Variable gain amplifier for use in communications

Abstract

A variable gain amplifier of low amplitude distortion, and low noise, having a large variable range, is provided. A variable gain differential amplifier that controls a gain by use of bias current is used as each of unit amplifiers (VGAs) making up the variable gain amplifier. A large variable gain range is obtained by series-connecting a plurality of the variable gain differential amplifiers. An attenuator is installed on the input side of the unit amplifier (VGA) at least in the initial stage. By doing so, it becomes possible to prevent amplitude distortion from occurring to the respective VGAs. An attenuator utilizing voltage division by capacitors, generating no noise, is used for lowering noise. Further, the variable gain amplifier is provided with a fixed gain amplifier installed in the final stage as necessary in order to obtain a total gain as desired. With the use of the variable gain amplifier as a variable gain amplifier for output power control of a polar loop transmitter, an excellent function for output power control can be achieved without causing significant deterioration in distortion characteristic and noise characteristic thereof.

Description

BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The invention generally relates to a variable gain amplifier and, more particularly, to a variable gain amplifier suitable for use in a semiconductor integrated circuit for communications. [0003] 2. Discussion of Background [0004] A conventional variable gain amplifier may employ a variable gain amplifier, shown in FIG. 2A and FIG. 2B, as a configuration block. For more detail see non-patent document 1 below. The variable gain amplifier controls a gain with bias current by taking advantage of the gain of a transistor being proportional to the bias current. [0005]FIG. 2A and FIG. 2B show a conventional variable gain amplifier 200. FIG. 2A is a block circuit diagram of the variable gain amplifier 200 comprising variable gain unit amplifiers (hereinafter referred to merely as unit amplifiers) that are series connected in three stages, and FIG. 2B is a circuit diagram of the unit amplifier making up the respective stage...

AI Technical Summary

Benefits of technology

[0014] Further, it is another object of the invention to provide a polar loop transmitter using the variable gain amplifier having such an excellent low noise characteristic, and having an excellent function for output power control, without causing significant deterioration in distortion characteristic and noise characteristic thereof.

[0015] The variable gain amplifier according to the invention is characterized in that a low noise attenuator is installed on the input side of a unit amplifier (VGA) at least in the initial stage. By installing the low noise attenuator on the input side of the VGA in the initial stage, an input signal to the VGA in the initial stage, significantly affected by noise, can be attenuated, so that it becomes possible to prevent amplitude distortion from occurring to the VGA in the initial stage. In this case, it is suitable for lowering noise to use an attenuator utilizing voltage division by capacitors that generates no noise as the low noise attenuator. Further, a fixed gain amplifier may be installed in the final stage as necessary in order to supplement insufficiency in total gain.

Problems solved by technology

Accordingly, it has not been possible to obtain a variable gain amplifier capable of satisfying a requirement for low distortion and low noise even at times of a large input signal.

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