ULTRA LOW NOISE INDUCTIVELY SOURCE DEGENERATED CASCODE PREAMPLIFIER FOR MRI APPLICATION
Publication Date: 2017-Feb-28
The IP.com Prior Art Database
An ultralow noise inductively source degenerated cascode preamplifier for MRI application is disclosed. The preamplifier overcomes the instability condition experienced during the LNA design at VHF frequency range for MRI application, by inserting a series inductance in common source (CS) and a series resistance in a gate of common-gate device of the cascode amplifier.
The present disclosure relates generally to low noise preamplifier (LNA) and more particularly to ultralow noise cascode preamplifier for magnetic resonance imaging (MRI).
A low noise preamplifier is a critical component of the radio frequency (RF) front end of the MR (magnetic resonance) receiver and plays a key role in the overall performance of the MRI receiver. The MR preamplifier is required to meet a number of technical requirements simultaneously. A typical cascode topology for a low noise amplifier (LNA) provides performance of a common-source (CS) stage with relatively smaller Miller effect and relatively larger output resistance. The simplified cascode topology comprises pseudomorphic high electron mobility transistor (pHEMT) Q1 in common source mode followed by a bipolar junction transistor (BJT) Q2 in common base (CB) mode. The L-C section provides the input and output impedance matching.
The cascode gain block is characterized by high output impedance - Zout (cascode), higher gain –S21 (cascode) and input impedance – Zin (cascode). For high output impedance- Zout (cascode), the output impedance looking into the collector of Q2 is very high. Hence, the effective Zout (cascode) is ~R1. For higher gain – S21 (cascode), the cascode configuration offers higher gain than that of Q1 as a single stage CS amplifier. The CS stage Q1 output is terminated by CB stage Q2 acting as a current buffer. The very low input impedance of the CB stage Q2 allows all of the transconductance current (gm.Vgs) of the CS stage to flow into the output load of 50 ohms. Hence, the S21 of the cascode is about 5-7 dB higher than the Q1.
For input impedance – Zin (cascode) assuming unilateral operation (S12=0), it is same as S11 of Q1. Typically, the Zin of GaAs FET transistors have small resistance (~10ohms) in series with capacitor (~ 3 to 4 pF). For input matching network, the L1-C1 network transforms the 50 ohm source to provide gamma optimum (ſ opt) impedance for minimum noise figure (NF). For output matching network, the L2-C2 network transforms the R1 to 50 ohms.
The GaAs-FET (gallium arsenide –field effect transistor) transistors have been used extensively in preamplifier designs due to their superior noise performance and amplification. However, the usage of high-gain and high-frequency GaAs-FET at very high frequency (VHF) poses a greater concern of designing the amplifier for unconditional stability.
The low noise preamplifiers are the front end blocks for several MR receive chains for deciding the signal to noise ratio (SNR). The low noise and high gain requirements of MR receive chain are often challenged with the conventional designs.
It would be desirable to have an efficient technique to overcome the stability challenge experienced with LNA design at very high frequency for MRI application.
BRIEF DESCRIPTION OF DRAWINGS
Figure 1 is a graph...