Amplifiers – With amplifier bypass means
Patent
1991-07-14
1994-06-21
Mullins, James B.
Amplifiers
With amplifier bypass means
H03F 126
Patent
active
053231198
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to amplifiers and in particular to high power amplifiers wherein feed forward cancellation is employed, such as those used in radio-frequency (r.f.) applications.
In amplifier design, there is a trade off to be made between distortion performance and efficiency. Amplifiers which operate under so-called `Class A` conditions have good distortion but low efficiency whereas an amplifier operated under class C conditions is reasonably efficient but introduces significant distortion. High efficiency and low distortion is the goal, but efficiency increasingly becomes a consideration at high power levels. For example, a typical cellular radio multi-carrier base station amplifier requirement would be 200 w average, 2 kw peak so clearly efficiency must be the best achievable if undue heat dissipation in the amplifier is to be avoided. Unfortunately, the common technique of negative feedback to correct distortion can only be considered for narrow band applications in r.f. high power amplifiers. For this reason many r.f. power amplifier operate in class A with the consequence heat dissipation tolerated.
As an alternative to class A operation, a more efficient class AB amplifier may be employed if feed-forward cancellation is applied. In this technique, the amplifier output (suitably scaled) is compared with the input signal to yield an error signal. The error signal is amplified and reintroduced to the output 180.degree. out of phase with the original distortion, the distortion products being thereby cancelled in the final output. Feed forward can yield a 30 dB improvement in distortion performance but only if both the first comparison loop and the second correcting loop are accurately aligned. Generally the loop parameters do not remain constant over the full operating range of the amplifier and some dynamic correction is required, particularly in wideband application. For example, the error signal may be subject to amplitude and phase control prior to amplification. To provide the required control signals, a pilot tone may be injected into the amplifier input. The pilot signal (at residual distortion level) is detected at the output and used to provide control signals.
There are a number of problems to be solved with this approach. Firstly, there is successful detection of the low level pilot tone at the output. This demands good filtering at the receiver. Secondly there is the problem of deriving the control signals themselves. Systems which have been tried are an amplitude only arrangement, with long time constants aimed at correcting long term drift only, and iterative microprocessor controlled phase and amplitude systems, the basis of the control algorithm being to introduce an amplitude or phase change in the event of increased distortion and monitor the effect at the output for improvement, then loop until optimized. Unfortunately, this technique requires expensive filtering techniques at the receiver and a processor resource making it impractical and non-cost effective in most applications. Also, settling time is likely to be long.
In accordance with the present invention there is provided an amplifier arrangement to which feed forward cancellation is applied by a comparison loop including comparison means for comparing amplifier input with amplifier output to provide an error signal, a cancellation loop including secondary amplifier means for amplifying the error signal and combining means for combining said amplified error signal with said amplifier output, a pilot generator coupled to said amplifier input to introduce a pilot tone therein, detector means for detecting a level of pilot tone in said amplifier output and correction means for correcting said cancellation loop performance as a function of said detection wherein said pilot generator is further coupled to a multiplier receiving said amplifier output, said multiplier producing an output signal arranged to control a loop parameter to effect said correction.
Preferably the loop parameter is amplitude, or phase, or both.
REFERENCES:
patent: 3922617 (1975-11-01), Denniston et al.
patent: 4885551 (1989-12-01), Myer
Ha Thomas
Luettgenau Georg
Powell Jack
Barbee Joe E.
Botsch Sr. Bradley J.
Motorola Inc.
Mullins James B.
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