MOSFETs do have very low input bias current, but they also have higher noise than bipolar or JFET transistors. In most high-side current sense applications, a very small resistor is used to minimize the voltage drop in series with the load. This gives a small voltage signal that we need to pick off with a low-noise input stage. There is usually a fairly high current we are trying to sense, so a few nanoamps or even microamps of input bias current with a bipolar device will be a negligible error. Take a look at the AD8418A current-sense amplifier. For very common mode voltages, you could try the AD8479.
MOSFETs do have very low input bias current, but they also have higher noise than bipolar or JFET transistors. In most high-side current sense applications, a very small resistor is used to minimize the voltage drop in series with the load. This gives a small voltage signal that we need to pick off with a low-noise input stage. There is usually a fairly high current we are trying to sense, so a few nanoamps or even microamps of input bias current with a bipolar device will be a negligible error. Take a look at the AD8418A current-sense amplifier. For very common mode voltages, you could try the AD8479.
