Wave transmission lines and networks – Plural channel systems – Having branched circuits
Reexamination Certificate
1999-08-10
2001-07-10
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Plural channel systems
Having branched circuits
C333S033000, C333S034000, C330S286000, C330S295000
Reexamination Certificate
active
06259335
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to combining networks for power amplifiers and, more particularly, to an amplitude and phase impedance matching network for use in monolithic microwave integrated circuit (MMIC) power amplifiers.
2. Discussion of Related Art
Power amplifiers are an integral part of a myriad of electronic systems. For example, power amplifiers are used in televisions, cellular phones, audio and video receivers and transmitters, radios, compact disc players, satellites, automotive electronics, as well as many other devices. Power amplifiers typically employ transistors that perform amplification and accommodate the high currents and high power dissipation within the amplifier. These transistors are typically field-effect transistors (FETs) fabricated on GaAs (Gallium-Arsenide) or InP (Indium-Phosphide) substrates, but can be other types of transistors, such as bipolar transistors and fabricated on other semiconductor materials. The transistors are connected to other circuit elements, such as resistors, capacitors and inductors, or to power supplies or common terminals to form connections with other transistors in an overall network to provide the power amplifier.
The power transistors have a dramatic impact on the efficiency and the overall performance of power amplifier circuits. An InP high electron mobility transistor (HEMT) is the preferred type of transistor for these applications. InP GaAs HEMTs or HBT provide higher cutoff frequencies and lower noise figures than their GaAs counterparts. InP HEMTs also operate at lower drain voltages, and, therefore, draw less power than GaAs HEMTs. Also, at the same output power level, InP HEMTs have a higher power-added efficiency (PAE) than GaAs HEMTs. PAE is a measure of the amplifier's performance, and it correlates to the amount of output signal power produced by the amplifier compared to the input signal power and the DC supply voltage and current.
The connections between transistors and other circuit elements in a power amplifier pose several challenges in circuit design. These challenges are especially evident in radio frequency (RF) applications. RF applications typically have operating frequencies from several hundred megahertz (MHz) to more than one hundred gigahertz (GHz). In order to effectively combine the power outputs from individual transistors to a higher power output, all the individual transistors have to be combined in phase with balanced amplitudes. The overall efficiency of an amplifier is partially dependent on correctly matching these properties. The more accurately the design matches the phase and amplitude of the various signals, the more efficiently the amplifier will operate. An efficient power amplifier has a high gain and a low internal power loss. When the phase and amplitude of the components' impedance are not properly matched, the amplifier loses efficiency and it operates at a lower gain and dissipates more power internally. Power amplifiers also experience the negative effect of a reduced gain at higher operating frequencies. This phenomenon makes it especially advantageous to optimize the gain of power amplifiers for high frequency applications.
Traditionally, circuit designers use the interconnects to the transistors to correct mismatches in phase and amplitude of the device impedances. By specializing the interconnects, circuit designers can correct many deficiencies. For example, the width of a transmission line is directly related to its impedance. Thus, varying the width of the lines by using wider transmission lines will allow for proper impedance matching. Additionally, varying the lengths of the interconnects or tapering the interconnects will also serve to match different phases and amplitudes of the individual transistor's impedance. Tapering the transmission lines is another technique that helps to match the phase and impedance.
These interconnect design techniques suffer from several drawbacks. For example, wide transmission lines allow transverse signal transmission, especially at high frequencies. Instead of being directed toward the next stage, a signal that travels transversely takes a longer and less efficient path along the interconnect. This inefficient signal propagation degrades the performance of the amplifier. Matching phase and amplitude of the transistors' impedances through longer interconnects also requires a larger chip area and significantly increases the size of the amplifier. Tapering the transmission lines introduces a high conductive loss in the network. Increasing the conductive loss reduces the power of the transmitted signal, and this reduces the gain of the amplifier. Because the conductive loss, from both tapering and from longer interconnects, degrades the performance of the network, there is a serious trade-off from the phase and amplitude matching benefits achieved by lengthening or widening the transmission lines.
In addition to the previously discussed limitations, current network matching techniques utilize square corners in their interconnects. Square corners cause additional power loss through radiation, and, therefore, reduce the efficiency of the amplifier.
A simpler method of interconnecting transistors in a power amplifier network is needed. The method must reduce the conductive losses incurred by current methods, reduce the chip area required by the interconnects and simultaneously reduce or eliminate transverse signal transmission.
SUMMARY OF THE INVENTION
In accordance with the teachings of the present invention, an umbrella-shaped matching network for matching phase and amplitude of the impedances of the power transistors in a power amplifier circuit is disclosed. The matching network is a specialized interconnect between the power transistors and other circuit components in the amplifier. The network serves as an impedance transforming network because it changes the low power-matched device impedance to an intermediate impedance before connecting the network to a load.
The matching network includes rounded corners to reduce microwave signal scattering losses, and to be less prone to signal radiation and subsequent scattering losses than the square corner designs used in traditional matching networks. The matching network further includes slits that prevent current from traveling in the transverse direction, and help direct the current forward towards the load. This prevents the impedance transformer from self-resonating. The slits are also contoured to provide phase and amplitude balancing for the signals transmitted to the transistors. Because the interconnects don't have to be lengthened or tapered to provide phase and amplitude balancing, the required chip area and the power loss associated with longer transmission lines is significantly reduced.
Additional objects, advantages and features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.
REFERENCES:
patent: 5132641 (1992-07-01), Khandavalli
patent: 5942957 (1999-08-01), Mohwinkel et al.
patent: 60-247303 (1985-12-01), None
patent: 2-274101 (1990-11-01), None
Ingram et al., “A 427 mW, 20% Compact W-Band InP HEMT MMIC Power Amplifier,” TRW, RF Products Center, Jun. 1999.
Chen et al, “A 95-GHz InP HEMT MMIC Amplifier with 427-mW Power Output,” Nov. 1998, IEEE Microwave and Guided Wave Letters, vol. 8, No. 11, pp. 399-401.
Stones et al., “Q. and V-Band Planar Combiners,” Jun. 1991, IEEE MTT-S Digest, pp. 1049-1052.
Chen Yaochung
Ingram Daisy L.
Yen Huan-Chun
Pascal Robert
Summons Barbara
Thousand Connie M.
TRW Inc.
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