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Cascode Amplifier with Common Gate Parallel RC

IP.com Disclosure Number: IPCOM000199901D
Publication Date: 2010-Sep-20
Document File: 2 page(s) / 157K

Publishing Venue

The IP.com Prior Art Database

Abstract

This paper describes the method used to achieve a broadband stability (DC up to 20GHz) for a high gain low noise amplifier. This method too is able to achieve a 38dBm OPI3 at 64mA biasing current with high operating gain and low noise figure.

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Cascode Amplifier with Common Gate Parallel RC

Jagadheswaran Rajendran, Avago Technologies (M) Sdn Bhd

Abstract

This paper describes the method used to achieve a broadband stability (DC up to 20GHz) for a high gain low noise amplifier. This method too is able to achieve a 38dBm OPI3 at 64mA biasing current with high operating gain and low noise figure.

Introduction

Figure 1 show a typical cascode amplifier topology used to achieve a low noise figure and wide bandwidth.

Figure 1 Typical Cascode Amplifier

However, in order for the cascode amplifier to have a high gain (more than 20dB), the stability becomes an issue for a wide bandwidth, especially at high frequencies (i.e more than 5GHz). The common gate stability resistor R6 provides a good stability for high frequencies at the expense of high noise figure. As the value is increased, the noise figure gets worse as shown in Figure 2.

Figure 2 The Noise figure gets worse if R6 is increased form 10 Ohm to 15 ohm

With R6 as 10ohm, the K-factor of the cascode amplifier is less than 1 from 5 to 10GHz as shown in Figure 3

Figure 3: Stability factor up to 20GHz

Concept

In order to resolve this issue, the proposed circuit contains a parallel RC for the common gate transistor which provides a good stability at high frequency without affecting the noise figure. Figure 4 illustrates the circuit of invention. With R11 and C8 acts as a parallel RC, R6 value can be reduced below 10ohm in order to improve the noise figure without compromising the stability. Figure 5 shows the K factor more than 1 up to 20 GHz for the circuit of invention.

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