Sample hold circuit and semiconductor device having the same

Abstract: To form a high-speed, high-precision sample hold circuit with the minimum number of elements and low current consumption, there is provided a sample hold circuit including an operational amplifier including differential input stage in which sources or emitters are commonly connected, a cascode current mirror circuit for receiving a differential output from the different input stage, and a push-pull output stage having a diamond circuit connected to the cascode current mirror circuit, wherein a hold capacitor is connected to the output of the operational amplifier, and the push-pull output stage is switched between a buffer operation mode and a high-impedance output operation mode in accordance with a logic signal.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a sample hold circuit formed in a semiconductor integrated circuit in a semiconductor device and a semiconductor device having the same. Related Background Art A conventional sample hold circuit is designed to hold an input level at every sample timing and used to measure the instantaneous value of a signal which changes with time. For example, this circuit is used on the input stage of an A/D converter to temporarily hold an analog value. That is, the circuit is used on the input stage of the A/D converter when the level of the analog value is to be converted into a digital value. This sample hold circuit includes a high-speed non-feedback type circuit (poor precision) like the one shown in Fig. 1, a low-speed, high-precision feedback type circuit like the one shown in Fig. 2, an integrating type circuit like the one shown in Fig. 3, which has performance intermediate between the above circuits, and the like. As shown in Fig. 1, the non-feedback type is the most basic sample hold circuit scheme, and constituted by an input buffer circuit 11, a switch circuit 12, a hold capacitor C H , and a high-input-impedance output buffer 13. An analog signal is input to an input terminal 10. The switch circuit 12 is turned on/off in accordance with a sampling timing. The output voltage of the input buffer circuit 11 which is set when the switch circuit 12 is on in the sample mode is charged into the hold capacitor C H in an acquisition time. When the switch circuit 12 is turned off, the hold mode is set, and a voltage corresponding to the charged voltage is output from the output buffer 13. For example, this output voltage is converted into a digital value before the arrival of a timing signal next to the sampling timing. Alternatively, the output voltage is shaped into a staircase waveform to be sampled. In this case, as the switch circuit 12, a switch circuit 19 using a diode bridge 18 like the one shown in Fig. 4 is generally used. Referring to Fig. 4, constant current sources I4 and I5 are repeatedly turned on/off under the control of a switch control signal 17, and an analog signal output from the input terminal 10 when the diodes of the diode bridge 18 are turned on is charged into a hold capacitor C H . When the diodes of the diode bridge 18 are not turned on, the charged voltage of the hold capacitor C H is held. The switch circuit 19 has a high speed because the switching speed of the diode bridge 18 as a switching element is high as compared with other elements such as a binary transistor, an FET, and a MOSFET. This is because, the diode bridge 18 has a small capacitance component. As shown in Fig. 5, the output buffer 13 is generally constituted by FET source followers M5 and M6 and emitter followers Q12 and Q13. Referring to Fig. 5, a hold capacitor C H for holding an analog value on the previous stage is connected to an input terminal 20, and the charged voltage of the hold capacitor C H is applied to the gates of the MOS transistors M5 and M6. The source outputs of the MOS transistors M5 and M6 of an FET source follower circuit arrangement are respectively connected to constant current sources I6 and I7 and are input to the bases of the transistors Q12 and Q13 of a push-pull emitter follower arrangement to be output to an output terminal 14 through the emitters of the transistors Q12 and Q13. In this case, since a voltage is input to the voltage-dependent MOSFET, a high-speed operation is performed. With the output arrangement using the emitter followers Q12 and Q13, high-speed output response can be realized in accordance with the input voltage. The arrangement of the non-feedback type shown in Fig. 1, however, provides no compensation for variations in power supply voltage, changes in temperature, and the like, and hence a high precision is difficult to attain. To satisfy both the demands for higher speed and precision, a high-speed operational amplifier with an FET input is used as an output buffer 21, and a switch circuit 19 is bootstrapped from the output of the amplifier to increase the precision. More specifically, the output of the output buffer 21 is fed back to the switch circuit 19. When the output increases, the levels of current sources I4 and I5 are decreased to decrease the charged voltage of a capacitor C H , thereby increasing the ON resistance of a switch diode 18. In this manner, a negative feedback operation from the output buffer 21 is performed to obtain an accurate charged voltage. With such a circuit arrangement, however, the current consumption may increase because of increases in circuit size and the levels of the constant current sources I4 and I5. According to the feedback type shown in Fig. 2, since a voltage is not fed back to the switch circuit but is fed back to the overall circuit up to the input terminal of the input buffer 15 including the switch unit 12, a high precision can be basically obtained, but a delay is caused by a feedback operation. That is, a high-speed circuit is difficult to realize. Although a circuit of this type is often used as a standard monolithic sample hold circuit arrangement for audio signal processing, this circuit is not suited for a video signal processing circuit arrangement and the like requiring high-speed processing. According to the integrating type in Fig. 3, which exhibits performance