Amplifiers – With semiconductor amplifying device – Including gain control means
Patent
1991-10-24
1993-05-11
Mottola, Steven
Amplifiers
With semiconductor amplifying device
Including gain control means
330277, H03G 330, H03F 316
Patent
active
052105083
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a high-frequency amplifier of a radio transmitter, comprising an amplifier stage the gain of which can be automatically controlled by a gain control feedback coupled from a subsequent amplifier stage or the output of the high-frequency amplifier.
2. Description of the Prior Art
An automatic power control is used in many radio transmitters to maintain the transmission power constant irrespective of the different factors affecting it. Moreover, it must be possible especially in mobile telephone systems to select the transmission power of a mobile phone depending on the current radio link, for instance according to the information transmitted from the base station of a mobile phone.
Previously, a series transistor which regulates the current passing through an amplifier transistor in a manner depending on the actual transmission power has been used in one of the amplifier stages of a transmitter for controlling the transmission power. The problem with a transmission power control of such kind is that a series transistor consumes an excessive amount of current, which reduces the efficiency of the transmitter and shortens the charging interval of battery-driven transmitters.
Another problem with known high-frequency amplifiers is that the power control may affect the frequency stability of the transmitter by changing the load impedance of a modulator or a voltage-controlled oscillator (VCO) coupled before the amplifier stages, and thereby the frequency. To remove this problem there have been previously been attempts to increase the separation between the amplifier stages and the VCO by means of attenuators coupled between them.
SUMMARY OF THE INVENTION
The object of the invention is to provide an automatic transmission power control for a transmitter, which control has low power consumption and by means of which control the effect of a change in the load of a transmitter on the frequency thereof can be prevented.
This is achieved by a high-frequency amplifier of a radio transmitter of the type disclosed in the introductory portion, the amplifier being characterized in that the amplifier stage comprises a two-gate field-effect transistor, the first gate of which constitutes the signal input, and the second gate of which constitutes the gain control input to which said feedback is coupled, and that the operating point of the transistor is set by biasing the first gate.
As the gate control of a field-effect transistor is an almost pure voltage control, and the gate current is infinitesimal, the gain control of a field-effect transistor consumes practically no power at all. However, there is considerable separation between the gates of a field-effect transistor and correspondingly between the output electrodes and the gates of a field-effect transistor. Therefore the variations in the control signal supplied to the second gate on the one hand and the variations in the output load of the transistor on the other hand do not affect the first gate, to which the input signal is supplied, and do not change the operating point of the transistor, which is set from this gate. Thus, the power control of the amplifier stage does not affect the frequency of the voltage-controlled oscillator preceding the amplifier stage, and the attenuators between them can be omitted.
The power consumption of the amplifier stage can be minimized by setting the operating point of the field-effect transistor in such a manner that the transistor is in a substantially non-conducting state when no high-frequency signal is supplied to the first gate.
BRIEF DESCRIPTION OF THE DRAWINGS
In the following, the invention will be described, by way of example, in greater detail by means of a specific embodiment and with reference to the attached FIG. 1, which shows a circuit diagram of a high-frequency amplifier according to the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In FIG. 1, the amplifier stage according to the invention is the pre-stage in a h
REFERENCES:
patent: 3644832 (1972-02-01), Sherman, Jr.
patent: 4338572 (1982-07-01), Schurmann
patent: 4392245 (1983-07-01), Mitama
patent: 4511854 (1985-04-01), Kishida
Mottola Steven
Telenokia OY
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