In Figure # 1, the input/output matching networks are responsible for matching the system 50 ohm impedance with the low impedances of the RF transistors for maximum power transfer.

The circuit level schematic at the bottom of Figure # 1 shows a basic amplifier stage which consists of input/output matching networks, an active device (Q1),  coupling/decoupling components (C1,C2/L1, respectively), DC supply and Gating voltages.

This is simply a number that the input power is multiplied by to solve for the output power. The problem with that is it takes far too long to turn the entire amplifier system on and off for it to be effectively used in an NMR/MRI system. Transistors operating in Class A amplify the full 360 degrees of the RF sinusoidal signal.

What can be discussed, however, is a rough estimate of what has been typically used to date. The reason for specifying in the voltage dimension is more than likely a legacy issue; i.e.


It’s more important to have better Gain flatness centered on the key nuclei frequency (+/- 500 kHz) than to try to sustain it over frequencies between nuclei where the amplifier won’t be operated.

It should be pointed out that an amplifier optimized for pulse operation is vastly different than one that is designed for Continuous Wave (CW) applications. The purpose of the inductive circuit is to pass DC power to the transistor easily while preventing the amplified RF signal from working its way back into the DC supply. Ideally, the perfect RFPA would perform exactly the same at one key nuclei frequency as it would at another. The Pre-Driver and Driver are low power amplifier stages that raise the power level of the small, low level input signal up from the milli-Watt range to a level high enough to drive the high power PA sections. It is truly beyond the scope of this discussion to quantitatively convey how to specify a maximum power output level for all applications.

Connect with NXP professionals and other knowledgeable designers ready to help.Software, documentation, evaluation tools. The Gain linearity specification comes with a definition of dynamic range requirement as it must be understood and clarified, a priori, where the amplifier will be expected to be operated in terms of output power level. Familiarity with its fundamental function, capabilities and limitations will prove beneficial to the various disciplines of technologists involved with the design, development and use of these machines.  The objective is to provide an overview of amplifier technology, terminology, specifications, testing, and evaluation along with future trends in RFPA architectures.A comprehensive definition of RF pulse parameters is initially provided as the sole purpose of the RFPA is the amplification of the RF pulse.  Amplifier specifications are delineated and explained in detail with reference to their impact on the RF pulse sequences. The microcontroller is essentially a micro-computer that continuously runs a fixed program loop that monitors several vital operating parameters throughout the RFPA chassis; DC voltages, currents, pulse width, duty factor, RF output power, load VSWR and temperature. A balanced compromise would be a sufficient working knowledge in several of the areas to gain a solid grasp on the NMR/MRI technology en masse.
If the RFPA was ideal, that is all it would do, the output would be an exact replica of the input waveform, and only the amplitude would be greater by some fixed multiple.

Amplificateur définition. Figure # 3 shows this graphically, which is a plot of output power vs. input power. Class AB offer a tradeoff of acceptable linearity reduced ruggedness, improved efficiency (approx 50%) and cost effectiveness. all the power from the RF signal source will enter the RF amplifier).

Élément d'une chaîne haute-fidélité qui précède les haut-parleurs.

It may not seem essential for an RFPA to have a low noise output while it is transmitting since the very thing it is tasked to do is to generate RF power and lots of it.

The duration of this transient is generally very fast and currently has not been shown to inhibit or corrupt images or data; therefore, while it is shown for completeness, it is not formally specified.This distortion occurs directly after an RFPA has been un-blanked (or the RF pulse is terminated) and appears in the time domain as a half or more cycles of a low frequency signal superimposed on the un-blanked noise voltage. Most NMR applications run less than 500-2 kW of peak power up to 400MHz, from 400 MHz to 1 GHz, 100-500 Watts is adequate and from 1-2 GHz, 30-100 Watt amplifiers have been deployed.MRIS RF power requirements can vary widely, as discussed. One can safely argue that this is the case as power meters that could visually display an Time Domain RF pulse waveform in terms of power dimensions (i.e.

They can be initially generated when an RF input is applied, yet can remain when the RF input is shut off. So you can stay connected. Another issue is the coupling between transmitter elements which can lead to high levels of reflected power into the RFPAs output and virtual VSWR fault triggering(i.e. A “domain” is simply the “x” axis of a basic “x-y” plot. How much power to use for a particular application in an NMR Spectrometer or MRI Scanner depends on many, many factors: coil (probe) limitations, SAR values, part of the anatomy to be scanned (i.e. Health Solutions From Our Sponsors.

An NMR Spectrometer/MRI Scanner is an extremely complex machine, an understanding of which requires knowledge of Physics, Chemistry, Analog/Digital/Radio Frequency Electronics, Digital Signal Processing etc, etc. They are generally non harmonic related and can be close in or far away in frequency from the carrier.