1 Introduction

Amplifying the small signals produced by sensors in a low-noise circuit is a very common but difficult problem. This was outlined in the JFE150 Ultra-Low-Noise Pre-Amp application note. High source impedance sensors such as a microphone, that produce signals on the order of a few thousandths of a volt, are loaded by low noise and low input impedance bipolar junction transistor (BJT) stages. Using a complementary metal-oxide semiconductor (CMOS) device is a good choice for a high input impedance; however, the noise performance is worse than that of a bipolar input. The discrete junction field-effect transistor (JFET) has better noise performance than the CMOS device and also has high input impedance. More details are found in the Trade-offs Between CMOS, JFET, and Bipolar Input Stage Technology application report. A discrete JFET such as TI's JFE2140, when followed by a bipolar op amp such as the OPA202, offers a way to achieve high input impedance and low noise with flexible biasing, see #FIG_Y2T_X5F_2VB.