Abstract:
An amplifier includes a differential amplifier and a first collector follower amplifier connected to the output terminal of the differential amplifier. A second collector follower amplifier is provided, the input terminal of which is connected to the common impedance of the differential amplifier to receive one part of the voltage thereacross, and the output terminal of which is connected to the output terminal of the first collector follower amplifier so as to reduce the common mode gain of the differential amplifier.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to an amplifier, and is directed more particularly to an amplifier with a differential amplifier. 
     2. Description of the Prior Art 
     In the art there has been known such an amplifier which includes a differential amplifier formed of two transistors and a collector follower transistor connected directly to the output side of the differential amplifier. Such a prior art amplifier is shown in FIG. 1. As shown in FIG. 1, a differential amplifier A is formed of two transistors Q 1  and Q 2  whose emitter electrodes are connected together through an emitter resistor R 2  to one terminal -V CC  of a voltage source, and whose base electrodes are connected to input terminals T 1  and T 2 , respectively. The collector electrode of the transistor Q 2  is connected to the other terminal +V CC  of the voltage source and the collector electrode of the transistor Q 1  is connected through a resistor R 1  to the terminal +V CC  and also to the base electrode of another transistor Q 3 , whose emitter electrode is connected through a resistor R 3  to the terminal +V CC  and whose collector electrode is connected to an output terminal T 3  and also to the terminal -V CC  through a resistor R 4 . 
     With such a prior art amplifier, if the differential amplifier A is formed completely or ideally, even when an input signal with components which are the same in phase are supplied to the input terminals T 1  and T 2 , respectively no signal components which are the same in phase are delivered to the output terminal T 3 . 
     However, due to the fact that the transistors Q 1  and Q 2  which form the differential amplifier A are not uniform in characteristics, when the same signal components in phase are fed to the input terminals T 1  and T 2 , respectively, the same signal components in phase appear at the output terminal T 3 . 
     The total gain of the amplifier for the same signal components in phase or the common mode gain CMG is expressed as follows: 
     
         CMG = R.sub.1 /2R.sub.2. R.sub.4 /R.sub.3 
    
     in the above expression, the factor R 1  /2R 2  represents the common mode gain of the differential amplifier A itself, and the factor R 4  /R 3  the gain of the transistor Q 3 . 
     With the prior art amplifier, the resistor R 2  is used as a constant current source which is made to have a high impedance to reduce the common mode gain. 
     However, in general the constant current source is formed of a transistor whose emitter electrode is grounded, so that it is impossible to select the impedance of the constant current source infinitely. Therefore, the prior art amplifier can not make its common mode gain zero. 
     If there is such a common mode gain in an amplifier, there occurs a defect that when the voltage source is made ON and OFF, and its source voltage is changed, noises are caused or the ripple in the voltage source is contained in the signal which deteriorates the S/N ratio. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a novel amplifier free from the defects inherent to the prior art amplifier. 
     It is another object of the invention to provide an amplifier with which the common mode gain caused by the nonuniformity of transistors forming a differential amplifier can be reduced substantially to zero. 
     It is a further object of the invention to provide an amplifier which is free from noises when the voltage source is made ON and OFF. 
     It is a further object of the invention to provide an amplifier which does not contain any appreciable ripple in its output signal and hence has a good S/N ratio. 
     It is a yet further object of the invention to provide an amplifier with which its voltage source ripple filter can be made simple or omitted. 
     In accordance with an aspect of the present invention, there is provided an amplifier which comprises a differential amplifier consisting of first and second transistor each having first, second and third electrodes, the first electrodes of said first and second transistors being connected to first and second input terminals, respectively, the second electrode of said first transistor being connected to one terminal of a voltage source through a first impedance and the third electrodes of said first and second transistors being connected together to the other terminal of said voltage source through a common impedance having an intermediate tap, a third transistor having first, second and third electrodes, said first electrode of the third transistor being connected to the collector electrode of said first transistor and said second electrode of the third transistor being connected to said one of said voltage source, said third electrode of the third transistor being connected to an output terminal, and a fourth transistor having first, second and third electrodes, said first electrode of the fourth transistor being connected to said intermediate tap of said common impedance, said second electrode of the fourth transistor being connected to the other terminal of said voltage source, said third electrode of the fourth transistor being connected to the third electrode of said third transistor, said fourth transistor being used for reducing the common mode gain of said differential amplifier. 
     The other objects, features and advantages of the invention will become apparent from the following description taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram showing the prior art amplifier; 
     FIG. 2 is a circuit diagram showing an example of the amplifier according to the present invention; and 
     FIG. 3 is a circuit diagram showing an audio main amplifier in which the amplifier of the invention is used. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will be hereinafter described with reference to FIGS. 2 and 3. 
     An example of the amplifier according to the invention will be now described with reference to FIG. 2, in which the parts corresponding to those of FIG. 1 are marked with the corresponding numerals and letters and their description will be omitted for the sake of brevity. 
     As shown in FIG. 2, in the amplifier of the invention, two resistors R 5  and R 6 , which are connected in series, are used in place of the emitter resistor R 2  of the transistors Q 1  and Q 2 , which form the differential amplifier A of the prior art amplifier shown in FIG. 1, and a series circuit of a transistor Q 4  and a resistor R 7  is used in place of the collector resistor R 4  for the transistor Q 3  of the prior art amplifier shown in FIG. 1. The connection point between the resistors R 5  and R 6  is connected to the base electrode of the transistor Q 4  whose emitter electrode is connected to the negative voltage source terminal -V CC  through the resistor R 7  and whose collector electrode is connected to the collector of the transistor Q 3 . The output terminal T 3  is led out from the connection point between the collector electrodes of the transistors Q 3  and Q 4 , and a load resistor R L  is connected between the output terminal T 3  and ground. The rest of the circuit construction of the amplifier of the invention shown in FIG. 2 is substantially the same as that of the prior art amplifier shown in FIG. 1. 
     With the amplifier of the invention shown in FIG. 2, when signal components which are the same in phase are supplied to the input terminals T 1  and T 2 , the signal components appear through the transistor Q 3  at its collector electrode. However, at this time the signal components which are the same in phase and supplied to the terminals T 1  and T 2  are added to each other without being cancelled and appear at the emitter electrodes of the transistors Q 1  and Q 2 . The signal components appeared at the collector electrodes of the transistors Q 1  and Q 2  are divided by the resistors R 5  and R 6 , then reversed in phase by the transistor Q 4 , and appear at the collector electrode thereof with the phase reversed from that appearing at the collector electrode of the transistor Q 3 . Thus, if the common mode gain is equal, the signal components appearing at the collector electrode of the transistor Q 3  are cancelled out by the signal components appearing at the collector electrode of the transistor Q 4  and hence no signal components which are the same in phase are delivered to the output terminal T 3 . 
     The common mode gain CMG of the amplifier of the invention shown in FIG. 2 is expressed as follows: ##EQU1## 
     In the above expression, the first factor ##EQU2## represents the common mode gains of the differential amplifier A and the transistor Q 3 , the former half (R 6  /(R 5  + R 6 )) of the second factor the dividing ratio of the resistors R 5  and R 6 , and the latter half (R L  /R 7 ) of the second factor the gain of the transistor Q 4 , respectively. 
     Accordingly, the common mode gain CMG of the amplifier shown in FIG. 2 can be rewritten as follows: ##STR1## 
     Therefore, if the resistance values R 1 , R 3 , R 6  and R 7  of the respective resistors are selected to satisfy ##EQU3## or 
     
