Summary
This video explains how to design a stable transimpedance amplifier (TIA) circuit to convert input currents from sensors into measurable output voltages. It details a three-step design process: selecting the feedback resistor (R1) for the desired gain, calculating the feedback capacitor (Cf) for stability and bandwidth, and verifying the amplifier's gain bandwidth product. The design example targets a 0-50 microamp input current to a 0-5 volt output voltage with a 10 kHz signal frequency, using TI's OPA170 operational amplifier.
Key Insights
Stability in TIAs is critically dependent on the feedback capacitor (Cf).
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.
Choose JFET or CMOS input amplifiers for low input bias current to minimize DC errors.
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.
Sections
Introduction to Transimpedance Amplifiers
Transimpedance amplifiers convert input currents to output voltages.
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.
Design goal: 0-50uA to 0-5V for a 10kHz signal.
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.
Transimpedance Amplifier Design Steps
Calculate R1 using output voltage swing and input current range.
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. [Equation implicit: R1 = Vout_swing / Iin_range]. The resulting value for R1 is 100 kilohms.
Select feedback capacitor (Cf) for stability and bandwidth.
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. [Equation implicit: Cf < 1 / (2 * pi * R1 * fp)]. This calculation results in a value of 159 picofarads, but we selected 150 picofarads because it is a standard capacitor value.
Verify amplifier's gain bandwidth product for stability.
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.
Design Verification and Notes
DC sweep confirms 0-50uA input yields 0-5V output.
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.
AC sweep confirms bandwidth meets 10kHz design goal.
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.
Stability in TIAs is critically dependent on the feedback capacitor (Cf).
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.
Choose JFET or CMOS input amplifiers for low input bias current to minimize DC errors.
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.
For single supply TIAs, bias the non-inverting input to set zero-current output voltage.
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.
Always check the amplifier's linear output swing in the datasheet.
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 Resources
TI offers extensive online resources for circuit design, including reference designs and tools.
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.
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