Simplify Transimpedance Applications with High-Bandwidth, Precision JFET Op Amps

Simplify Transimpedance Applications with High- bandwidth, Precision JFET Op Amps Raphael Puzio, Luis Chioye

Modern JFET-input operational amplifiers (op amps) Transimpedance Op Amp Gain, Bandwidth, and offer a combination of high input impedance, excellent Stability DC and AC performance, low noise, high bandwidth, and wide voltage supply range, making them a natural When selecting an op amp for a photodiode TIA choice for transimpedance op amp (TIA) applications. application, it is important to carefully consider the A TIA is a current-to-voltage converter, typically used three factors that dictate the minimum gain bandwidth as a front-end for optical sensors such as photodiodes, product of the op amp (fGBW) necessary for circuit commonly found in Optical Line Cards, Light Level stability: the TIA required V/I gain, the required closed- Sensors, PM 2.5 Detectors, and many more. Silicon loop TIA bandwidth, and the parasitic junction photodiodes produce a current output that changes capacitance of the photodiode (CJ). linearly with incident light, where the typical photo current ranges from a few pico-amps to a few The stability of an op amp is related to its closed-loop milliamps. The combination of high input impedance, gain and phase response over frequency. The closed- with low input bias currents of a few pico-amps at loop gain is defined as the product of the open-loop room temperature, and very low current and voltage gain of the op amp (AOL) and the feedback factor of noise, allow JFET op amps to be used in precision, the op amp (β), defined as AOL × β. Figure 2 shows high-resolution photodiode applications. Furthermore, the bode plots of the open loop gain (AOL) and 1/β JFET op amps operate over a wide voltage range and plot of a typical TIA. See the TI Precision Labs - Op are able to cover a broad spectrum of photodiode Amps: Stability – Lab Video Series for more output current, which is beneficial in TIA applications information about stability analysis and simulation. as this increases the overall resolution and accuracy of the system. Figure 1 shows a typical photodiode AOL: High BW OPA transimpedance application. AOL: Lower BW OPA 1/t

CF 1 RF fP = Total parasitic capacitance: 2pCRFF GND Rate of closure = 20dB CT = CJ + CDIFF +CCM2 (dB)Gain STABLE C Rate of closure = 40dB Photodiode Current (I) CM2 UNSTABLE 1 fz = œ VOUT = I x RF 2p (CCRFTF+ ) CF OPA145 ffC= GBW CDIFF CCFT+ + CJ fz fP fC fGBW Frequency

CCM1 Figure 2. AOL and 1/β Plot for Transimpedance Op GND GND Op amp‘s parasitic input Photodiode Capacitance: Neglect CCM1 since Amp Circuit parasitic junction capacitance non-inverting input is grounded Figure 1. Photodiode Transimpedance Amplifier The 1/β curve of the Figure 2 presents a zero (fZ) and a pole (fP) on its frequency response. Above the zero The feedback resistor (RF) across the op amp (fZ), the 1/β curve increases a rate of +20 dB per converts the photodiode current (I) to a voltage decade. At frequencies above the pole (fP), the 1/β (VOUT) using Ohm’s law, shown in Equation 1: curve remains flat. The 1/β curve intersects the AOL curve at frequency fC is shown in Equation 2: VOUT = I × RF (1)

The feedback resistor (RF) determines the gain of the (2) transimpedance op amp, and the feedback capacitor In Equation 2, f is the unity gain bandwidth of the (C ) defines the closed-loop bandwidth of the circuit. In GBW F op amp. By analyzing the rate of closure of AOL and addition, the feedback capacitor (C ) is required for F 1/β when the curves intersect, you can determine the stability and is used to compensate for the total stability of the circuit. A handy rule of thumb for this parasitic capacitance (C ) at the inverting input of the T method is that the rate of closure must equal 20-dB for op amp: the photodiode junction capacitance (C ) and J optimal stability. Therefore, to maintain stability, the the input capacitance of the op amp (C + C ). DIFF CM2 AOL curve must intersect the 1/β curve when the 1/β SPACER curve is flat (assuming a unity gain stable op amp). If the AOL curve intersects the 1/β curve when the 1/β

