Amplifiers – With plural amplifier channels – Redundant amplifier circuits
Reexamination Certificate
2000-04-12
2001-11-20
Pascal, Robert (Department: 2817)
Amplifiers
With plural amplifier channels
Redundant amplifier circuits
C330S295000, C330S302000
Reexamination Certificate
active
06320462
ABSTRACT:
GOVERNMENT RIGHTS
Not applicable.
RELATED APPLICATIONS
Not applicable.
FIELD OF THE INVENTION
This invention relates to RF circuits and more particularly to RF power amplifier circuits.
BACKGROUND OF THE INVENTION
As is known in the art, in telecommunications systems, satellite systems and other systems, it is desirable for radio frequency (RF) power amplifiers (PAs) to linearly amplify RF signals in a highly efficient manner. Efficiency is generally proportional to input drive level. High efficiency is typically not attained until an amplifier approaches its maximum output power. This, however, is not consistent with linear operation. Thus, a tradeoff must typically be made between achieving maximum efficiency and high linearity in RF power amplifier circuits.
As is also known, so-called Doherty-type amplifiers or more simply Doherty amplifiers have been used to overcome this problem. Generally, a Doherty amplifier includes a pair of transmission paths connected in parallel between a source and a load. Each of the transmission paths includes an amplifier.
In one signal path, the amplifier is provided as a class “B” or a class “AB” amplifier (also referred to as “carrier amplifier” in the Doherty amplifier design). The carrier amplifier is designed and biased to amplify signals having relatively low signal levels. In the other signal path, the amplifier is provided as a class “C” amplifier (also referred to as a peak amplifier in the Doherty amplifier design). The peak amplifier is designed and biased such that it is off when the instantaneous value of the RF signals provided by the signal source have a signal level below a predetermined threshold. When the signal level of the RF signal fed to the peak amplifier input port reaches a predetermined threshold level, the peak amplifier is biased on and both the peak and carrier amplifiers deliver RF power to the load.
With this approach, the Doherty amplifier is able to provide an optimum power added efficiency (PAE) over a desired range of amplifier output power (P
out
). An ideal Doherty amplifier has a constant PAE value over a 6 decibel (dB) range of P
out
.
The carrier and peak amplifiers provided in the parallel signal paths of the Doherty amplifier may be built with high-power field effect transistors (FETs). In general, the optimum load impedance of a high power FET is relatively low at high power levels. This makes it relatively difficult to combine the signals from the carrier and peak amplifiers while at the same time maintaining a relatively high gain and PAE over a range of Doherty amplifier output power.
It would, therefore, be desirable to provide an RF power amplifier that provides a desired PAE over a relatively wide range of amplifier output power. It would also be desirable to provide an RF power amplifier that is linear and efficient for multi-carrier noise-like signals.
SUMMARY OF THE INVENTION
In accordance with the present invention, a method for providing a Doherty amplifier includes the steps of optimally matching an input impedance of a first amplification device in a first signal path, power matching an output impedance of the first amplification device to provide maximum power output, power matching the output impedance of a second amplification device in a second signal path to provide maximum power output, optimally matching the input impedance of the second amplification device under low signal conditions and providing a time delay circuit at the output of the second amplification device with the time delay circuit having a time delay selected to optimize the PAE of the Doherty amplifier. With this particular arrangement, a Doherty amplifier having a relatively high PAE over a relatively wide range of output power is provided. In one embodiment, the first signal path corresponds to a carrier amplifier signal path and the second signal path corresponds to a peak amplifier signal path. To provide an equal propagation time between the carrier amplifier signal path and the peak amplifier signal path a second delay line can be used. It should be noted that the delay in the peak amplifier signal path is comprised of input matching networks, the amplification devices, the output matching network and the delay line. Similarly, the delay in the carrier amplifier signal path is provided from the input matching network, the first amplification device and the output matching network. It should be noted that the delay required to equalize the delay through the two signal paths might be positive or negative. If the delay needed in the carrier amplifier signal path is negative, then such a delay is typically realized as a positive delay in the peak amplifier signal path. On the other hand, if the delay needed in the carrier amplifier signal path is positive, then the delay is typically realized as a positive delay in the carrier amplifier signal path. Thus, a further source of delay in the carrier amplifier signal path can be due to a delay line provided in the carrier amplifier signal path. Thus the Doherty amplifier can be provided having a first delay line which ensures that signals propagating through the carrier amplifier signal path experience the same delay as signals propagating through the peak amplifier signal path and a second delay line which provides the Doherty amplifier having an optimum PAE over a desired range of Doherty amplifier output power.
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Toshio Nojima et al. “High Efficiency Transmitting Power Amplifiers for Portable Radio Units”, 2334 The Transactions of the Institute of Electronics, Information and Comm. Engineers E74., No. 6, Jun. 6, 1991, pp. 1563-1570.
Emrys Williams, “Thermionic Valve Circuits”, Sir Isaac Pitman & Sons, Ltd, 3rdEd, 1952, pp. 124-125.
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Daly, Crowley & Mofford LLP
Nguyen Khanh Van
Pascal Robert
Raytheon Company
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