Browse Prior Art Database

USING SWITCHING MOSFETs IN LOW-NOISE FRONT END AMPLIFIERS

IP.com Disclosure Number: IPCOM000007845D
Original Publication Date: 1996-Nov-01
Included in the Prior Art Database: 2002-Apr-29
Document File: 2 page(s) / 107K

Publishing Venue

Motorola

Related People

Eliav Zipper: AUTHOR [+3]

Abstract

High dynamic range of the HF SSB receiver is one ofits important design goals. High dynamic range can be achieved by using RF elements that have high IP3, high 1 dB compression point and low noise figure. RF amplifiers are such elements, and increas- ing their IP3 or their 1 dB compression point gener- ally increases their noise figure. Therefore, high dynamic range amplifiers are not trivial and special schemes are adopted for high dynamic range RF amplifiers.

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Technical Developments

USING SWITCHING MOSFETs IN LOW-NOISE FRONT END AMPLIFIERS

by Eliav Zipper, Yehuda Eder and Yair Shapira

  High dynamic range of the HF SSB receiver is one ofits important design goals. High dynamic range can be achieved by using RF elements that have high IP3, high 1 dB compression point and low noise figure. RF amplifiers are such elements, and increas- ing their IP3 or their 1 dB compression point gener- ally increases their noise figure. Therefore, high dynamic range amplifiers are not trivial and special schemes are adopted for high dynamic range RF amplifiers.

  HF and IF amplifiers were traditionally realized by Bipolar Junction Transistors (BJT) or Junction Field-effect Transistor (JFET). In high dynamic range application, JFETs (such as MMBFJ910) offer bet- ter linearity than BJTs due to their square-low trans conductance. The exponential nature of BJTs con- tains power of three elements, that result in third order intermodulation products (3'rd IMD). There- fore, the IP3 point of a basic BJT amplifier is lower than that of a JFET amplifier IP3. It is possible to design a feedback amplifier with BJTs and to achieve high IP3. Still, the feedback amplifier has its draw- backs. When a resistive feedback amplifier is employed, the increase in IP3 and 1 dB compres- sion point cause an increase in the NE If a loss-less feedback amplifier is employed, high cost transform- ers are required. Therefore, there seems to be no configuration of a low cost BJT amplifier with low noise figure and high IP3 and 1 dB compression point.

  The JFET offers better linearity and is superior to the BJT in the aspect of IP3. Still, it has some drawbacks that eliminate its performance in the aspect of 1 dB compression point. These drawbacks are related to the limited range of the JFET's gate voltage and to the low transconductance (g,) values

(10 mS) that RF JFET's achieve. Another field-effect transistor, the MOSFET, is more appropriate for high dynamic range applications. It is superior to the JFET in its 1 dB compression point since its gate voltage is not limited as in the case of the JFET Besides, the linearity ofthe MOSFET is equal to the JFET's linearity since they both show square-low transcon- ductance. MOSFET's with high g, exist today....