lmproved Low Noise Amplifier with Bypass Switch (27-Jan-2012)

Avago Technologies                                                        

lmproved Low Noise Amplifier with Bypass Switch

Abstract

This invention enables a low noise amplifier with a bypass switch feature to achieve negligible degradation in a noise figure and minimum degradation in isolation between the Radio Frequency (RF) input and RF output at amplifier mode. This is done without compromising the performance at the bypass mode.

Description

A.           Prior solutions and their disadvantages

Figure 1 illustrates one prior art method to accomplish a low noise amplifier with an RF bypass switch feature.

Figure 1: One scheme to accomplish RF bypass switch in a low noise amplifier

AMP1 is a low noise amplifier (LNA). C1, C2, C4, and C5 are RF coupling capacitors. X1 and X2 are Field-Effect Transistors (FETs) that turn ON during the bypass mode and turn OFF during the amplifier mode via Vbyp. This configuration can achieve the RF bypass switch feature but it has the following disadvantages. With this conventional RF bypass switch introduced to an LNA at amplifier mode:

·         The noise figure is degraded

·         The isolation be1Ween the RFin and RFout is degraded

·         Tile gain is reduced

B.           Problems solved by the invention

A method is needed which enables a low noise amplifier with a bypass switch feature to achieve negligible degradation in a noise figure and minimum degradation in isolation between the Radio Frequency (RF) input and RF output at amplifier mode. The solution must accomplish this without compromising the performance at the bypass mode.

C.           Advantages of the invention over what has been done before

With this implementation of an improved bypass switch feature added to an LNA.

1.    At amplifier mode, any signal from RFout that is going to be feedback to RFin will be shunt to ground via the shunt capacitor, C3, as it provides a lower impedance path compared to the path from Xl to C, thus:

·         The noise figure of this Circuit at the amplifier mode is not degraded

·         The change of the input impedance at the amplifier mode of this circuit is negligible

·         The gain of this circuit at the amplifier mode is not reduced

·         The isolation between the RFout and RFin of this circuit is not degraded, thus making the device easier to match for optimum noise figure

2.    At bypass mode, X1 and X2 are turned ON via Vbyp; the signal goes through C1 to X1 to X2 to C2 because the path from X2 to C2 provides a lower impedance path compared to the path provide by the shunt capacitor, C3 to ground. Thus, the bypass mode performance is not compromised.

This circuit has been proven in the dice used in the Wolffia 3 project implemented in enhancement-mode pHEMT technology. However, the application of this disclosure is not dependent upon Integrated Circuit (IC) technology and is applicable to other IC technologies.

D.           Construction and operation of the invention

The invention in this disclosure solves the above-mentioned pro...