Electronic variable delay line filters using two in-line varactor-controlled four-input couplers allowing variable delay

A feed-forward power amplifier, comprising:

• a main signal path;
• and
• a secondary signal path having a delay circuit comprising
• a first 3 dB quadrature hybrid coupler having a first node receiving an input signal, a second node that is coupled to a first resonator circuit, a third node that is coupled to a second resonator circuit and a fourth node that is an output node, the first and second resonators respectively including a first and a second varactor circuit, the first and second resonators each being resonant at a first predetermined resonant frequency that is a function of a first control voltage applied to each of the first and second varactor circuits, the first 3 dB quadrature hybrid coupler having a delay response as a function of frequency that is substantially linearly decreasing for increasing frequency greater than the first resonant frequency;
• and
• a second 3 dB quadrature hybrid coupler having a first node coupled to the fourth node of the first 3 dB quadrature hybrid coupler, a second node that is coupled to a third resonator circuit, a third node that is coupled to a fourth resonator circuit, and a fourth node that is an output node, the third and fourth resonators respectively including a third and a fourth varactor circuit, the first and second resonators each being resonant at a second predetermined resonant frequency that is a function of a second control voltage applied to each of the third and fourth varactor circuits, the second predetermined resonant frequency being greater than the first predetermined resonant frequency, the second 3 dB quadrature hybrid coupler having a delay response as a function a frequency that is substantially linearly decreasing for increasing frequency less than the second resonant frequency,
• the overall combined delay response of the first 3 dB quadrature hybrid coupler and the second 3 dB quadrature hybrid coupler appearing at the fourth node of the second 3 dB quadrature hybrid coupler being substantially constant between the first resonant frequency and the second resonant frequency.