Automatic power control circuit for controlling transmitting power of modulated radio frequency signal

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The preferred embodiments of the present invention will be described with reference to the drawings in which parts coincident with or corresponding to parts described in connection with the related art are designated by similar reference numerals and will not be further described.
FIG.
2 shows the first embodiment of the present invention in which a signal 1 is a modulated RF signal to be amplified by the circuit of the first embodiment.
The RF signal 1 may mainly include a signal having envelope fluctuations such as .
pi.
/4 shift QPSK signal, that is, a signal required to be linearly amplified.
A variable gain amplifier 2 may be in the form of an automatic gain control (AGC) amplifier which can vary its own gain depending on a control signal applied to a gain regulating terminal 2a.
A power amplifier 3 can linearly operate to amplify the RF signal 1 to a given level.
A signal 4 is the output signal of the power amplifier 3.
A detector 5 detects the RF output signal 4 to output an envelope 6.
A switching device 20 is responsive to a switching control signal 24 from a control unit 21 to select a low-pass filter into which the envelope 6 is to be inputted.
The control unit 21 may comprise a CPU or digital signal processor (DSP).
A low-pass filter 22 is one having a smaller time constant to secure a short rise time of a transmitting burst while a low-pass filter 23 is one having a larger time constant sufficient to remove fluctuation components in the envelope 6 which may be generated by the modulation.
A comparator 9 compares the output signal of one of the low-pass filters 22 and 23 with a reference voltage 10 outputted from a reference voltage generator 14 to form a feedback control signal.
Since it is required to increase or decrease the transmitting power level in the RF signal depending on a condition such as a distance between the circuit of the present invention and a base station or the like, the reference voltage 10 has been regulated into a value corresponding to the transmitting power level of the RF signal