Amplifier having complete isolation of power sources
BACKGROUND OF THE INVENTION 
     The present invention relates to an amplifier operating from a plurality of power source systems in which there is no current flow between the power source systems. 
     An example of an amplifier operating from a plurality of power sources is shown in FIG. 1. In FIG. 1, reference symbols +V 1  and -V 1  designate voltage amplifying stage power sources, and +V 2  and -V 2  power amplifying stage power sources. A bias resistor R 1  is connected between input terminals 1 and 1&#39;. The terminal 1 is connected to the gate of an FET Q 1 . The FET Q 1  and an FET Q 2  form a differential amplifier circuit. The sources of the FETs are connected together and are connected to a constant current source I 1 . The drain of the FET Q 1  is connected to one terminal of a load resistor R 2  and to the base of a transistor Q 3 . The other terminal of the load resistor R 2  and the emitter of the transistor Q 3  are connected to the power source +V 1 . The collector of the transistor Q 3  is connected to a constant current source I 2 , and also to the base of a transistor Q 4 . The transistor Q 4  and a transistor Q 5  are Darlington connected. The emitters of the transistors Q 4  and Q 5  are connected to constant current sources I 3  and I 4 , respectively, and the collectors thereof are connected to the power source +V 2 . The emitter of the transistor Q 5  is connected through a resistor R 3  to the gate of the FET Q 2 . A resistor R 4  is connected between the gate of the FET Q 2  and the terminal 1&#39;. The connecting point of the power sources +V 1  and -V 1  is connected to the input terminal 1&#39; and is grounded at a grounding point G 1 . The emitter of the transistor Q 5  is connected to a terminal 2 which is connected to one terminal of a load 3, the other terminal of which is connected to a terminal 2&#39;. The terminal 2&#39; is connected to the connecting point of the power sources +V 2  and -V 2  and is grounded at a grounding point G 2 . The grounding points G 1  and G 2  are connected together through a ground bus or chassis. Further in FIG. 1, reference numeral 4 designates a voltage amplifying stage, and 5 a power amplifying stage. 
     The operation of the amplifier thus arranged will be described. 
     The circuit is fundamentally a negative feedback amplifier. If the loop gain is sufficiently high, then the closed loop voltage gain can be represented substantially by (R 3  +R 4 )/R 4 . 
     Currents flow in the circuit as follows: The base current (signal current) of the transistor Q 4  flows from the power source +V 1  through the transistor Q 3 , the base and emitter of the transistor Q 4  and the base and emitter of the transistor Q 5  to the load 3, and it returns from the load 3 through the terminal 2&#39;, the connecting point of the power source +V 2  and -V 2  and the grounding points G 2  and G 1  to the connecting point of the power sources +V 1  and -V 1 . Similarly, almost all the current flowing in the feedback resistors R 3  and R 4  flows as indicated by a route i 2  in FIG. 1: the power source +V 2  --the transistor Q 5  --the resistor R 3  --the resistor R 4  --the grounding point G 1  --the grounding point G 2 . 
     In the conventional amplifier constructed as described above, if there is some nonzero impedance between the grounding points G 1  and G 2 , a potential difference is produced between the grounding points which adversely affects amplification of the signal. In the case of the negative feedback amplifier, the magnitude of the current i 1  is set in such a manner as to correct the nonlinearity of the transistors Q 4  and Q 5 , and therefore, the current i 1  itself includes considerable nonlinear components. Accordingly, the potential difference between the grounding points G 1  and G 2  causes a serious problem. Especially in an audio power amplifier, the input voltage is often very low, and accordingly even a small potential difference between the grounding points G 1  and G 2  adversely affects the performance and harms the tone quality. 
     Instead of the transistor Q 4 , an FET Q 4  may be employed as shown in FIG. 2. In this case, usually cascade connection is employed by using a transistor Q 6 . However, the circuit thus modified suffers from the same difficulties as the circuit in FIG. 1 because little gate current flows in the FET Q 4  but the same current i 1  flows to the base of the transistor Q 6 . 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to eliminate the above-described difficulties accompanying a conventional amplifier. 
     In accordance with the above and other objects, the invention provides an amplifier in which an element such as an FET is provided for preventing the flow of current to the following stage. The FET is arranged between the current loops of the power source system so that the current loops are isolated from each other, whereby the effect of the nonzero impedance of the ground bus or chassis is eliminated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram showing an example of a conventional amplifier; 
     FIG. 2 is a circuit diagram showing a modification of a part of the amplifier of FIG. 1; 
     FIG. 3 is a block diagram showing a first preferred embodiment of the invention; 
     FIG. 4 is a circuit diagram showing a more specific example of an amplifier of the invention; and 
     FIGS. 5 through 8 are circuit diagrams, partly as block diagrams, showing other embodiments of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will be described with reference to preferred embodiments shown in the drawings. 
     FIG. 3 is a block diagram showing a first preferred embodiment of the invention. A signal applied across input terminals 1 and 1&#39; is supplied to a voltage amplifying stage 4, the output of which is applied to a power amplifying stage 5 having an FET input buffer 6 on its input side. The output of the power amplifying stage 5 is applied to output terminals 2 and 2&#39;, and is subjected to voltage division by resistors R 3  and R 4 . The divided voltage is applied to an FET input buffer 7, the output of which is applied to the inverting input terminal of the voltage amplifying stage 4. The &#34;cold&#34; input terminal 1&#39; is grounded at a grounding point G 1 . The connecting point of the power sources +V 1  and -V 1  provided for the voltage amplifying stage 4 and the FET input buffer 7 is also grounded at the grounding point G 1 . The connecting point of the power sources +V 2  and -V 2 , one terminal of the resistor R 4 , and the &#34;cold&#34; output terminal 2&#39; are grounded at a grounding point G 2 . A bias resistor R 1  is connected between the input terminals 1 and 1&#39;. 
