Wideband distributed differential power amplifier utilizing metamaterial transmission line conception with impedance transformation
According to one embodiment, a differential power amplifier includes a pair of transistors, a transformer coupled to the drain terminals of the transistors, and an output transmission line. The differential power amplifier operates in a range of frequencies from a lower operating frequency to an upper operating frequency to provide a relatively linear gain between the lower operating frequency and the higher operating frequency. The drains of the transistors are coupled to the primary winding of the transformer. The output transmission line is coupled to the secondary winding of the transformer. The output transmission line further includes at least one inductor-capacitor (LC) circuit that is configured to match predetermined output impedance in view of the lower and upper operating frequencies of the differential power amplifier.
1. A differential power amplifier used in a wideband radio frequency (RF) circuit implemented in an integrated circuit (IC), the power amplifier comprising:
a pair of a first transistor and a second transistor coupled to each other to form a differential amplifier that operates in a range of frequencies from a lower operating frequency (f 1 ) to an upper operating frequency (f 2 ) to provide a relatively linear gain between the lower operating frequency and the upper operating frequency;
a transformer having a primary winding and a secondary winding with a turn ratio (n) and a coupling coefficient (k), wherein the primary winding is coupled to a first drain terminal of the first transistor and a second drain terminal of the second transistor; and
an output metamaterial transmission line coupled to the secondary winding of the transformer, wherein the output metamaterial transmission line comprises:
a first inductor and a second inductor having an identical inductance (L 1 ) coupled in series to a first terminal and a second terminal of the secondary winding of the transformer,
a third inductor coupled to the first terminal of the secondary winding of the transformer, wherein the second inductor is configured to have an inductance (L 1_1 ) that is determined based on L 1 , n, and k,
a first capacitor coupled to the third inductor in series and a first output terminal of the differential power amplifier, the first capacitor having a capacitance of C 2 ,
a fourth inductor coupled to the second terminal of the secondary winding of the transformer, wherein the fourth inductor is configured to have an inductance (L 1_1 ) that is determined based on L 1 , n, and k, and
a second capacitor coupled to the fourth inductor in series and a second output terminal of the differential power amplifier, the second capacitor having a capacitance of C 2 .
2. The power amplifier of claim 1 , wherein the inductance (L 1_1 ) of the third inductor is determined based on: L 1_1 =((1−k 2 )*L 1 )/k 2 .
3. The power amplifier of claim 1 , wherein the inductance (L 1_1 ) of the fourth inductor is determined based on: L 1_1 =((1−k 2 )*L 1 )/k 2 .
4. The power amplifier of claim 1 , wherein output impedance (Z 0 ) across the first and second output terminals is determined based on: Z 0 =SQRT (L 1 /C 2 ).
5. The power amplifier of claim 1 , wherein the output metamaterial transmission line further comprises a third capacitor coupled to the first and second output terminals of the differential power amplifier, wherein the third capacitor has a capacitance of C 1 /2.
6. The power amplifier of claim 5 , further comprising a fifth inductor and a sixth inductor having an identical inductance (L 3 ) coupled in series and then to the first and second output terminals of the differential power amplifier.
7. The power amplifier of claim 6 , wherein output impedance (Z 0 ) across the first and second output terminals is determined based on: Z 0 =SQRT (L 3 /C 2 ).
8. The power amplifier of claim 6 , further comprising:
a seventh inductor having an inductance of L 1A coupled in series between the second inductor and the first capacitor; and
an eighth inductor having an inductance of L 1A coupled in series between the third inductor and the second capacitor.
9. The power amplifier of claim 8 , wherein output impedance (Z 0 ) across the first and second output terminals is determined based on: Z 0 =SQRT ((L 1_1 +L 1 )/C 1 ).
10. The power amplifier of claim 8 , wherein the upper operating frequency f 2 <=1/(2*PI*SQRT ((L 1_1 +L 1A )*C 1 )).
11. The power amplifier of claim 1 , wherein the lower operating frequency f 1 >=1/(2*PI*SQRT (L 1 *C 2 )).
12. The power amplifier of claim 1 , further comprising an input metamaterial transmission line coupled to a first gate terminal of the first transistor and a second gate terminal of the second transistor.
13. The power amplifier of claim 12 , wherein the input metamaterial transmission line comprises:
a ninth inductor having an inductance of L R2 coupled between the first gate terminal of the first transistor and a first input terminal of the differential power amplifier;
a tenth inductor having an inductance of L R2 coupled between the second gate terminal of the first transistor and a second input terminal of the differential power amplifier; and
a fourth capacitor having a capacitance of C R2 /2 coupled to the first and second input terminals.
14. The power amplifier of claim 13 , wherein the upper operating frequency f 2 <=1/(2*PI*SQRT (L R2 *C R2 )).
15. The power amplifier of claim 1 , further comprising a fifth capacitor having a capacitance value of C 1 /2 across the primary winding of the transformer.
16. The power amplifier of claim 1 , wherein a first source terminal of the first transistor is coupled to a second source terminal of the second transistor.