Amplifiers – With semiconductor amplifying device – Including temperature compensation means
Patent
1994-06-22
1996-07-02
Mottola, Steven J.
Amplifiers
With semiconductor amplifying device
Including temperature compensation means
330290, H03F 130
Patent
active
055326480
DESCRIPTION:
BRIEF SUMMARY
FIELD OF INVENTION
The present invention relates to radio frequency amplifiers and more particularly, but not exclusively, to radio frequency amplifiers for use in television transmitting apparatus.
BACKGROUND TO THE INVENTION
The devices used in such amplifiers, which may be semiconductor devices such as transistors, or therminoic tubes, may be required to handle large radio frequency (RF) currents which may fluctuate, for example with the degree of modulation of the RF signal being amplified.
It is an unfortuante feature of such devices that their internal temperature will vary as a function of the average RF power handled, due to the heating effect of the RF current. The resulting changes of internal temperature produce corresponding changes in the condustion point of the device and therefore related changes of gain.
For example, in a silicon transistor, the base-emitter voltage necessary to initiate collector current flow, Vbe, changes by 2.1 mV for each degree centigrade change in the temperature of the silicon structure of the device.
Radio frequency amplifiers in television transmitters are commonly run in Class AB mode, in which a degree of non-linearity in the power-in/power-out characteristic of the amplifier is tolerated at high signal levels. However to achieve optimum performance from such Class AB amplifiers, the biassing level of the amplifying device is critical. It will be appreciated that variation in the conduction point of the device, resulting from internal temperature changes caused by radio frequency Dower level variation can give rise to unwanted and varying distortion of the output signal from the device, as the average radio frequency power level varies.
In a radio frequency amplifier within television transmitting apparatus the problem is particularly severe as the amplitude of the modulated radio frequency signal and hence the average radio frequency power handled by the device can undergo rapid swings as the signal varies from levels corresponding to white (minimum power) to black (maximum power). The effect of resulting non-linearities on the transmitted vision signal due to thermal changes within the device can be most noticeable.
The thermal changes associated with the internal heating of the device are rapid, and occur in the internal structure of the device. It is therefore not possible to monitor and correct for such changes by means such as an externally attached temperature sensor forming part of a compensation arrangement, such as may be used with radio frequency power amplifying devices to correct for relatively long-term thermal changes due to ambient operating conditions.
It is thus an object of the present invention to obviate at least in part the deleterious effect of such rapid internal temperature changes due to variations in radio frequency power level, on the performance of devices employed in radio frequency amplifiers.
SUMMARY OF THE INVENTION
According to one aspect of the invention, a radio-frequency amplifier incorporating at least one amplifing device comprises means for applying to the device a signal related to changes in the radio frequency power handled by the device, thereby to compensate for changes in the conduction point of the device due to temperature changes within the structure of the device resulting from changes in the radio frequency power handled by the device.
The compensation signal related to the changes in the radio frequency power handled by the device may be derived from a sensing element in the circuit of the device.
The sensing element may be connected in the principal current path of the device, and may be a resistor.
Alternatively the compensation signal may be derived from elsewhere in a signal chain of which the amplifier forms part, either from a point in the signal chain prior to the amplifier, or from a point in the signal chain following the amplifier.
The compensation signal may be applied to a biassing circuit of the device, preferably by means of a signal filter the time constant of which is related to the intern
REFERENCES:
patent: 3497822 (1970-02-01), Beurrier
patent: 4547746 (1988-10-01), Erickson et al.
patent: 4849712 (1989-07-01), Jarrett
patent: 4924194 (1990-05-01), Opas et al.
patent: 5311143 (1994-05-01), Soliday
patent: 5345192 (1994-09-01), Green
8th European Microwave Conference 78, Sep. 4 through 8, 1978, Sep. 4, 1978, Paris FR, pp. 135-139.
J. A. Vaughan et al. "Bipolar and F.E.T. Power Amplifier For A 2,6 GHZ Community Television Transponder".
Harris Corporation
Mottola Steven J.
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