intermediate between the above two types, a hold capacitor C H is connected between the inverting input terminal and output terminal of an operational amplifier 16, and a switch 12 is arranged between an input resistor R IN and a feedback resistor R F . Since the switch unit is in the negative feedback system, a high precision can be obtained. It is, however, difficult to realize high-speed signal processing because a band is limited by the time constants of the input resistor R IN and the hold capacitor C H . With the above circuit arrangements, it is difficult to satisfy both the demands for higher speed and precision. To satisfy the demands, for example, the circuit size and the current consumption must be increased. U.S. Patent No. 5378938 describes a sample-and-hold circuit in which switching transistors are added to a push-pull transconductance input stage, the output of the push-pull stage being connected to current mirror circuits. U.S. Patent No.5130572 describes a track-and-hold circuit having a differential input stage driving a current mirror circuit and provided with an output buffer stage. SUMMARY OF INVENTION The present invention has been made in consideration of the above points, and has as its object to provide a high-speed, high-precision sample hold circuit with the minimum number of elements and low power consumption, and a low-cost monolithic type sample hold circuit with a small number of manufacturing steps. The present invention as defined by claim 1, provides a sample hold circuit comprising an operational amplifier including a differential input stage in which main electrodes are commonly connected, a cascode current mirror circuit for receiving a differential output from said differential input stage, and a push-pull output stage having a diamond circuit connected to said cascode current mirror circuit, wherein a hold capacitor is connected to an output of said operational amplifier, and said sample hold circuit further comprises switching means for switching said push-pull output stage between a buffer operation mode and a high-impedance output operation mode in accordance with a logic signal, and wherein either an output from said operational amplifier or an output from said cascode current mirror circuit is negatively fed back to one input terminal of said differential input stage. According to the above arrangement, the push-pull output stage itself also serves as the switching unit of the sample hold circuit so that the sample mode is set when the output transistor is in the operative state, and the hold mode is set when the output transistor is in the cutoff state. That is, the operational amplifier serves as a sample hold circuit. For this reason, the overall circuit is included in the feedback system, and hence a high precision can be attained. In addition, when the push-pull output transistor is set in the operative state by a logic signal, the operational amplifier is set in the buffer operation mode, and the operation speed is maximized. When a shift from the cutoff state to the operative state occurs, necessary charges are supplied to the base capacitance of the output transistor to form the feedback system again. As a result, the output follows the input owing to the slewing rate of the operational amplifier, and the sample mode is set. With a circuit arrangement having a high slewing rate, therefore, the operation speed can be increased. To further increase the operation speed, feedback may be performed from the input terminal of the push-pull output state to the inverting input terminal of the operational amplifier. With this arrangement, a circuit designed to always perform a buffer operation and a push-pull output stage serving as both an output buffer and a switch are formed. That is, an operation is performed while a high slewing rate is maintained, and hence a higher operation speed can be attained. In addition, the output transistors of the push-pull output stage preferably have the same input capacitance. As the operation speed increases, clock feedthrough with respect to an output passing through the parasitic capacitance of each output transistor becomes noticeable owing to a logic signal for controlling the switch unit, and the performance of the sample hold circuit deteriorates in terms of the hold steps. However, since logic signals for controlling the transistors of the push-pull output stage as the switch unit have opposite phases, the clock feedthrough in the output can be canceled out by equalizing the input parasitic capacitances of the output transistors. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1 to 3 and Fig. 6 are circuit diagrams for respectively explaining sample hold circuits; Fig. 4 is a circuit diagram for explaining a switch circuit which can be applied to a sample hold circuit; Fig. 5 is a circuit diagram for explaining an output buffer circuit which can be applied to a sample hold circuit; Figs. 7 and 9 are circuit diagrams for respectively explaining sample hold circuits according to preferred embodiments of the present invention; and Figs. 8A, 8B, 8C, 8D and 8E are timing charts for explaining an operation of the circuit in Fig. 7 or 9. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the accompanying drawings. (First Embodiment) Fig. 7 is a circuit diagram for explaining a sample hold circuit according to an embodiment of the present invention. Referring to Fig. 7, this circuit includes a power supply line 1 for supply power, a GND line 2 set at a ground potential, a signal input terminal 3, an output terminal 4 from which a sampled/held result is output, a logic signal input terminal 5 to which a sampling logic signal for a sampling timing is input, an inverter 6 for inverting and outputting an input signal, a differential input stage 7 of this sample hold circuit in which main electrodes such as source and emitter electrodes are commonly connected, a bias circuit 81 for a cascode current mirror circuit, a cascode circuit 82 of a cascode current mirror circuit arrangement with an emitter input for receiving a differential output from the differential input stage 7, a diamond push-pull output stage/switch