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MRI RF Coil Low Noise Amplifier Protection Circuit with Input Impedance Adjustment

IP.com Disclosure Number: IPCOM000114821D
Publication Date: 2005-Mar-29
Document File: 7 page(s) / 257K

Publishing Venue

The IP.com Prior Art Database

Related People

John Bushert: INVENTOR

Abstract

A method is described for protecting amplifiers (Low Noise Amplifiers, LNAs), or other circuits, from excessive voltage. The method also provides a means for adjusting the input impedance to the amplifier.

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     Invivo Corporation Publication of Intellectual Property www.ip.com

Date: 3/23/05 ID#: INVD-014

Publication Title: MRI LNA Protection Circuit with Input Impedance

Adjustment

Author(s)/Inventor(s): John Bushert

Invivo Corporation N27 W23676 Paul Road, Pewaukee, WI 53072 Tel.: 262-524-1402 e-mail: jbushert@invivocorp.com

Copyright: Invivo Corporation Country of Origin: USA

Document Abstract:

A method is described for protecting amplifiers (Low Noise Amplifiers, LNAs), or other circuits, from excessive voltage. The method also provides a means for adjusting the input impedance to the amplifier.

1

Page 2 of 7

TABLE OF CONTENTS

Theory.................................................................................................................................3
Schematics and Diagrams .................................................................................................5
Measurements ....................................................................................................................6
Discussion ...........................................................................................................................7
Conclusion ..........................................................................................................................7

2

Page 3 of 7

THEORY

A receiver front-end circuit for a transmit-receive MRI surface coil may be that shown in Figure 1:

+V

Figure 1

At 64MHz the impedance of the 1000pF capacitors is about -j2.5 Ohms, so the Quad diode and the preamplifier will see about the same RF voltage as the PIN Diode. If the transmitter were to source 1000W peak to a 50 Ohm coil what would be the voltage across the PIN diode if the Quad diode and Preamp were not in place?

1000W = V2/50Ω V = 224V

XL = 2πfL = 2 * 3.1416 * 64MHz * 124nH = 50Ω

iPIN = 224V/50Ω = 4.47A

Say the resistance of the pin diode is guaranteed to be less than or equal to one Ohm. If we use one ohm to find the peak RF voltage across the PIN diode:

4.47A * 1Ω = 4.47V

With the 1000pF coupling capacitors in place, the current in the tank circuit (4.47Apeak) would be shared between the PIN diode, the Quad diode and the preamplifier because the small capacitive reactance will not significantly limit the flow of current to the Quad diode and preamplifier.

An alternative circuit is this one:

3

   50 Ohm Lambda over Four

1000pF 1000pF 1000pF

PIN

Preamplifier

  To System

Coil

TX - IN

Figure 1

4.7uH

PIN Quad

PIN Drive

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+V

   50 Ohm Lambda over Four

100pF 62nH

-j25 +j25

A

Figure 2

During transmit the -j25 Ohm reactance serves to limit the current to the Quad diode and the +j25 Ohm reactance serves to limit the current to the preamp.

During receive the net reactance of the -j25 and the +j25 are negligible since the two are at resonance and the ESR of these two components is small compared to the source resistance (50Ω). However, the parasitic capacitance of the quad di...