Amplifiers – With semiconductor amplifying device – Including frequency-responsive means in the signal...
Reexamination Certificate
2003-03-11
2004-12-07
Choe, Henry (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including frequency-responsive means in the signal...
C330S277000, C333S033000
Reexamination Certificate
active
06828862
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to power amplifiers, and specifically to reduction of intermodulation distortion and its dependence on difference frequency in wideband RF power amplifiers.
BACKGROUND OF THE INVENTION
Wideband radio frequency (RF) power amplifiers must commonly amplify signals on multiple, adjacent carriers with high gain and linearity. For example, power amplifiers used in WCDMA base stations are required to amplify signals over a 20 MHz bandwidth in the 2 GHz range. Intermodulation distortion (IMD) is a well-known problem in such amplifiers, stemming from third-order (and higher-order) interaction between different carriers. IMD results from undesired amplification of intermodulation products at the difference frequencies &Dgr;f between different carriers in the wideband signal. Another, related problem in power amplifiers is the memory effect, which causes the amplifier output to depend not simply on the present input signal, but on the history of the signal over a certain time period. The dependence of IMD in a power amplifier on the difference frequency &Dgr;f is indicative of the presence of memory effect in the power amplifier, as well.
Power amplifiers based on Field Effect Transistors (FETs) commonly use a DC bias on the drain of the FET to improve linearity. In order to prevent loading of the FET output at the RF carrier frequency, the DC bias source is connected to the drain by a transmission line having a high impedance at the carrier frequency. Typically, to satisfy this objective, the length of the transmission line is one-quarter wavelength at the carrier frequency. On the other hand, to reduce IMD, it is desirable that the drain bias circuit have a low impedance at the difference frequency &Dgr;f, so that intermodulation products are shunted to ground through this circuit. These impedance requirements tend to conflict.
Various drain bias circuit designs have been proposed for the purpose of reducing IMD. For example, U.S. Pat. No. 5,272,450, to Wisherd, whose disclosure is incorporated herein by reference, describes a DC feed network for a wideband RF power amplifier, using a shunt capacitor in parallel with a choke coil to connect a DC power source to the active device of the amplifier. The cutoff frequency of the bias network can be adjusted so that sum and difference frequencies of multiple-input signals to the amplifier are not attenuated by the feed network. The sum and difference frequencies are thus shunted to ground.
As another example, U.S. Pat. No. 6,081,160, to Custer et al., whose disclosure is also incorporated herein by reference, describes a the use of a bias feed line to couple a DC bias circuit to an active device, wherein a dielectric member is placed adjacent to the bias feed line. The dielectric member is designed to maintain high RF impedance in the bias feed line, while reducing impedance at lower frequencies. The physical length of has an effective length of a quarter wavelength at the RF carrier frequency.
Takenaka et al. have studied the relationship between drain bias circuit impedance and IMD, as described in their article, “Improved IMD Characteristics in L/S-Band GaAs FET Power Amplifiers by Lowering Drain Bias Circuit Impedance,” IEICE Transactions on Electronics E82-C(5), pages 730-736 (May, 1999), which is incorporated herein by reference. The authors showed experimentally that distortion characteristics, such as IMD, are degraded when the absolute value of the drain bias circuit impedance at low frequency is high. Measured IMD in the output of an experimental amplifier was found to increase with bias circuit impedance above a critical impedance value Rc, which was determined empirically. The IMD was also found to increase with increasing frequency spacing &Dgr;f between carrier tones.
It is also possible to use an additional filtering circuit, coupled to the drain of the active device in parallel with the drain bias circuit, in order to suppress beat frequencies between different RF carriers. Such arrangements are described, for example, in U.S. Pat. No. 5,999,058, to Saitou et al., and in U.S. Pat. No. 6,346,859, to Saitou, both of which are incorporated herein by reference. In the amplifiers disclosed in these patents, a DC bias voltage is coupled to the drain through a quarter-wavelength transmission line. The additional filtering circuit (referred to as a filter block or beat-smoothing circuit) comprises a microstrip line or low-pass filter, which is coupled to ground through a capacitor, and which provides a low impedance at the beat frequency between carrier frequencies.
SUMMARY OF THE INVENTION
Embodiments of the present invention provide a method for designing the drain circuit of a RF power amplifier so as to minimize IMD and its dependence on the frequency spacing &Dgr;f. This method is thus also useful in reducing the memory effect in such amplifiers. The power amplifier comprises an active device, typically a FET, with a drain bias circuit coupling the output port of the active device to a DC supply. The method of the present invention is based on the realization that the baseband load impedance of the output port and bias circuit, taken together, can be modeled as a pi-type network, having a resonant frequency. IMD suppression is achieved by setting the parameters of the network so that the resonant frequency is greater than the total bandwidth of the RF signal.
In some embodiments of the present invention, IMD suppression is achieved by means of a suitable transmission line in the bias circuit, or in parallel with the bias circuit, which is typically coupled to a bypass capacitor. In order to achieve the desired resonant frequency in the pi-network, characteristic impedance of the transmission line is set to be less than or equal to a threshold value, which is determined by the operating parameters of the amplifier, including the center frequency, bandwidth, peak power and output capacitance of the active device. Alternatively, a suitable inductor may be used for this purpose. As a result, third-order (and higher-order) intermodulation products are suppressed in the amplifier output.
There is therefore provided, in accordance with an embodiment of the present invention, a radio-frequency (RF) power amplifier, including:
an active device, which includes:
an input port, which is coupled to receive an RF input signal having a center frequency f
0
and a single-sided bandwidth BW; and
an output port, which is characterized by an output capacitance C
1
and is coupled to output an amplified RF signal, responsive to the input signal, with a peak output power P
out
; and
a distortion suppression circuit, which is coupled between the output port and a ground, and includes a transmission line having a characteristic impedance Z
0
that is selected so as to substantially satisfy a condition that
Z
0
≤
f
0
π
BW
·
1
P
out
11.3
+
4
π
BW
·
C
1
.
Typically, the transmission line has a length that is approximately equal to one quarter wavelength at the center frequency f
0
.
Additionally or alternatively, the distortion suppression circuit includes a bypass capacitor, including first and second terminals, wherein the transmission line is coupled between the output port of the active device and the first terminal of the capacitor, and the second terminal of the capacitor is connected to the ground. Typically, the bypass capacitor has a capacitance that is at least twice the output capacitance C
1
.
In a disclosed embodiment, a direct current (DC) bias is supplied to the output port of the active device via the transmission line included in the distortion suppression circuit.
The distortion suppression circuit may include a plurality of cascaded transmission lines, which includes the transmission line having the characteristic impedance Z
0
.
Typically, the distortion suppression circuit has an input impedance Z
L
in a baseband frequency range satisfying
&LeftBracketingBar;
Z
L
&RightBracketingBar;
&leq
Choe Henry
Welsh & Katz Ltd.
Wiseband Communications Ltd.
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