1 Introduction

Amplifying the small signals produced by sensors in a low-noise circuit is a very common but difficult problem. Designers often use an operational amplifier (op amp) with bipolar inputs to achieve this amplification, given their inherently low flicker (1/f) and broadband noise. Bipolar op amps present another challenge when the small signal of interest is generated by a sensor with high source impedance that cannot deliver sufficient current to the input of the amplifier. Bipolar op amps have high input bias currents in the nanoampere range or greater, and lower input impedance relative to their complementary metal-oxide semiconductor (CMOS)-input and junction field-effect transistor (JFET)-input counterparts.

The bipolar input loads a sensor, such as a high source impedance microphone, that produces signals on the order of a few thousandths of a volt. This loading reduces audio sonic quality and dynamic range and distorts the signal. You can choose an op amp with a JFET front end such as the OPA145 from Texas Instruments. However, you will not be able to bias the circuit as flexibly as you could with discrete components and may sacrifice extra current for unwanted bandwidth relative to the audio bandwidth of up to 20 kHz. While CMOS and JFET input stages have comparable bias currents, JFET devices have much better noise performance. Moreover, JFETs also have higher gain (transconductance) than CMOS devices. A discrete JFET such as TI’s JFE150, when followed by a bipolar op amp such as the OPA202, does offer a way to achieve high input impedance and low noise with flexible biasing, see Figure 1-1.