     The operation of the amplifier thus arranged will be described. The signal current i 1  flowing between the voltage amplifying stage 4 and the power amplifying stage 5 is substantially zero because of the presence of the FET input buffer 6. Furthermore, the current i 2  flowing from the output 2 to the FET input buffer 7 is also substantially zero. That is, the power sources +V 1  and -V 1  and the power sources +V 2  and -V 2  are completely isolated from each other, and no current flows between the grounding points G 1  and G 2 . Accordingly, even if there is a large impedance between the grounding points G 1  and G 2 , and even if that impedance is nonlinear, the grounding points G 1  and G 2  are maintained at the same potential, and the gain and linearity of the amplifier are not affected at all. 
     FIG. 4 shows a specific example of an amplifier obtained by applying the technical concept of the invention to the circuit in FIG. 1. The input circuit of the power amplifying stage 5 is formed by a buffer 6 with an FET Q 4 . A transistor Q 6  is cascade connected using a constant current source I 5  and a resistor R 5  to prevent the mirror effect due to the gate-drain capacitance of the FET Q 4 . In FIG. 4, an FET input buffer 7 is formed by FETs Q 7  and Q 8 , constant current sources I 6  and I 7 , and resistors R 6 , R 7 , R 8  and R 9 . With this arrangement, the currents i 1  and i 2  are substantially zero, and accordingly no current flows between the grounding terminals G 1  and G 2 . 
     In the circuit shown in FIG. 4, the output circuit of the power amplifying stage 5 is the emitter-follower circuit of the transistor Q 5 . However, it may be a push-pull circuit formed by transistors Q 5a  and Q 5b  as shown in FIG. 5. In FIG. 5, the outputs of transistors Q 3a  and Q 3b  are applied to a buffer 6 composed of FETs Q 4a  and Q 4b . As in the case of FIG. 4, transistors Q 6a  and Q 6b  are cascade connected to the FETs Q 4a  and Q 4b  via constant current sources I 5a  and I 5b  and resistors R 5a  and R 5b  to prevent the mirror effect. The outputs of the buffer 6 are supplied to the push-pull connected transistors Q 5a  and Q 5b . 
     In the above-described circuits, the voltage divided by the resistors R 3  and R 4  on the output side of the power amplifying stage is applied to the buffer 7. In addition to this, as shown in FIG. 6, a resistor R 3  &#39; and a resistor R 4  &#39; may be connected respectively between the output terminal of the FET input buffer 7 and the inverting input terminal of the voltage amplifying stage 4 and between the same inverting input terminal and the &#34;cold&#34; input terminal 1&#39; so that a division voltage provided by the resistors R 3  &#39; and R 4  &#39; is applied to the inverting input terminal of the voltage amplifying stage 4. 
     In the above-described embodiments of the invention, the buffers 6 and 7 are implemented with FETs in view of the fact that the gate current of an FET is substantially zero. The same effect can be obtained by, as shown in FIG. 7, optically coupling the voltage amplifying stage and the power amplifying stage using photocouplers Q 74a  and Q 74b  and optical cables. 
     In the above-described amplifiers, the circuit is divided into a voltage amplifying stage and a power amplifying stage. However, the invention is not limited thereto or thereby. That is, the objects of the invention can be achieved by the provision of an amplifier in which a current loop is formed for each of the power source systems so that the flows of current between the power source systems are made substantially zero. 
     Another example of an amplifier embodying the inventive concept is shown in FIG. 8. In this amplifier, an input signal is applied to a first differential amplifier composed of FETs Q 81  and Q 82  driven by power sources +V 1  and -V 1 . The outputs of the first differential amplifier are applied to a second differential amplifier composed of FETs Q 83  and Q 84  so that the outputs of the second differential amplifier drive push-pull connected transistors Q 85a  and Q 85b  are powered by power sources +V 2  and -V 2 . The outputs of the transistors Q 85a  and Q 85b  are applied to MOSFETs Q 4a  and Q 4b  to drive push-pull connected transistors Q 5a  and Q 5b . The MOSFETs Q 4a  and Q 4b  and the transistors Q 5a  and Q 5b  are operated by power sources +V 3  and -V 3 . 
     The output is subjected to voltage division by resistors R 3  and R 4 . The divided voltage is applied to a buffer 7 driven by the power sources +V 1  and -V 1 . The output of the buffer 7 is applied to the FET Q 82 . With the above-described arrangement, the currents I 11 , I 12  and I 2  flowing respectively between the initial amplifying stage 8 (the first differential amplifier FETs Q 81  and Q 82 ) and the voltage amplifying stage 4 (the second differential amplifier FETs Q 83  and Q 84  and the push-pull transistors Q 85a  and Q 85b ), between the voltage amplifying stage 4 and the power amplifying stage 5, and between the power amplifying stage 5 and the buffer 7 are made substantially zero. 
     As is apparent from the above description, according to the invention, isolated current loops are formed for each of the power source systems so that no current flows between the grounding points. Therefore, the amplifier provided by the invention is an ideal one which is not affected by the impedance of the ground bus or chassis.