Sample-and-hold circuit
BACKGROUND OF THE INVENTION 
     The present invention relates to a sample-and-hold circuit for sampling and holding the output signals of an A/D converter used in communications systems, audio systems, video systems and the like. 
     Such a sample-and-hold circuit is disclosed in Rudy J. Van de Plassche, et al. A Monolithic High-Speed Sample-and-Hold Amplifier for Digital Audio in IEEE Journal of Solid-State Circuits, Vol. SC-18, No. 6, December 1983, p. 717 (FIG. 1). FIG. 1 shows the structure of this circuit. In this sample-and-hold circuit, an input signal V IN  is supplied to the non-inverting input terminal (+) of operational amplifier 11 through resistor R1 and switch S1. The output of operational amplifier 11 is fed back to the inverting input terminal (-) of amplifier 11. It is also supplied to the inverting input terminal (-) of operational amplifier 12 through switch S3. The non-inverting input terminal (+) of operational amplifier 12 is grounded. The output of amplifier 12 is fed back to the inverting input terminal (-) of amplifier 12 through holding capacitor C H . It is also supplied to node N1 of resistor R1 and switch S2 through resistor R2. Switch S2 is connected between node N1 and the ground. Switches S1, S2 and S3 are selectively turned on and off, thereby controlling the operation of sampling and holding input signal V IN  and thus obtaining output signal V OUT  at the output terminal of operational amplifier 12. 
     In the sample-and-hold circuit (SHC) of FIG. 1, an imaginary short-circuit occurs when switches S1 and S3 are simultaneously turned on. Potentials V1, V2 and V3 of node N1, node N2 (i.e., the output terminal of amplifier 11) and node N3 (i.e., the non-inverting input terminal of amplifier 12) thereby fall to the ground level. As a result, output signal V OUT  becomes -V IN . In other words, an inverse of input signal V IN  is obtained at the output terminal of operational amplifier 12. When switches S1 and S3 are turned off and switch S2 is turned on, holding capacitor C H  maintains the output of operational amplifier 12 at a level attained in the previous sampling cycle. Since switch S2 is on, potential V1 of node N1 is reliably held at the ground level. 
     It will now be explained how sample-and-hold circuit SHC shown in FIG. 1 functions to sample and hold the output of D/A converter 13 when circuit SHC is coupled to D/A converter 13 as is illustrated in FIG. 2. 
     When switch S4 connected between D/A converter 13 and sample-and-hold circuit SHC is turned on, thus coupling the output terminal of D/A converter 13 to the input terminal of sample-and-hold circuit SHC, input signal V IN  of circuit SHC has the following value: 
     
         V.sub.IN =VDAC·R/(Z+R) 
    
