Amplifiers – Hum or noise or distortion bucking introduced into signal...
Patent
1991-06-17
1992-10-13
Mullins, James B.
Amplifiers
Hum or noise or distortion bucking introduced into signal...
330 52, 330 51, H03F 132
Patent
active
051554488
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 wide band 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, as used in fixed frequency intermediate frequency amplifiers for example, to correct distortion can only be considered for narrow band applications in r.f. high power amplifiers. For this reason, many r.f. power amplifiers operate in class A with the consequent 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 in a first comparison loop to yield an error signal. The error signal is amplified and reintroduced to the output 180.degree. out of phase with the original distortion in a second correction loop, 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.
Generally, attempts at improving the performance of feed forward amplifiers have centred upon balancing parameters of the comparison loop and the cancellation loop. For an account of such techniques, reference may be made to the applicant's co-pending patent applications reference SC00095EB and SC00097EB both entitled "Improvements in or relating to amplifiers," and filed on the same day as the present application.
A particular problem with feed forward amplifiers is that the power rating of the error amplifier required to correct the worst case conditions may be many times that required to correct average conditions and to avoid the use of large error amplifiers this design aspect is often compromised.
It is well known that the distortion performance of an amplifier may be improved if an input signal is acted upon by a network having a distortion characteristic chosen to be complementary to the distortion characteristic of the following amplifier. U.S. Pat. No. 4,453,133 for example describes a technique in which a predistorting amplifier closely matched to a following main amplifier is employed. An alternative arrangement is described in U.S. Pat. No. 3,732,502 wherein a plurality of narrow band networks is used to match the distortion, although this latter arrangement lacks the advantage of the former in that signal processing at high power levels is required. Either way, reduced distortion is achieved by matching the static characteristic of the predistortion network with that of the distorting amplifier over the range of use. Clearly, it is important that the match be as close as possible over the entire operating range else the process itself introduces distortion. Distortion may also arise due to the dynamic behaviour of the amplifier not being tracked by the static model, although a system as taught by U.S. Pat. No. 4,453,133 can accommodate a degree of dynamic shift if the performance of the main amplifier can be adequately mirrored.
Feed forward amplifiers, particularly those operating in class AB can show signi
REFERENCES:
patent: 3732502 (1973-05-01), Seidel
patent: 4453133 (1984-06-01), Travis
patent: 4629996 (1986-12-01), Watanabe et al.
patent: 4943783 (1990-07-01), Nojima
Botsch Bradley J.
Motorola Inc.
Mullins James B.
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