Amplifiers – With amplifier bypass means
Patent
1991-06-19
1992-10-20
Mottola, Steven
Amplifiers
With amplifier bypass means
330107, 330149, H03F 132
Patent
active
051573466
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 no remain constant over the full operating range of the amplifier and some dynamic correction is required, particularly in wideband applications. 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.
For an account of such a correction technique reference may be made to the applicants co-pending application Ser. No. 07/690,929, now U.S. Pat. No. 5,155,448.
In considering the first comparison loop, the objective is that only distortion power is present at the input of the error amplifier, that is to say that the signals due to the input at the point of comparison must cancel. Thus the path via the amplifier to the comparator must balance the undistorted reference path to the comparator. To ensure that this condition is met, amplitude and phase networks may be introduced into the amplifier path. In some arrangements, feed back control of the networks is employed, the control voltages being derived either from comparator output (i.e. distortion power) or separate phase and amplitude comparisons of the signals reaching the feed forward loop comparator, with the objective of maintaining balance. The control becomes active when imbalance occurs for example due to amplifier drift with temperature or if amplifier characteristics are sensitive to average power input. essentially slowly varying parameters are balanced, a typical response time being of the order of 1 mS. Typically even a low bandwidth amplifier has a signal bandwidth of 1MHz, which is a period of 1 .mu.S. Hence control loop response is relatively very slow compared with signal bandwidth. Indeed, low pass filtering of control signals or even use of temperature detectors to give average power feedback is used.
It is an objective of the present invention to provide a feed forward amplifier with improved comparison loop correction compared with the prior art techniques described above.
According to the present invention an ampl
REFERENCES:
patent: 4916407 (1990-04-01), Olver
Ha Thomas
Powell Jack
Botsch Bradley J.
Motorola Inc.
Mottola Steven
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