Internet Explorer is not a supported browser for TI.com. For the best experience, please use a different browser.
Video Player is loading.
Current Time 0:00
Duration 4:17
Loaded: 3.88%
Stream Type LIVE
Remaining Time 4:17
 
1x
  • Chapters
  • descriptions off, selected
  • en (Main), selected

Hello. Welcome to our short video on how to design transimpedance amplifier circuits. Transimpedance amplifier circuits are used to convert and amplify input currents to output voltages. The current to voltage gain, or transimpedance gain, is set by the value of the feedback resistor, R1. This circuit is commonly used to convert low level currents from sensors, such as photodiodes, into output voltages, which can then be measured by an analog to digital converter, or ADC. This video explains how to design a robust and stable transimpedance amplifier using TI's OPA170 in just a few easy steps.

The design goal for this circuit is to convert an input current that ranges from 0 to 50 microamps to an output voltage that ranges from 0 to 5 volts. The frequency of the input signal is 10 kilohertz. Finally, a dual 15-volt supply is used because it removes the need for biasing at the non-inverting input. The first design step is to calculate the value of the feedback resistor, R1, based on the required transimpedance gain. We calculate this by dividing the output voltage swing by the input current range, as shown in the equation. The resulting value for R1 is 100 kilohms.

The next step is to calculate the feedback capacitor. Transimpedance circuits require a feedback capacitor to remain stable. The feedback capacitor also defines the closed loop bandwidth and can be calculated based on the feedback resistor and the circuit bandwidth. The value of the feedback capacitor should be less than 1 divided by 2 times pi times R1 times fp, where fp is the required bandwidth. This calculation results in a value of 159 picofarads, but we selected 150 picofarads because it is a standard capacitor value.

The last step in this design is to verify that the gained bandwidth of the amplifier is sufficiently large to ensure stability. To do this, we must take into consideration the device's input capacitance, feedback capacitance, and feedback resistor. Note that the input capacitance, Ci, is the summation of the source's capacitance and the amplifier's differential and common mode input capacitances. For this design, the minimum required gained bandwidth is 11.03 kilohertz. Since the typical gained bandwidth of the OPA170 1.2 megahertz, we easily meet this requirement.

A DC sweep of the input current from 0 to microamps yields an output voltage from 0 to 5 volts. This verifies the functionality of the circuit. Running an AC sweep analysis, we find that the bandwidth of the circuit is 10.57 kilohertz, which meets the design goal of 10 kilohertz. When designing transimpedance amplifier circuits, there are a few design notes to be aware of.

First, it is recommended to use a JFET or CMOS input amplifier with low input bias current to reduce DC errors. Next, if a single supply design is desired, you can supply a voltage to the non-inverting input of the amplifier to set the output voltage when the input current is 0 amps. Finally, be sure to always check the linear output swing of the amplifier, which is usually given in the conditions section of the AOL datasheet specification.

Texas Instruments has many online resources to help you design circuits Op amps. This includes reference designs and guides, educational videos, simulation and prototyping tools, support resources, and search tools. Thank you for taking the time to watch this short presentation on how to design transimpedance amplifier circuits. Please visit www.ti.com for additional information and resources.