Patent ID: 6172543
Filing Date: 2001-01-09
Classification: H03H

Abstract:
A 90.degree. phase shift circuit comprising:a high-pass filter including a first capacitor connected between an input terminal and a first output terminal and a first resistor connected between said first output terminal and a reference line,said high-pass filter outputting a first signal having a first amplitude and a first phase;a low-pass filter including a second resistor connected between said input terminal and a second output terminal and a second capacitor connected between said second output terminal and said reference line,said low-pass filter outputting a second signal having a second amplitude and a second phase which is 90.degree. behind with respect to said first phase; anda level comparator for comparing said first amplitude against said second amplitude to feed-back a control signal to said high-pass filter and said low-pass filter, said control signal controlling cut-off frequencies of said high-pass filter and said low-pass filter based on a difference between said first amplitude and said second amplitude,wherein said first and second capacitors have a variable capacitance controlled by said control signal, and said first and second resistors have a fixed resistance, andwherein each of said first and second capacitors includes a single fixed capacitor and a plurality of additional capacitors selectively connected in parallel with said fixed capacitor.