SBOA354–May 2019 Simplify Transimpedance Applications with High-bandwidth, Precision JFET 1 Submit Documentation Feedback Op Amps Raphael Puzio, Luis Chioye Copyright © 2019, Texas Instruments Incorporated www.ti.com curve is rising, as shown by the lower bandwidth op In contrast, the higher fGBW OPA828 is able to support amp AOL curve in Figure 2, the circuit may be the TIA gain of 50 k V/A with 1-MHz bandwidth while unstable, leading to many unfavorable circuit offering 55 degrees of phase margin using a single- behaviors. Equation 3 gives us the necessary stage as shown on Figure 5. A single stage TIA condition to avoid these problems: implemented with a precision, high bandwidth op amp (3) offers better noise performance and accuracy than a dual stage version built with two lower bandwidth Substituting the equations for fC and fP into the amplifiers of similar accuracy, since only one amplifier inequality provided on equation 3, and solving for the contributes to noise, offset, and drift errors in the amplifiers unity gain bandwidth (fGBW), provides a system. This greatly simplifies your design while useful equation. offering lower component count, simpler routing and a Equation 4 determines the minimum required smaller solution size. bandwidth of the amplifier to guarantee stability for a 3.2 pF T 140 OPA828 Single-Stage TIA TIA design. Therefore, higher bandwidth amplifiers 50 k TIA Gain = 50kV/A, BW = 1MHz 120 OPA828 fGBW = 45-MHz support higher gain and bandwidth TIA circuits, and -15V 100 tolerate a higher parasitic photodiode capacitance 100nF AOLAOL 80 Photodiode œ GND while remaining stable. Consider the case of a Vout OPA828 60 photodiode application with the following Gain(dB) + RateRate of of Closure 100nF 40 == 20-dB 20-dB specifications: Transimpedance Amplifier Gain (50-K 1/Beta1/Beta Stable!Stable! 20 GND V/A), Transimpedance Amplifier Bandwidth, 1-MHz, GND +15V and Photodiode Junction Capacitance (Cj) (100 pF). 0 OPA828 Single Stage: Transimpedance G=50k V/A -20 1 10 100 1k 10k 100k 1M 10M (4) Frequency (Hz)

For comparison, consider two implementations with op Figure 5. Single-stage OPA828 (fGBW = 45-MHz) TIA amps with different bandwidths: the OPA140 offers 11- Stability Analysis (Stable) MHz of gain bandwidth product, while the OPA828 offers 45-MHz. Using the previously derived equation Summary for minimum bandwidth, the minimum fGBW for the Modern JFET op amps combine high input impedance, transimpedance op amp is ~37-MHz. Using a single excellent noise performance, high bandwidth, and wide stage, the OPA140 is unstable as shown on Figure 3. output voltage range, making JFET amplifiers an

T 3.2 pF 140 optimal choice in the use of high gain, and high OPA140 Single-Stage TIA TIA Gain = 50kV/A, BW = 1MHz 120 resolution transimpedance photodiode circuits. 50 k OPA140 fGBW = 11-MHz

100 -15V 100nF AOLAOL 80 Table 1. TIA Op Amps

Photodiode œ GND 60 Vout DESCRIPTION OP AMP Gain(dB) OPA140 RateRate of of Closure 40 = = 40-dB40-dB 36-Volt, High-speed (45 MHz GBW and 150 V/µs SR), + Unstable!!Unstable!! OPA828 100nF low-noise (4 nV/√Hz) RRO JFET Op Amplifier 20 1/Beta1/Beta GND 5.5 MHz, High Slew Rate, Low-Noise, Low-power, RRO GND +15V 0 OPA145 Precision JFET Op Amplifier -20 OPA140 Single Stage: Low-Offset, Low-Drift, Low-Noise, 11-MHz, 36-V JFET- Transimpedance G=50k V/A 1 10 100 1k 10k 100k 1M 10M OPA140 Frequency (Hz) Input, RRO Op Amplifier

Figure 3. Single-stage OPA140 (fGBW = 11-MHz) Stability Analysis (Unstable) Table 2. Related Documentation

TYPE TITLE To meet the TIA requirements, two cascaded stages of Green-Williams-Lis: Improved Op Amp Spice Application Brief the OPA140 are required. A transimpedance stage Model with a lower gain of 10-kΩ cascaded a with a non- Application Report Cookbook Circuit: Transimpedance Amplifier inverting gain stage of 5 V/V as shown on Figure 4.

16 pF 1 k 4.02 k T 140 10 k OPA2140 Dual Stage TIA -15V 120 First TIA Stage Stability GND 100nF TIA Gain = 10kV/A, BW=1-MHz -15V 100 OPA2140 fGBW = 11-MHz 100nF GND œ 80 Vout AOLAOL Photodiode GND œ OPA2140 60 Gain(dB) + OPA2140 100nF 40 RateRate of of Closure 1/Beta1/Beta = = 20-dB20-dB + StableStable ! 100nF 20 GND 0 GND GND +15V +15V -20 100 10 100 1k 10k 1M 10M st nd k 1 Stage: Transimpedance 2 Stage: Non-Inverting Gain Frequency (Hz) G=10k V/A G=+5 V/V Figure 4. Alternate Two-stage OPA140 TIA Amplifier (Stable)

2 Simplify Transimpedance Applications with High-bandwidth, Precision JFET SBOA354–May 2019 Op Amps Raphael Puzio, Luis Chioye Submit Documentation Feedback Copyright © 2019, Texas Instruments Incorporated IMPORTANT NOTICE AND DISCLAIMER

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