RF Receiver: Low-Noise Preamplifiers

RF Receiver · Electronics

The analog front end between the receive coil and the digital receiver, where a tenth of a decibel of noise figure is worth having.

Diagram of three parallel preamplifier routes between the receive coil and the digital receiver — Three preamplifier routes carried in parallel to first measurement.

Two small preamplifier circuit boards inside an open aluminum enclosure — Two preamplifier boards in a shielded enclosure (October 2026).

The question

The preamplifier sets the noise floor of everything downstream. At 3 tesla the proton frequency is 123.25 MHz, and the target is a noise figure well under one decibel while presenting the impedance the coil needs for preamplifier decoupling. Commercial MRI preamplifiers reach this, but they are closed parts; we want a front end we can specify, measure and change.

Three routes in parallel

Rather than commit to one design, we are carrying three to first measurement and letting the bench decide.

Route	Approach	Main risk
Integrated MRI preamplifier	A commercial MRI preamplifier chip on our own board	The part is discontinued and its input impedance is unmeasured
Discrete	A pHEMT first stage followed by an MMIC second stage	Reaching the noise figure target through the second stage
Custom integrated circuit	A CMOS low-noise amplifier taped out on a multi-project wafer run	Schedule, and that fabrication is one shot

The receiver board reserves resources for four channels, so the front end is specified four channels at a time.

Measurement

Noise figure at this level is not something to take from a datasheet. Noise measurement on our own bench, against a commercial benchmark amplifier, is part of the project rather than a check at the end.