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The transimpedance amplifier is a common op amp application with an output voltage that depends on the input current and a feedback resistor:

Quite often I see this circuit used to amplify the output current of a photodiode as shown in Figure 1. Almost all transimpedance amplifier circuits require a feedback capacitor (CF) in parallel with the feedback resistor to maintain stability by compensating for parasitic capacitances at the inverting node of the amplifier.

**Figure 1: Feedback capacitor C _{F} compensates for the photodiode junction capacitance and op amp input capacitance.**

Numerous articles exist on how to select a feedback capacitor when using a certain op amp, but I think this is the wrong approach. Despite what we semiconductor manufacturers love to believe, engineers don’t start with an op amp and go looking for a circuit to build with it! Most engineers start with a list of performance requirements, and look for parts that will meet those requirements.

With that in mind, a better approach would be to determine the largest feedback capacitor allowable in our circuit and then select an op amp with a sufficient gain bandwidth product (GBW) to be stable with this feedback capacitor.

Here’s a simple step-by-step approach to determine the required op amp bandwidth for a transimpedance amplifier.

**Step 1: Determine the maximum allowable feedback capacitance. **

The feedback capacitor, in combination with the feedback resistor, forms a pole in the frequency response of the amplifier:

Above this pole frequency, the amplification of the circuit will decline. The maximum feedback capacitor value can be determined from the feedback resistor and the desired bandwidth:

By keeping the feedback capacitor at or below the value calculated in equation 3, we ensure that our circuit will meet our bandwidth requirements.

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**Step 2: Determine the capacitance at the inverting input of the amplifier. **

In Figure 2, the circuit in Figure 1 has been redrawn to show the junction capacitance of the photodiode (C_{J}) and the differential (C_{D}) and common-mode (C_{CM1}, C_{CM2}) input capacitances of the amplifier. These values are typically given in the op amp and photodiode datasheets.

**Figure 2: Transimpedance amplifier circuit showing capacitances at the inverting node.**

From this illustration it is apparent that C_{J}, C_{D}, and C_{CM2} are in parallel and the capacitance at the inverting input is:

C_{CM1} does not contribute to the input capacitance because the non-inverting terminal is grounded. C_{D}, and C_{CM2 }may not be known at this time since we haven’t yet selected a specific op amp. I often use 10pF as a reasonable guess for the sum of these values. The exact value can be substituted later to confirm the appropriateness of a specific op amp.

Now that we’ve determined our values for C_{F} and C_{IN }we’re ready to calculate the required op amp bandwidth. In the next part of this blog I will explain this calculation and use this process in a design example.

Stay tuned!