1. Field of the Invention
The present invention relates to the technology of a differential signal circuit, and more specifically to the technology of stabilizing an output common mode direct-current voltage.
2. Description of the Related Art
An analog IC (integrated circuit) for outputting an analog signal of a high frequency (for example, several tens of megahertz) is considered. Since a substrate for implementing the IC or a following circuit to which the IC outputs a signal has a large capacitance, it is necessary to provide a high-speed and high driving capability buffer amplifier at the final stage of signal output.
The buffer amplifier can be an operational amplifier. However, when an operational amplifier is used, it is necessary to guarantee a sufficient phase margin by passing a large current to an operational amplifier to suppress an oscillation caused by a large load capacity connected to a subsequent stage. That is, when an operational amplifier is used as a buffer amplifier, the current consumption of the IC becomes large.
Under the circumstances, a source follower can be used as a high-speed and high load driving capability buffer amplifier with small current consumption.
FIG. 1 shows an example of a circuit of a source follower.
In FIG. 1, a transistor 101 is an NMOS transistor, and its drain is connected to the power line (voltage Vcc) of the circuit. The source of the transistor 101 is connected to the ground line (reference potential GND) of the circuit through a current source 102, and a load 103 is connected to a point between the connection point between the source of the transistor 101 as the output of the circuit and the current source 102, and the ground line of the circuit.
The input voltage Vi and the output voltage Vo of the circuit shown in FIG. 1 has the relationship expressed by the following equation.
  Vo  =            Vi      -      Vgs        =          Vi      -              (                                                            2                ⁢                                                                  ⁢                Id                            β                                +          Vth                )            
In the equation above, Vgs indicates the voltage between the gate and the source of the transistor 101, Id indicates the drain current of the transistor 101, β indicates a physical parameter of the transistor 101, and Vth indicates the threshold voltage of the transistor 101.
Since the transistor 101 has the threshold voltage Vth that largely depends on the production condition and the operation temperature condition (for example, ±0.2 volt), the voltage level of the direct-current component of the output of the source follower shown in FIG. 1 is not stable. Therefore, there has been the problem that a source follower cannot be used as a buffer amplifier when it is necessary to have a direct-current component at a stable voltage level in the circuit at a subsequent stage.
Relating to the present invention, for example, Japanese Published Unexamined Patent Application No. H8-228115 discloses the technology of matching a common mode output signal with a reference common mode input signal with high accuracy in a differential amplification circuit having a common mode feedback.