         R.sub.1 .sup.. R.sub.7 = 2R.sub.3 .sup.. R.sub.6 
    
     the common mode gain CMG is made zero (CMG = 0) and hence no signal components same in phase appear at the output terminal T 3 . 
     An audio main amplifier, in which the amplifier of the invention is employed, will be now described with reference to FIG. 3, in which reference numerals being the same as those used in FIG. 2, which designate the same element, and their detailed description will be omitted. 
     In the audio main amplifier shown in FIG. 3, a series connection of diodes D 1 , . . . D n  of the same polarity is connected between the collector electrodes of the transistors Q 3  and Q 4 . Transistors Q 5  and Q 6  are connected in a Darlington connection, and transistors Q 7  and Q 8  are connected also in a Darlington connection. The transistors Q 5  to Q 8  are further connected as an SEPP (single-ended push-pull) stage. In this case, the base electrodes of the transistors Q 5  and Q 7  are connected through resistors to the collector electrodes of the transistors Q 3  and Q 4 , respectively, and a resistor R 8  is connected between the base electrode of the transistor Q 2  and the output terminal T 3 , which is led out from the connection point between the emitter electrodes of the transistors Q 6  and Q 8 , to apply a negative feedback. The series connection of the diodes D 1  to D n  serves as the bias diode for the transistors Q 5  to Q 7 . In FIG. 3, S p  designates a loudspeaker. 
     With the amplifier shown in FIG. 3, if the resistors R 1 , R 3 , R 6  and R 7  are selected in resistance value as in the case of FIG. 2, there appear no signal components the same in phase. As a result, there is no fear that when the voltage source is made ON and OFF, that the loudspeaker S p  will be damaged by noises, or that uncomfortable sounds will be emitted from the loudspeaker S p  or that the S/N ratio will be deteriorated by ripples. 
     In general, in the main amplifier, it is difficult to eliminate ripple components as much as desired due to the current capacity of the voltage source, but with the main amplifier using the amplifier of the invention no ripples are contained in the output signal and its S/N ratio is good. 
     Further, with the present invention even if the ripple filter for the voltage source is made simple or the ripple filter is omitted, no ripple is contained in the output signal. 
     The above description is given on only one preferred embodiment of the invention, but it will be apparent that many modifications and variations could be effected by one skilled in the art without departing from the spirit and scope of the novel concepts of the present invention.