Frequency Response & Transimpedance Circuit

1. What are Photops™?
2. What are the advantages of having the op-amp in the same package with the photodiode?
3. Why aren't the circuits completed in the Photops™ by adding resistors and/or capacitors?
4. Can additional components be added to the existing Photops™?
5. How do I determine what values for Rf and Cf should be chosen?
6. How does the TIA bandwidth vary with load resistance?

1. What are Photops™?
The Photops™ Series combine either a Photoconductive or UV enhanced photodiode with an operational amplifier die in the same package.

2. What are the advantages of having the op-amp in the same package with the photodiode?
By having a signal amplified as close to the detector as possible, the effects of noise picked by the pins, wire leads, and other PCB conductor traces can be minimized. This is in addition to saving a lot of space on your board or any other substrate.

3. Why aren't the circuits completed in the PhotopsTM by adding resistors and/or capacitors?
Photops™ are very versatile components. They can be utilized for various gains and bandwidths determined by the external feedback components (resistors and capacitors). Since bandwidth and gain characteristics are application specific, adding a specific value resistor and/or capacitor will limit the operating of these devices.

4. Can additional components be added to the existing Photops™?
Yes, any modification is possible. OSI Optoelectronics can not only provide any additional electrical components in the same package, but can also include an optical filter. Please contact our Applications Group with any of your specific requirements.

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5. How do I determine what values for Rf and Cf should be chosen?
The feedback resistor, Rf, directly determines the gain in a transimpedance amplifier configuration; e.g. a 10kW resistor will amplify the signal, including noise, by a factor 10,000. The feedback capacitor, Cf, will determine the -3dB-frequency response as following:

The Gain Bandwidth Product (GBP) of the op-amp limits the desired frequency response of the detector/amplifier combination. Rf and Cf should be chosen such that the -3dB frequency response to be less than the maximum frequency of the op-amp:

where CA is the amplifier's input capacitance and CJ is the photodiode's junction capacitance.

A large value RF, even though achieving a large gain, will reduce the bandwidth as well as introducing additional noise due to the thermal noise in the resistor. An increase of Cf, however, will increase the stability of the system, but at the expense of a slower response time.

6. How does the TIA bandwidth vary with load resistance?
The Transimpedance Amplifier (TIA) bandwidth is related to the load resistance per following relationship:

where RL is the load resistance.

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Frequently Asked Questions

• Basic PIN Photodiode Characteristics
• Input Light Intensity
• Temperature and Bias Effects
• Detector/ Filter Combination
• Avalanche Photodiodes
• High Speed Devices
• Frequency Response & Transimpedance Circuit
• Noise and Lower Limit of Detectivity
• UV / Soft X-Ray Enhanced Series
• Position Sensing Devices (PSDs)
• Packaging-Mechanical