Simplify Transimpedance Applications with Highbandwidth, Precision JFET Op Amps Raphael Puzio, Luis Chioye Modern JFET-input operational amplifiers (op amps) offer a combination of high input impedance, excellent DC and AC performance, low noise, high bandwidth, and wide voltage supply range, making them a natural choice for transimpedance op amp (TIA) applications. A TIA is a current-to-voltage converter, typically used as a front-end for optical sensors such as photodiodes, commonly found in Optical Line Cards, Light Level Sensors, PM 2.5 Detectors, and many more. Silicon photodiodes produce a current output that changes linearly with incident light, where the typical photo current ranges from a few pico-amps to a few milliamps. The combination of high input impedance, with low input bias currents of a few pico-amps at room temperature, and very low current and voltage noise, allow JFET op amps to be used in precision, high-resolution photodiode applications. Furthermore, JFET op amps operate over a wide voltage range and are able to cover a broad spectrum of photodiode output current, which is beneficial in TIA applications as this increases the overall resolution and accuracy of the system. Figure 1 shows a typical photodiode transimpedance application. Total parasitic capacitance: CT = CJ + CDIFF +CCM2 GND Photodiode Current (I) CCM2 CDIFF ± When selecting an op amp for a photodiode TIA application, it is important to carefully consider the three factors that dictate the minimum gain bandwidth product of the op amp (fGBW) necessary for circuit stability: the TIA required V/I gain, the required closedloop TIA bandwidth, and the parasitic junction capacitance of the photodiode (CJ). The stability of an op amp is related to its closed-loop gain and phase response over frequency. The closedloop gain is defined as the product of the open-loop gain of the op amp (AOL) and the feedback factor of the op amp (β), defined as AOL × β. Figure 2 shows the bode plots of the open loop gain (AOL) and 1/β plot of a typical TIA. See the TI Precision Labs - Op Amps: Stability – Lab Video Series for more information about stability analysis and simulation. AOL: High BW OPA AOL: Lower BW OPA 1/t Gain (dB) CF RF Transimpedance Op Amp Gain, Bandwidth, and Stability fP Rate of closure = 20dB STABLE Rate of closure = 40dB UNSTABLE fz VOUT = I x RF OPA145 1 2S CFRF 1 2S (CF CT ) RF fC + CJ fz fP fC CF fGBW CF CT fGBW Frequency CCM1 GND GND Photodiode parasitic junction capacitance Figure 2. AOL and 1/β Plot for Transimpedance Op Amp Circuit 2S DPS¶V SDUDVLWLF LQSXW Capacitance: Neglect CCM1 since non-inverting input is grounded Figure 1. Photodiode Transimpedance Amplifier The feedback resistor (RF) across the op amp converts the photodiode current (I) to a voltage (VOUT) using Ohm’s law, shown in Equation 1: VOUT = I × RF (1) The feedback resistor (RF) determines the gain of the transimpedance op amp, and the feedback capacitor (CF) defines the closed-loop bandwidth of the circuit. In addition, the feedback capacitor (CF) is required for stability and is used to compensate for the total parasitic capacitance (CT) at the inverting input of the op amp: the photodiode junction capacitance (CJ) and the input capacitance of the op amp (CDIFF + CCM2). SPACER SBOA354 – May 2019 Submit Documentation Feedback The 1/β curve of the Figure 2 presents a zero (fZ) and a pole (fP) on its frequency response. Above the zero (fZ), the 1/β curve increases a rate of +20 dB per decade. At frequencies above the pole (fP), the 1/β curve remains flat. The 1/β curve intersects the AOL curve at frequency fC is shown in Equation 2: (2) In Equation 2, fGBW is the unity gain bandwidth of the op amp. By analyzing the rate of closure of AOL and 1/β when the curves intersect, you can determine the stability of the circuit. A handy rule of thumb for this method is that the rate of closure must equal 20-dB for optimal stability. Therefore, to maintain stability, the AOL curve must intersect the 1/β curve when the 1/β curve is flat (assuming a unity gain stable op amp). If the AOL curve intersects the 1/β curve when the 1/β Simplify Transimpedance Applications with High-bandwidth, Precision JFET Op Amps Raphael Puzio, Luis Chioye Copyright © 2019, Texas Instruments Incorporated 1 www.ti.com curve is rising, as shown by the lower bandwidth op amp AOL curve in Figure 2, the circuit may be unstable, leading to many unfavorable circuit behaviors. Equation 3 gives us the necessary condition to avoid these problems: (3) Substituting the equations for fC and fP into the inequality provided on equation 3, and solving for the amplifiers unity gain bandwidth (fGBW), provides a useful equation. Equation 4 determines the minimum required bandwidth of the amplifier to guarantee stability for a TIA design. Therefore, higher bandwidth amplifiers support higher gain and bandwidth TIA circuits, and tolerate a higher parasitic photodiode capacitance while remaining stable. Consider the case of a photodiode application with the following specifications: Transimpedance Amplifier Gain (50-K V/A), Transimpedance Amplifier Bandwidth, 1-MHz, and Photodiode Junction Capacitance (Cj) (100 pF). In contrast, the higher fGBW OPA828 is able to support the TIA gain of 50 k V/A with 1-MHz bandwidth while offering 55 degrees of phase margin using a singlestage as shown on Figure 5. A single stage TIA implemented with a precision, high bandwidth op amp offers better noise performance and accuracy than a dual stage version built with two lower bandwidth amplifiers of similar accuracy, since only one amplifier contributes to noise, offset, and drift errors in the system. This greatly simplifies your design while offering lower component count, simpler routing and a smaller solution size. 3.2 pF T 140 OPA828 Single-Stage TIA 50 k TIA Gain = 50kV/A, BW = 1MHz OPA828 fGBW = 45-MHz 120 -15V 100 AOL AOL 100nF GND ± Vout OPA828 + Gain (dB) 80 Photodiode 60 Rateof of Closure Closure Rate 20-dB ==20-dB Stable! Stable! 