typical bias for GHz-class speed [1] [2]. Hamamatsu adds that a FET-input op amp's bias current doubles every 10 °C while a bipolar one falls, so it suggests considering a bipolar op amp for high-temperature operation, and notes that circuit-board leakage can exceed the op amp's bias current [4]. Ask the supplier for the bias current at your maximum temperature and how the board guards the input node.

FEMTO's DLPCA-200 switches transimpedance gain from 10³ to 10¹¹ V/A with bandwidth falling as gain rises [3]. Hamamatsu's Rf × Cf rule gives the same trade-off for a discrete design [4]; give the supplier your photodiode's shunt resistance and terminal capacitance so the gain resistor and feedback capacitor can be checked against both.

Editorial calculation: in Hamamatsu's example, Rf × Cf = 10 MΩ × 10 pF = 100 µs, a −3 dB frequency of about 1.6 kHz (1 ÷ 2π × 100 µs), and with Rsh = 100 MΩ the noise gain factor 1 + Rf/Rsh is 1.1. [4]

Values are from Hamamatsu's Si photodiodes technical note (text extracted from the PDF); the example circuit is illustrative.

04

Compare what suppliers offer

Suppliers linked to this page list transimpedance amplifiers or amplified photodetector modules; each card links to the page reviewed.

Listings include amplified photodetector modules (PIN and APD) and high-speed and balanced photodetector modules.

05

Trace the TIA components

This is a functional reference built from the sources cited; it is not one maker's design.

06

Fill the specification fields with values for your sensor

A TIA quotation needs values for your sensor. The table explains each field, gives published values where a reviewed source states them, and says what to obtain from the supplier.

07

Send the sensor, signal and accuracy data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Sensor: [type; capacitance; dark current]
Signal: [current range; DC or pulsed]
Output: [range; bandwidth; ADC]
Accuracy: [offset; noise]
Supply: [voltages; power]
Form: [IC; module; package; quantity]

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