1. Field of the Invention
The present invention relates to a gain control circuit and a radio communication apparatus using such gain control circuit, and more particularly the present invention relates to a gain control circuit to be used for an output power control in a mobile radio communication apparatus and to a radio communication system using such mobile radio communication apparatus.
2. Description of the Related Art
In a mobile radio communication system, for example, in a mobile telephone system, it is desirable that the output power of each mobile terminal is controlled so as to be received with the same signal strength at a base station, in order to increase the communication capacity of the base station.
In particular, in a communication system in which the mobile telephone system employs a spectrum scattering method referred to as a CDMA (Code Division Multiple Access) system, plural stations are assigned to the same frequency band, and a received signal is recovered by means of a predetermined scattering code. And accordingly, output power control of each mobile terminal becomes a necessary requirement.
There are two types of power control systems that perform the output power control of the mobile terminal. One of the two types of the systems is to determine the output power of the mobile terminal in accordance with the signal strength of a received signal by the mobile terminal, wherein the signal is transmitted from the base station. This type depends on the hypothesis that there is a strong correlation between the signal propagation from the base station to the mobile terminal and the signal propagation from the mobile terminal to the base station. This type of control is named an open loop control.
The other type of system is to determine for information about an actually received the strength of the radio wave at the base station, wherein such information is transmitted from the base station to the mobile terminal. This type of control is named a closed-loop control.
A gain control circuit is necessary in order to control the output power. The performance of such gain control circuit requires a wide gain control range, a wide dynamic range, a good controllable linearity, an absolute gain accuracy, a good temperature characteristic and a broad frequency band.
For example, as the gain control range, a gain of about 90 dB is necessary in a receiving side, and a gain of about 80 dB is necessary in a transmitting side. As for the dynamic range, it is necessary to consider, particularly in the receiving side, a situation where the radio signal wave received is very weak and a strong interfering radio wave enters. And accordingly, as for the gain control circuit, tolerance to a very large input signal and a low noise characteristic are simultaneously required.
It is necessary to match the characteristic of the receiving side gain control circuit and the transmitting side gain control circuit about the controllable linearity, the absolute gain accuracy, the temperature characteristic in order to raise the accuracy of the previously described the open loop control. About the frequency bandwidth, it is different by a system, but it is easiest to do such an operation by an IF (Intermediate Frequency) stage. As for the typical frequency of that purpose, there are many cases that are around 100 MHz.
FIG. 6 is a circuit diagram which shows a conventional embodiment of a variable gain circuit constituting a gain control circuit. The variable gain circuit of this conventional embodiment has a differential amplifying circuit 101, a bias circuit 102, a pair of current dividing circuits 103 and 104 and a pair of resistive circuit meshes 105 and 106.
The differential amplifying circuit 101 comprises npn-type differential pair transistors Q101 and Q102, in which each emitter electrode of the transistors Q101 and Q102 is grounded through respective emitter resistors R101 and R102, respectively. An input voltage Vi is supplied to input terminals Vin+, Vin− connected to each base electrode of the differential pair transistors Q101 and Q102.
The bias circuit 102 comprises bias resistors R103 and R104 connected to each base electrode of the differential pair transistors Q101 and Q102 and a bias voltage supply 107 connected between the bias resistors R103 and R104 and the ground and which supplies a fixed bias voltage Vbias to each base electrode of the differential pair transistors Q101 and Q102 through the bias resistors R103 and R104.
One current dividing circuit 103 comprises npn-type differential pair transistors Q103 and Q104, in which each emitter electrode of the transistors Q103 and Q104 is connected commonly to a collector electrode of the transistor Q101. The other current dividing circuit 104 comprises npn-type differential pair transistors Q105 and Q106, in which each emitter electrode of the transistors Q105 and Q106 is connected commonly to a collector electrode of the transistor Q102.
In these current dividing circuits 103 and 104, each base electrode of transistors Q103 and Q105 is connected to each other, each base electrode of transistors Q104 and Q106 is connected to each other and a control voltage Vc is applied to a pair of input terminals Vc+, Vc− connected between these base electrodes of the transistors Q103, Q105, Q104 and Q106. And, an output voltage Vo is provided from a pair of output terminals Vout+, Vout− connected to each collector electrode of the transistors Q103 and Q105.
One resistive circuit mesh 105 comprises resistors R105 and R106 connected between the differential pair transistors Q103, Q104 and a power source voltage VCC and a resistor R107 connected to the collector electrodes of the differential pair transistor Q103 and Q104. The other resistive circuit mesh 106 comprises resistors R108 and R109 connected between the differential pair transistors Q105, Q106 and the power source voltage VCC and a resistor R110 connected to the collector electrodes of the differential pair transistor Q105 and Q106.
The transmission gain G of the variable gain circuit, as shown in FIG. 6, is now explained. At first, the control voltage Vc from the control voltage supply circuit 108 is supplied to the input terminals Vc+, Vc−connected between the base electrodes of the differential pair transistors Q103 and Q104 and the base electrodes of the differential pair transistors Q105 and Q106. This control voltage supply circuit 108 generates an internal control voltage Vc varying in linearity relative to the external control voltage VC supplied from an external control voltage generating source 109.
The transmission gain G varies by changing the ratio of flowing currents of the current dividing circuits 103 and 104 in accordance with the internal control voltage Vc generated at the control voltage supply circuit 108 based on the external control voltage VC from the external control voltage generating source 109, wherein the potential difference ΔVbe between base electrodes of the differential pair transistors Q103 and Q104 and the differential pair transistors Q105 and Q106 are changed by means of the internal control voltage Vc supplied from the control voltage supply circuit 108.
The transmission gain G is expressed by the next expression:G=Gmax/{1+exp(−qVc/kt)}+Gmim/{1+exp (qVc/kt)}
Gmax shows the maximum transmission gain of the variable gain circuit, Gmin shows the minimum transmission gain of the variable gain circuit, q shows the charge of an electron, k shows the Boltzmann's constant and t shows the absolute temperature.
As described above, in the conventional variable gain circuit, the transmission gain G is controlled by means of the internal control voltage Vc that varies in linearity relative to the external control voltage VC. As shown in FIG. 7, as the external control voltage VC approaches the maximum transmission gain Gmax or the minimum transmission gain Gmin, the characteristic curve tends to bend, and the linearity of the variable gain circuit becomes deteriorated.
This kind of variable gain circuit composes a gain control circuit by providing a plural number of the variable gain circuits in a cascade connection by way of buffer circuits. For example, this kind of gain control circuit is used as an AGC (Automatic Gain Control) amplifier for amplifying an IF (Intermediate Frequency) signal of a transmission stage in an RF (Radio Frequency) front-end section of the CDMA-type mobile telephone apparatus.
In such an application, a multistage-type variable gain circuit, such as mentioned above, is used as the AGC amplifier in order to satisfy a request for a wide variable gain range, but if the linearity of the gain control characteristic is bad, it is necessary to increase the number of stages of the variable gain circuit to be cascade-connected thereto. As a result, the circuit scale of the AGC amplifier becomes large and current consumption increases too.