     where Z is the output impedance of D/A converter 13, V DAC  is the output which D/A converter 13 produces when the output terminal of the converter is opened by turing off switch S4, and R is the input resistance of sample-and-hold circuit SHC. 
     Obviously, the higher the impedance (Z), the more the input signal (V IN ) attenuates. The condition of Z&lt;&lt;R is therefore required. To satisfy this requirement, output impedance Z of D/A converter 13 must be lowered, or input resistance R of sample-and-hold circuit SHC must be raised. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a sample-and-hold circuit which can reliably sample and hold the output of a D/A converter even if the D/A converter has a high output impedance. 
     To achieve this object, a sample-and-hold circuit according to the present invention comprises a first switched capacitor supplied with an input signal, an impedance converter for receiving the output of the first switched capacitor, an amplifier for receiving the output of the impedance converter through a switch, a capacitor coupled between the input and output terminals of the amplifier, and a second switched capacitor coupled between the output terminal of the amplifier and a node between the first switched capacitor and the impedance converter. 
     The input resistance of the sample-and-hold circuit of the invention can be equivalently regarded as being infinitely high since the input signal is supplied to the first switched capacitor. Therefore, the circuit can reliably sample and hold the output of an D/A converter without attenuating it, however high the output impedance of the D/A converter is. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of a conventional sample-and-hold circuit; 
     FIG. 2 shows the conventional electrical connection between a D/A converter and a sample-and-hold circuit; 
     FIG. 3 is a block diagram of the sample-and-hold circuit according to a first embodiment of this invention; 
     FIG. 4 shows the equivalent input impedance of the sample-and-hold circuit shown in FIG. 3; 
     FIG. 5 is a circuit diagram showing the sample-and-hold circuit of FIG. 3 in detail; 
     FIGS. 6 and 7 are timing charts explaining the operation of the circuit shown in FIG. 3; 
     FIGS. 8 and 9 are circuit diagrams showing the main sections of two other sample-and-hold circuits according to the present invention; 
     FIG. 10 shows an impedance converter which can be used in place of the impedance converter employed in the sample-and-hold circuit shown in FIG. 5; and 
     FIG. 11 shows another impedance converter which can be used in place of the impedance converter used in the sample-and-hold circuits shown in FIGS. 8 and 9. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3 is a block diagram showing the sample-and-hold hold circuit according to an embodiment of the present invention. This circuit has switched capacitor 15 for receiving the output signal of a D/A converter. The output of switched capacitor 15 is supplied to the input terminal of an amplifier 17 through impedance converter 16 and switch S5. The output of amplifier 17 is fed back to the input terminal of amplifier 17 through holding capacitor C H , and is also supplied to the input terminal of impedance converter 16 through switched capacitor 18. 
     The voltage gain (V OUT  /V IN ) of this sample-and-hold circuit (FIG. 3) is determined by the two switched capacitors 15 and 18. The sample-and-hold circuit is set to the sample mode when switch S5 is closed, and amplifier 17 generates output signal V OUT  which is exactly an inverse to input signal V IN  as long as the voltage gain is -1. On the other hand, the sample-and-hold circuit set to the hold mode when switch S5 is opened, and holding capacitor C H  maintains the level of output signal V OUT , which has been obtained in the preceding sampling cycle. 
     As mentioned above, in the sample-and-hold circuit of FIG. 1, it is necessary to raise the input resistance (R). In contrast, the input resistance of the sample-and-hold circuit of FIG. 3 is indefinitely high since the input impedance is basically equal to capacitance C IN  as is illustrated in FIG. 4. Therefore, it suffices to take only this input capacitance (C IN ) into consideration. Hence, the sample-and-hold circuit of FIG. 3 can prevent the attenuation of an input signal, without increasing the pattern area, even if the input signal is the output of a high output-impedance D/A converter such as a D/A converter of charge-redistribution type. The sample-and-hold circuit can, therefore, be applied to various uses. 
     FIG. 5 is a circuit diagram showing the circuit of FIG. 3 in greater detail. In this figure, the same numerals and symbols are used, designating the same elements as shown in FIG. 3. Input signal V IN  is supplied to the non-inverting input terminal (+) of operational amplifier (impedance converter) 16 through switch S11, capacitor C1 and switch S12. Switch S21 is coupled between the ground and the node between switch S11 and capacitor C1. Switch S22 is coupled between the ground and the node between switch S12 and capacitor C1. The output terminal of operational amplifier 16 is connected to the inverting input terminal (-) thereof, and is coupled to switch S5 which in turn is connected to the inverting input terminal (-) of operational amplifier 17. The non-inverting input terminal (+) of amplifier 17 is grounded. The output terminal of amplifier 17 is connected to the inverting input terminal (-) through capacitor C H . The output terminal of amplifier 17 is also connected to switch S14 which in turn is connected to one end of capacitor C2. The other end of capacitor C2 is coupled to switch S13 which in turn is connected to the non-inverting input terminal (+) of operational amplifier 16. Switch S24 is connected between the ground and the node between switch S14 and capacitor C2. Switch S23 is coupled between the ground and the node between switch S13 and capacitor C2. 
     In the circuit of FIG. 5, switches S11, S12, S21 and S22 and capacitor C1 form first switched capacitor 15, and switches S13, S14, S23 and S24 and capacitor C2 constitute second switched capacitor 18. Switches S11-S14, switches S21-S24 and switch S5 are controlled in response to clock signals φ1-φ3 shown in FIG. 6, which are generated by control signal generator CSG. More precisely, switches S11-S14 are controlled by clock signal φ1, switches S21-S24 are controlled by clock signal φ2, and switch S5 is controlled by clock signal φ3. Each switch is closed when the clock signal supplied to it is at &#34;1&#34; level. 
     As shown in the timing chart of FIG. 6, only clock signal φ2 is at &#34;1&#34; level during period T1 which ends at time t1. Therefore, switches S21-S24 are kept on, and switches S11-S14 and switch S5 are kept off, during period T1. Hence, capacitors C1 and C2 are discharged during this period T1. When clock signal φ1 rises to &#34;1&#34; level at time t2 after clock signal φ2 has fallen to &#34;0&#34; level, switches S11-S14 are turned on. At time t2, switches S21-S14 and switch S5 are kept off. When clock signal φ3 rises to &#34;1&#34; level at time t3 under this condition, switch S5 is turned on, thereby setting the sample-and-hold circuit into an imaginary short-circuited condition. In consequence, nodes N1, N2 and N3 are set at the ground potential. Electrical charge Q (=C1·V IN ) is accumulated in capacitor C1. The same charge is accumulated also in capacitor C2. Therefore: 
     
