Active double-balanced mixer
The present invention describes active double-balanced mixers. The mixers use an isolation and matching section that provides RF section input matching and superior isolation between the LO and RF port. The mixers can be implemented using an inexpensive semiconductor processing technology, obviating the need for matching inductors and facilitating a low-cost, low-power fully integrated wireless receiver.
1. A mixer comprising:
a local oscillator switch coupled to a local oscillator;
an inductorless transconductor coupled to an RF signal input, the inductorless transconductor further comprising four transistors arranged in a first and second stage, each of the two transistors in the first stage being coupled to a first stage load and to the local oscillator switch, the second stage of two transistors being capacitively coupled to bases of the two transistors in the first stage, and the RF signal input being capacitively coupled to bases of the two transistors in the second stage, whereby the inductorless transconductor provides isolation between the local oscillator and the RF signal input and reduces DC offsets.
2. The mixer of claim 1 wherein the local oscillator has a frequency which is a fundamental-tone frequency of the RF signal input.
3. The mixer of claim 2 wherein the local oscillator switch comprises two differential pairs of transistors.
4. The mixer of claim 1 wherein the local oscillator has a frequency which is a subharmonic frequency of the RF signal input.
5. The mixer of claim 4 wherein the local oscillator switch comprises four differential pairs of transistors coupled to four component signals from the local oscillator, where the four component signals are each phase shifted from each other.
6. The mixer of claim 1 wherein the first stage loads coupled to the two transistors in the first stage further comprise resistive elements, where the resistive elements are arranged in parallel with capacitive elements between the two transistors in the second stage and the two transistors in the first stage and where the resistive elements and capacitive elements are sized so as to provide impedance matching for the RF signal input.
7. The mixer of claim 6 wherein the first stage of transistors is coupled at the bases of the transistors to a voltage source.
8. The mixer of claim 7 wherein the first stage of transistors is capacitively coupled at emitters of the transistors to ground.
9. The mixer of claim 1 wherein the transistors are selected from the group of bipolar junction transistors, heterojunction bipolar transistors, junction field effect transistors,III–V compound field effect transistors, metallic semiconductor field effect transistors, and pseudomorphic high electron mobility transistors.
10. The mixer of claim 1 wherein the mixer is a component in a direct conversion receiver.