The gain of an operational amplifier becomes infinitely high when in an open-loop configuration. When a feedback resistor is connected across it—from its output terminal to its negative input terminal or it’s summing point—all of the current goes through this feedback resistor due to the high gain. The output in this case becomes, VOUT = -(IIN × RF).
This configuration is known as a Transimpedance Amplifier, whose transimpedence or gain equals RF.
This configuration of an operational amplifier is extremely important as it’s used in a wide variety of applications. One important application involves the amplification of the output of a sensor, like a photodiode by the op-amp. Photodiodes output current at high impedances and have high capacitances. If a photodiode is connected directly to a resistor, there can be two problems:
- In case of large sensor resistors, the time constant and output gain will be large, but with slower response.
- If you opt for small sensor resistors to get smaller time constant, it will decrease the gain. Moreover, it will give you a poor signal-to-noise ratio (SNR).
To prevent this situation, a transimpedance amplifier is incorporated in the circuit. In our example, the sensor’s output will be connected to the transimpedance amplifier’s summing point. This can significantly increase the response time and gain by using larger RF.
Factors For Selecting A Transimpedance Amplifier For Optimal Performance
These are a few factors that you need to consider when looking for a transimpedance amplifier for you particular circuit:
- Look for an operational amplifier that has less voltage noise rating (nV/vHz).
- Find operational amplifiers with low current noise (pA/vHz). However, most bipolar operational amplifiers have higher current noise ratings than standard FETs. It’s extremely rare to find an operational amplifier that is better than an FET, except for where Rs is low; the circuit has a capacitive input and narrower bandwidth.
- Avoid operational amplifiers that have large input capacitances. However, this is not properly mentioned in many op-amp data sheets, but you can assume that the op-amps with low noise have larger input capacitances than regular op-amps.
- While choosing sensors for your circuit, find one that has lower capacitances involved. Moreover, you can create a layout of low capacitance between the op-amp and you sensor.
Are you looking for reliable transimpedance amplifiers? Get in touch with ADSANTEC.
We offer various transimpedance amplifiers with offset control, input peak detectors and gain control.
Our range of products also includes clock phase shifters, active power dividers, frequency dividers, etc. Call us at (310) 530-9400 for further details.