Radio-frequency mixer having transformer with passive termination
An electronic device may include wireless circuitry having a radio-frequency mixer. The mixer may include an input stage, an output stage, and a transformer coupled between the input and output stages. The input stage may include input transistors forming a transconductance cell. The output stage may include two pairs of mixer transistors configured to receive an oscillating signal. The transformer may include a primary coil coupled between the input transistors and a secondary coil coupled between the two pairs of mixer transistors. A harmonic rejection circuit may be coupled to a center tap of the secondary coil. The harmonic rejection circuit may be configured to mitigate harmonic signals that are associated with the oscillating signal and that are generated at the inputs of the output stage. The harmonic rejection circuit may include an inductor, an optional capacitor, an optional current source transistor, or other passive component.
1 . Mixer circuitry comprising:
a first mixer transistor configured to receive an oscillating signal;
a transformer having a primary coil and a secondary coil, the secondary coil being coupled to the first mixer transistor; and
an inductor directly coupled to a center tap of the secondary coil in the transformer, the center tap of the secondary coil being coupled to a power supply line via the inductor.
2 . The mixer circuitry of claim 1 , further comprising:
a second mixer transistor configured to receive the oscillating signal, the secondary coil having a first terminal that is coupled to source terminals of the first and second mixer transistors.
3 . The mixer circuitry of claim 2 , further comprising:
a third mixer transistor configured to receive the oscillating signal; and
a fourth mixer transistor configured to receive the oscillating signal, the secondary coil having a second terminal that is coupled to source terminals of the third and fourth mixer transistors.
4 . The mixer circuitry of claim 3 , wherein:
the first mixer transistor has a drain terminal coupled to a first output terminal of the mixer circuitry;
the second mixer transistor has a drain terminal coupled to a second output terminal of the mixer circuitry;
the third mixer transistor has a drain terminal coupled to the second output terminal of the mixer circuitry; and
the fourth mixer transistor has a drain terminal coupled to the first output terminal of the mixer circuitry.
5 . The mixer circuitry of claim 4 , further comprising:
an output inductor having a first terminal coupled to the first output terminal of the mixer circuitry, a second terminal coupled to the second output terminal of the mixer circuitry, and a center tap coupled to an additional power supply line.
6 . The mixer circuitry of claim 1 , further comprising:
a first input transistor coupled to a first terminal of the primary coil in the transformer; and
a second input transistor coupled to a second terminal of the primary coil in the transformer.
7 . The mixer circuitry of claim 6 , further comprising:
an input transformer coupled to gate terminals of the first and second input transistors;
a signal strength indicator circuit coupled to the input transformer; and
a passive component coupled to the input transformer.
8 . The mixer circuitry of claim 1 , further comprising:
a capacitor coupled in parallel with the inductor, the capacitor having a first terminal coupled to the first terminal of the inductor and having a second terminal coupled to the power supply line.
9 . The mixer circuitry of claim 1 , further comprising:
a capacitor coupled in series with the inductor, the capacitor having a first terminal coupled to the inductor and having a second terminal coupled to the power supply line.
10 . The mixer circuitry of claim 9 , further comprising:
a current source transistor coupled to a node disposed between the secondary coil and the inductor.
11 . Mixer circuitry comprising:
a first input transistor;
a second input transistor;
a transformer having a primary coil coupled between the first and second input transistors and having a secondary coil;
a passive component coupled to a center tap of the secondary coil of the transformer; and
a current limiting transistor coupled to the passive component.
12 . The mixer circuitry of claim 11 , further comprising:
a first pair of mixer transistors coupled to a first terminal of the secondary coil and configured to receive an oscillator signal; and
a second pair of mixer transistors coupled to a second terminal of the secondary coil and configured to receive the oscillator signal.
13 . The mixer circuitry of claim 12 , wherein the passive component comprises an inductor.
14 . The mixer circuitry of claim 13 , further comprising a capacitor coupled to the inductor.
15 . Circuitry comprising:
an input port;
an output stage configured to receive an oscillating signal;
a transformer having a primary winding coupled to the input port and having a secondary winding coupled to the output stage;
a first harmonic rejection circuit coupled to a center tap of the primary winding; and
a second harmonic rejection circuit coupled to a center tap of the secondary winding.
16 . The circuitry of claim 15 , further comprising:
a signal strength indicator (SSI) circuit coupled to the input port.
17 . The circuitry of claim 15 , wherein the output stage comprises:
a first pair of mixing transistors coupled to a first terminal of the secondary winding; and
a second pair of mixer transistors coupled to a second terminal of the secondary winding.
18 . The circuitry of claim 15 , wherein the harmonic rejection circuit comprises one or more passive components.