         Q=C1·V.sub.IN =C2·(-V.sub.OUT) 
    
     Hence, voltage gain G is given: 
     
         G=V.sub.OUT /V.sub.IN =-C1/C2 
    
     If C1=C2, voltage gain G will be -1. 
     During period T2 when clock signal φ3 remains at &#34;1&#34; level, input signal V IN  is sampled, and amplifier 17 generates output signal V OUT  corresponding to input signal V IN . 
     When clock signal φ3 falls to &#34;0&#34; level at time t4, switch S5 is turned off. Output signal V OUT  generated during period T2, i.e., the sampling cycle, is thereafter held by capacitor C H  until clock signal φ3 rises to &#34;1&#34; level again. 
     FIG. 7 shows the timing relationship of input signal V IN , clock signal φ3, and output signal V OUT  in the sample-and-hold circuit of FIG. 5. As this figure clearly shows, amplifier 17 produces an output signal at negative high level when input signal V IN  is at positive high level, and clock signal φ3 is simultaneously at &#34;1&#34; level. Output signal V OUT  is held at the same level until clock signal φ3 rises to &#34;1&#34; level again. When input signal V IN  falls or rises in the next sampling cycle, the output signal generated by amplifier 17 proportionately falls or rises. In the sample-and-hold circuit, the input signal is sampled while clock signal φ3 is at &#34;1&#34; level, and the voltage stored in capacitor C H  while clock signal φ3 is at &#34;1&#34; level is held until clock signal φ3 rises to &#34;1&#34; level again. 
     FIG. 8 shows another embodiment of the invention. The sample-and-hold circuit shown in this figure is similar to the circuit of FIG. 5 except that switches S21 and S23 are grounded at one end, and switches S22 and S24 are coupled at one end to a source of voltage VR. In this regard, it should be noted that switches S21-S24 are grounded at one end in the circuit of FIG. 5. In FIG. 8, the same numerals and symbols are used to designate the same elements as shown in FIG. 5. In the sample-and-hold circuit of FIG. 8, capacitors C1 and C2 are charged to voltage VR when switches S22 and S24 are on, and a bias of VR is added to output signal V OUT . 
     FIG. 9 illustrates still another embodiment of this invention. The sample-and-hold circuit shown in this figure is similar to the circuit of FIG. 5 except that switched capacitor 15A, which functions as a negative resistor, is used in place of switched capacitor 15 (FIG. 5). To be specific, switches S11 and S21 take each other&#39;s position. Hence, when clock signal φ2 is at &#34;1&#34; level, switches S21 and S22 are on, thereby connecting capacitor C1 between the input terminal (V IN ) and the ground, and thus charging capacitor C1. When clock signal φ1 is at &#34;1&#34; level (and clock signal φ2 is at &#34;0&#34; level), switches S11 and S12 are on, thereby supplying the electrical charge from capacitor C1 to node N1. In this manner, switched capacitor 15A operates as a negative resistor. As a result, output signal V OUT  is inphase with input signal V IN . 
     The present invention is not limited to the embodiments described above. For example, either the source follower shown in FIG. 10 or the source follower shown in FIG. 11 can be used as an impedance converter in place of the voltage follower 16 shown in FIG. 5. The source follower of FIG. 10 comprises N-channel MOS transistors Q1 and Q2 connected in series between power supply V DD  and the ground. Output signal V1 of switched capacitor 15 is supplied to the gate of MOS transistor Q1, thereby making this MOS transistor conductive or non-conductive. Predetermined bias VB is applied to the gate of MOS transistor Q2. Output voltage V2 is applied from the node between MOS transistors Q1 and Q2 to amplifier 17 through switch S5. On the other hand, the source follower of FIG. 11 comprises P-channel MOS transistors Q3 and Q4 coupled in series between power source V DD  and the ground. Predetermined bias VB is applied to the gate of MOS transistor Q3. Output voltage V1 of switched capacitor 15 is applied to the gate of MOS transistor Q4, thereby rendering MOS transistor Q4 conductive or non-conductive. Output voltage V2 is applied from the node between MOS transistors Q3 and Q4 to amplifier 17 through switch S5.