OPA2210: Operational Amplifier's Input

Hi John, 

It comes down mainly to source impedance. If you have a high source impedance sensor you want a high input impedance amplifier. JFET and CMOS devices have higher input impedance and very low input bias currents. Bipolar inputs have lower input impedance relative to JFET and CMOS devices and higher input bias currents. 

Bipolar devices have very low voltage noise but higher current noise. When you pair a bipolar device with a high source impedance sensor the current noise gets multiplied by the source impedance value and translates into additional voltage noise. 

An example of a high source impedance sensor is a humbucker guitar pickup. The pickup is inductive and over frequency the impedance goes up. Inductor impedance is          Z = jwL. As frequency goes up so does the source impedance. For this reason an amplifier with a JFET or CMOS input would be better suited to used as the amplifier interfacing to the guitar. Less current noise getting multiplied by the source impedance.

I recently wrote an application note (link below) comparing two topologies and various source impedances. The plot below shows that as the source impedance rises the JFET circuit will win. For low source impedance sensors the INA with bipolar input wins. 

https://www.ti.com/lit/ab/sboa589/sboa589.pdf?ts=1718394922624&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FINA849

Here is another app note discussing the current noise impact in much more detail. 

https://www.ti.com/lit/an/sboa570/sboa570.pdf?ts=1718394964368

I hope this helps. 

Best Regards, 

Chris Featherstone

Hi John, 

It is really complicated and not straight forward to say one is faster than another when discussing op amps. In general when discussing discrete transistors CMOS are generally slower than bipolar transistors due to CMOS parasitics. We do have high speed op amps that use CMOS and Bipolar or a combination of both. 

Best Regards, 

Chris Featherstone