Not Applicable.
Not Applicable.
1. Field of Invention
The present invention relates generally to power amplifiers and, more particularly, to circuits and methods for suppression of noise and spurious signals in power amplifier systems.
2. Description of the Background
In wireless communications systems, the reception quality of the received signal is related to the signal to noise ratio (SNR), i.e., the ratio of the signal strength to the noise level, at the receiver. Typically, the amount of noise introduced by the power amplifier at the transmitter is negligible compared to the thermal noise at the receiver. This is especiaIly true for long-range applications, such as on the order of hundreds of miles or more. Consequently, it is ordinarily not necessary to utilize noise suppression techniques at the transmitter. Rather, to improve the SNR, it is often sufficient to merely increase signal power at the transmitter.
However, for short-range applications, the amount of noise introduced at the transmitter becomes increasingly important. This is because the noise introduced by the transmitter may approach, or even exceed, the thermal noise floor at the receiver. Moreover, for narrow-band applications, such as with radar systems, the SNR cannot be improved merely by increasing the signal power from the transmitter because of the non-linearity of the power amplifier at saturation, which may cause intolerable spectral regrowth (mostly in pulse modulated radar systems). Moreover, because noise is random, pre-distortion techniques cannot be used.
Accordingly, there exists a need for a manner to suppress noise and other spurious signals in a power amplifier. There further exists a need for a manner to inexpensively suppress noise and other spurious signals in a cascaded amplifier system.
The present invention is directed to an amplifier system. According to one embodiment, the amplifier system includes a vacuum tube amplifier having an input signal terminal and an output signal terminal, wherein the vacuum tube amplifier is for amplifying an input signal supplied to the input signal terminal, a phase comparator having a first input terminal responsive to the input signal and having a second input terminal coupled to the output signal terminal of the vacuum tube amplifier, and a phase shifter having a first input terminal responsive to the input signal, a second input terminal coupled to an output terminal of the phase comparator, and an output terminal coupled to the input signal terminal of the vacuum tube amplifier.
In contrast to prior techniques, the present invention provides an efficient and inexpensive technique for suppressing noise and other spurious signals for vacuum tube amplifiers. Moreover, the techniques of the present invention are applicable for amplifier systems having two or more cascaded amplifiers. In addition, the noise suppression technique of the present invention does not require modification of the vacuum tube amplifier to accommodate serrodyne modulation of the interaction region of the vacuum tube amplifier, such as the helix of a traveling wave tube (TWT). These and other benefits of the present invention will be apparent from the detailed description of the invention hereinbelow.