40 100nF 1/Beta 1/Beta GND 20 GND +15V 0 OPA828 Single Stage: Transimpedance G=50k V/A -20 1 10 100 (4) For comparison, consider two implementations with op amps with different bandwidths: the OPA140 offers 11MHz of gain bandwidth product, while the OPA828 offers 45-MHz. Using the previously derived equation for minimum bandwidth, the minimum fGBW for the transimpedance op amp is ~37-MHz. Using a single stage, the OPA140 is unstable as shown on Figure 3. T 140 3.2 pF OPA140 Single-Stage TIA TIA Gain = 50kV/A, BW = 1MHz 120 1k 10k Frequency (Hz) 100k 1M 10M Figure 5. Single-stage OPA828 (fGBW = 45-MHz) TIA Stability Analysis (Stable) Summary Modern JFET op amps combine high input impedance, excellent noise performance, high bandwidth, and wide output voltage range, making JFET amplifiers an optimal choice in the use of high gain, and high resolution transimpedance photodiode circuits. OPA140 fGBW = 11-MHz 50 k 100 -15V 100nF GND ± Vout OPA140 + Table 1. TIA Op Amps AOL AOL 80 Gain (dB) Photodiode 60 Rateof of Closure Closure Rate 40-dB ==40-dB Unstable!! Unstable!! 40 100nF 20 1/Beta 1/Beta GND GND 0 +15V -20 OPA140 Single Stage: Transimpedance G=50k V/A 1 10 100 1k 10k Frequency (Hz) 100k 1M 10M DESCRIPTION OP AMP 36-Volt, High-speed (45 MHz GBW and 150 V/µs SR), low-noise (4 nV/√Hz) RRO JFET Op Amplifier OPA828 5.5 MHz, High Slew Rate, Low-Noise, Low-power, RRO Precision JFET Op Amplifier OPA145 Low-Offset, Low-Drift, Low-Noise, 11-MHz, 36-V JFETInput, RRO Op Amplifier OPA140 Figure 3. Single-stage OPA140 (fGBW = 11-MHz) Stability Analysis (Unstable) Table 2. Related Documentation To meet the TIA requirements, two cascaded stages of the OPA140 are required. A transimpedance stage with a lower gain of 10-kΩ cascaded a with a noninverting gain stage of 5 V/V as shown on Figure 4. TYPE TITLE Application Brief Green-Williams-Lis: Improved Op Amp Spice Model Application Report Cookbook Circuit: Transimpedance Amplifier 16 pF 1k 4.02 k T 10 k -15V GND OPA2140 + Vout OPA2140 + 100nF Gain (dB) GND ± GND ± OPA2140 Dual Stage TIA First TIA Stage Stability TIA Gain = 10kV/A, BW=1-MHz OPA2140 fGBW = 11-MHz 100 100nF Photodiode 140 120 100nF -15V 80 AOL AOL 60 40 1/Beta 1/Beta 100nF 20 Rateof of Closure Closure Rate 20-dB == 20-dB Stable !! Stable GND 0 GND GND +15V st 1 Stage: Transimpedance G=10k V/A +15V nd 2 Stage: Non-Inverting Gain G=+5 V/V -20 10 100 1k 10k Frequency (Hz) 100 k 1M 10M Figure 4. Alternate Two-stage OPA140 TIA Amplifier (Stable) 2 Simplify Transimpedance Applications with High-bandwidth, Precision JFET Op Amps Raphael Puzio, Luis Chioye Copyright © 2019, Texas Instruments Incorporated SBOA354 – May 2019 Submit Documentation Feedback IMPORTANT NOTICE AND DISCLAIMER TI PROVIDES TECHNICAL AND RELIABILITY DATA (INCLUDING DATASHEETS), DESIGN RESOURCES (INCLUDING REFERENCE DESIGNS), APPLICATION OR OTHER DESIGN ADVICE, WEB TOOLS, SAFETY INFORMATION, AND OTHER RESOURCES “AS IS” AND WITH ALL FAULTS, AND DISCLAIMS ALL WARRANTIES, EXPRESS AND IMPLIED, INCLUDING WITHOUT LIMITATION ANY IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE OR NON-INFRINGEMENT OF THIRD PARTY INTELLECTUAL PROPERTY RIGHTS. These resources are intended for skilled developers designing with TI products. You are solely responsible for (1) selecting the appropriate TI products for your application, (2) designing, validating and testing your application, and (3) ensuring your application meets applicable standards, and any other safety, security, or other requirements. These resources are subject to change without notice. TI grants you permission to use these resources only for development of an application that uses the TI products described in the resource. Other reproduction and display of these resources is prohibited. No license is granted to any other TI intellectual property right or to any third party intellectual property right. TI disclaims responsibility for, and you will fully indemnify TI and its representatives against, any claims, damages, costs, losses, and liabilities arising out of your use of these resources. TI’s products are provided subject to TI’s Terms of Sale (www.ti.com/legal/termsofsale.html) or other applicable terms available either on ti.com or provided in conjunction with such TI products. TI’s provision of these resources does not expand or otherwise alter TI’s applicable warranties or warranty disclaimers for TI products. Mailing Address: Texas Instruments, Post Office Box 655303, Dallas, Texas 75265 Copyright © 2019, Texas Instruments Incorporated

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertising