Abstract:
A voltage-controlled attenuator achieves lower distortion and noise through a construction wherein collector currents of a driving differential amplifier are linearized by employing a current mirror configuration. The circuit design lends itself to IC fabrication by lowering current requirements.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a voltage-controlled attenuator which is made low in noise and distortion. 
     A conventional voltage-controlled attenuator is arranged as shown in FIG. 1. 
     More specifically, an input voltage E IN  is converted into a current by a differential amplifier having transistors Q 1  and Q 2 , and the current is used to drive the common emitters of a pair of differential amplifiers which include transistors Q 3  and Q 4 , and Q 5  and Q 6 , respectively. The base biases of the transistors Q 3  through Q 6  forming the pair of differential amplifiers are controlled by a voltage Vc, so that the collector current of one of the transistors Q 3  and Q 4  is increased as the collector current of the other is decreased, and so that the collector current of one of the transistors Q 5  and Q 6  is increased while that of the other is decreased. 
     That is, the currents in the load resistors R 1  and R 2  are controlled, and the output voltages are also controlled by the bias voltage. 
     Let us consider the operation when the transistors Q 4  and Q 5  are conductive (on), i.e. where the attenuation factor of the attenuator is 0 dB. The distortion and noise characteristics are determined by the transistors Q 1  and Q 2 , the load resistors R 1  and R 2  are emitter resistors R 3  and R 4  only. That is, the distortion characteristic is determined by the steady voltage V R  applied to the emitter resistors R 3  and R 4  and the ratio of the V BE  characteristics KT/q of the transistors Q 1  and Q 2 . The noise characteristic is governed by the square by the square average value of the terminal noises Vn=√4KTBR of the load resistors R 1  and R 2  and the emitter resistors R 3  and R 4 . 
     Accordingly, a level which meets both distortion and noise characteristics cannot be obtained without increasing the current value of the constant current source I. However, increasing the current of the constant current source I is disadvantageous in that the transistors must be increased in size. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, an object of this invention is to provide a voltage-controlled attenuator in which the above-described difficulty is eliminated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of a conventional voltage-controlled attenuator; 
     FIG. 2 is a circuit diagram of a first example of a voltage-controlled attenuator according to the invention; and 
     FIG. 3 is a circuit diagram of a second example of the voltage-controlled attenuator according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2 is a circuit diagram illustrating a first embodiment of the invention. 
     In FIG. 2, a pair of differential amplifiers comprising transistors Q 3  through Q 6  are similar to those shown in FIG. 1, and are driven by a differential amplifier comprising transistors Q 1 , Q 2  and Q 7  through Q 10 . 
     That is, emitter followers comprising transistors Q 7  and Q 8  are connected to the input terminals of the transistors Q 1  and Q 2  forming the differential amplifier, respectively. The transistor Q 9  is connected so that the operating currents of the transistors Q 7  and Q 2  are coupled in a current mirror mode, and the transistor Q 10  is connected so that the operating currents of the transistors Q 1  and Q 8  are coupled in a current mirror mode. 
     On the other hand, the collector outputs of the transistors Q 4  and Q 5  are applied to an operational amplifier OP 1 , so that the difference between these collector outputs is provided at the output terminal OUT of the operational amplifier OP 1 . 
     In the voltage-controlled attenuator thus organized, the input voltage E in  can be represented by the following expression (1): 
     
         E.sub.in =-V.sub.BE7 -V.sub.BE1 +2(R.sub.E I.sub.e)+V.sub.BE2 +V.sub.BE8 (1) 
    
     where R E  =r 3  =r 4 , resistances of the emitter resistors R 3  and R 4  being r 3  and r 4 , respectively; and I e  =I e1  =I e2 , the emitter currents of the transistors Q 1  and Q 2  being I e1  and I e2 , respectively. 
     When only the AC variation components are taken into consideration, then expression (1) can be rewritten as follows: 
     
         ei=-ΔV.sub.BE7 -ΔV.sub.BE1 +2(R.sub.E ·i.sub.e)+ΔV.sub.BE2 +ΔV.sub.BE8     (2) 
    
     where ΔV BE7  =ΔV BE2  and ΔV BE8  =ΔV BE1 , because the operating currents of the transistors Q 2  and Q 7  are coupled in current mirror fashion through the transistor Q 9 , and the operating currents of the transistors Q 1  through Q 8  are coupled in current mirror fashion through the transistor Q 10 . 
     Therefore, expression (2) can be rewritten as follows: 
     
         E.sub.in =2R.sub.E ·i.sub.e 
    
     Thus, the V BE  characteristics of the transistors are cancelled out, and the collector currents of the differential amplifier comprising the transistors Q 1  and Q 2 , which is adapted to drive the pair of differential amplifiers having the transistors Q 3  through Q 6  are linearized. 
     Accordingly, even when the resistances of the emitter resistors R 3  and R 4  of the transistors Q 1  and Q 2  and those of the emitter resistors R 5  and R 6  of the transistors Q 9  and Q 10  are decreased and the current of the constant current source I is reduced, no distortion is produced, and the noise is low. 
     Thus, the pair of differential amplifiers comprising the transistors Q 3  through Q 6  are driven by the collector currents of the transistors Q 1  and Q 2 , which are made distortion-less. Therefore, voltages developed across the load resistors R 1  and R 2  suffer mainly from secondary distortions. 
     However, since the outputs of the pair of differential amplifiers are subjected to subtraction in the operational amplifier OP 1 , the secondary distortions are cancelled out; that is, no distortion is provided at the output of the operational amplifier OP 1 . Thus, the voltage-controlled attenuator is, as a whole, low in noise and distortion. 
     FIG. 3 is a circuit diagram illustrating another embodiment of the invention. 
     In this embodiment, a pair of differential amplifiers, which are made up of transistors Q 3  and Q 4  and transistors Q 5  and Q 6 , respectively, are driven by transistors Q 11  and Q 12 , respectively. A linearized difference amplifier comprising transistors Q 1  &#39;, Q 2  &#39;, Q 7  &#39;, Q 8  &#39;, Q 9  &#39; and Q 10  &#39; constructed similarly to that in the embodiment of FIG. 2, employs PNP transistors. The transistors Q 1  &#39;, Q 2  &#39;, Q 7  &#39;, Q 8 , Q 9  &#39; and Q 10  &#39; correspond to the transistors Q 1 , Q 2 , Q 7 , Q 8 , Q 9  and Q 10  in FIG. 2, respectively. The transistor Q 11  is driven by the output of the transistor Q 1  &#39;, and the transistor Q 12  is driven by the output of the transistor Q 2  &#39;. 
     In the embodiment of FIG. 3, the transistors Q 7  &#39; and Q 8  &#39; form emitter followers connected to the input terminals of the transistors Q 1  &#39; and Q 2  &#39;, respectively. The operating currents of the transistors Q 2  &#39; and Q 7  &#39; are coupled in the current mirror mode through the transistor Q 9  &#39;. Similarly, the operating currents of the transistors Q 1  &#39; and Q 8  &#39; are coupled in the current mirror mode through the transistor Q 10  &#39;. Transistors Q 11  and Q 12 , in combination with diodes D 1  and D 2 , form current mirror circuits, respectively. 
     The effect of the second embodiment of FIG. 3 is similar to that of the first embodiment of FIG. 2, and therefore a detailed description thereof will be omitted. 
     However, it should be noted that the power utilization rate is improved, because the transistors Q 1  &#39;, Q 2  &#39;, Q 7  &#39;, Q 8  &#39;, Q 9  &#39; and Q 10  &#39; forming the differential amplifier are PNP transistors. 
     As is apparent from the above description, the voltage-controlled attenuator of the invention has less distortion and noise, and can be operated with a small current. As the attenuator can be operated with a small current, it can easily be provided in the form of an integrated circuit. Furthermore, the voltage-controlled attenuator can be operated with a low voltage, because the voltage loss due to the emitter resistors is small.