1. Field of the Invention
The present invention relates to a variable gain amplifier used by a cable or radio communication or a magnetic recorder, and more particularly relates to a variable gain amplifier having dc-offset cancel function.
2. Description of the Related Art
Generally, in many amplifiers, it does not avoid that a dc-offset which is error components of bias voltage and bias current occurs in an output side of the amplifier.
In prior art, various methods are taken in order to cancel the dc-offset.
According to the conventional system for canceling the dc-offset of the amplifier, low frequency component to be near to dc is removed from the output signal of the main amplifier circus by a feedback amplifier circuit. The frequency component is fed back to the input of the main amplifier circuit. Thus, the dc-offset is removed in the input port of the main amplifier circuit.
The feedback amplifier circuit includes an integrator. The integrator amplifies the dc-components, but does not amplify ac-components.
The amplifier circuit having a fixed voltage gain can cancel dc-offset. However, in a variable gain amplifier circuit, the variation of gain brings about a change of offset. Since the amplifier includes an integrator, a long time requires until the transient influence of the gain variation is removed. As a result, there is problem to make signal quality deteriorate over long hour.
As described above, the conventional offset cancel system for canceling the offset using a feedback amplifier circuit includes the following problems. In other words, when the main amplifier circuit comprises a variable amplifier and the gain varies, the offset drift cannot follow a change of gain promptly. As a result, the signal quality has deteriorated over comparatively long hour.
It is an object of the present invention to provide a variable gain amplifier wherein the good offset cancel that the offset does not fluctuates even if gain varies is possible.
According to a first aspect of the present invention, there is provided a variable gain amplifier receiving an input signal and outputting an output signal, comprising a main amplifier circuit whose gain is controllable and which amplifies a difference signal between the input signal and a feedback signal corresponding to the output signal and outputs the output signal, a feedback amplifier circuit whose gain is controllable, the feedback amplifier circuit configured to amplify the output signal and generate the feedback signal; and a gain control circuit which complementarily controls the gain of the main amplifier circuit and the gain of the feedback amplifier circuit.
According to a second aspect of the present invention, there is provided a variable gain amplifier apparatus, comprising: a plurality of variable gain amplifiers connected in cascade, each of the variable gain amplifiers including a main amplifier circuit whose gain is controllable and which amplifies a difference signal between an input signal and a feedback signal, and a feedback amplifier circuit whose gain is controllable and which amplifies an output signal of the amplifier apparatus and outputs the feedback signal; and a gain control circuit configured to supply a gain control signal to each of the variable gain amplifiers and complementarily control the gain of the main amplifier circuit and the gain of the feedback amplifier circuit according to the gain control signal, the gain control circuit setting a given delay time between the gain control signal supplying to a pre-stage one of the variable gain amplifiers and that to a rear-stage one thereof.