unit 9 constituted by a push-pull output stage using a diamond circuit, and a constant current source circuit 10. The sample hold circuit also includes constant current sources I1 to I4 for supplying constant currents, PNP transistors Q1, Q2, Q7, Q9, and Q11, NPN transistors Q3, Q4, Q5, Q6, Q8. and Q10, NMOS transistors M1, M2, and M4, a PMOS transistor M3, resistors R1, R2, and R3, a hold capacitor C H , and parasitic base capacitances C P1 and C P2 of the transistors Q10 and Q11. The operation of this circuit will be described in detail next with reference to Figs. 8A to 8E. An analog input signal is input to the input terminal 3. Assume that a logic signal (Fig. 8A) input to the logic signal input terminal 5 is at Hi level, i.e., the switch circuit 12 in Fig. 1 is in the OFF state, and this state will be referred to as a hold state. At this time, the PMOS transistor M3 and the NMOS transistor M4 are in the ON state by an inverted signal obtained by inverting the logic signal in the inverter 6, and the output stage of the diamond push-pull output stage/switch unit 9 is in the inoperative state. In addition, the bases of the transistors Q10 and Q11 are respectively set at the potentials of the GND line 2 and the power supply line 1, i.e., in a cutoff region. For this reason, the output of the output terminal 4 is set in a high-impedance state, and set at a given potential V CH owing to the charges stored in the hold capacitor C H . In addition, at this time, no feedback system is formed by the sample hold circuit constituting an operational amplifier, and hence the differential input stage 7 and the cascode circuit 82, which have an output at a point A in Fig. 7, serve as a comparator. Therefore, if V CH > V in then, the A point potential as the output of the comparator constituted by the differential input stage 7 and the cascode current mirror unit 8 is at Lo level (Fig. 8C). Consider a case in which the logic signal at the input terminal 5 changes from Hi level to Lo level. At this time, the PMOS transistor M3 and the NMOS transistor M4 change from the conductive (ON) state to the nonconductive (OFF) state, and the B and C point potentials are changed by current sources I3 and I4 from the A point potential to a potential set by the push-pull transistors Q8 and Q9 by a level shift operation (Fig. 8D). When thePMOS transistor M3 and the NMOS transistor M4 are turned off, the output stage/switch unit 9 is set in the operative state. As a result, the output impedance decreases, and a negative feedback system is formed again. The sample hold circuit in Fig. 7 then operates as an operational amplifier. The output of the output terminal 4 is therefore changed by the slewing rate of the operational amplifier which is the sample hold circuit such that the negative input of the differential input stage 7 is set to be equal to the potential of the signal input terminal 3 (Fig. 8E). When the PMOS transistor M3 and the NMOS transistor M4 are turned off, the B and C point potentials change in the form of pulses owing to the parasitic base capacitances C P1 and C P2 between the base/emitter paths of the transistors Q10 and Q11 (Fig. 8D). For this reason, waveforms formed by differentiating pulses like those indicated by the dotted lines on the output waveform in Fig. 8E appear in the output from the output terminal 4. By equalizing the parasitic capacitance values of the transistors Q10 and Q11, the differential waveforms are canceled out. As a result, the output waveform becomes identical to the input analog signal in Fig. 8B, and no impulse removal period is required in a switching operation. Consequently, the acquisition time can be shortened, and high-speed sampling can be performed. The operation of each unit will be described next in association with a transition from the sample mode to the hold mode. In this case, the PMOS transistor M3 and the NMOS transistor M4 change from the OFF state to the ON state. A point B is set at the GND potential by the NMOS transistor M4, and a point C is set at the power supply potential by the PMOS transistor M3. Since the potentials abruptly change as shown in 8D, differential waveforms appear in the output from the output terminal 4 as in the case of the transition from the hold mode to the sample mode. These differential waveforms are, however, canceled out (Fig. 8E). In addition, since the transistors M3 and M4 are respectively PMOS and NMOS transistors, when the gate potentials change from the points C and B by threshold voltages, these transistors are turned on. As a result, the output terminal 4 is set at a high impedance, and no negative feedback system is formed. The hold capacitor C H is connected to the output terminal 4 to trace the same potential as the input analog voltage in a sample hold operation with a low-impedance output. In addition, the logic signal changes to Hi level (Fig. 8A), and the input analog voltage at the moment of this switching change can be maintained. As a result, a short aperture and a small number of hold steps can be realized. In this case, the cascode current mirror circuit 82 is used for the following reason. Outputs from the differential stages M1 and M2 are current outputs, and input voltages are converted into current signals by the MOS transistors M1 and M2. Thereafter, the signals are transmitted to the mirror circuit of the transistors Q1 to Q6 with a current. The current is converted into a voltage at the point A. Since conversion from a current to a voltage which causes phase rotation is performed only once, phase compensation is not basically required. For this reason, a broadband arrangement can be realized. In this state, however, since the load driving ability is low, a diamond circuit serving as a buffer is arranged after the above arrangement to increase the driving ability. In addition, since the diamond circuits use complementary output transistors, a high-impedance state can be easily obtained by simultaneously controlling the bases of the transistors. Furthermore, since the operation of this circuit is an emitter follower operation or a cutoff operation, it can operate at the highest speed as a transistor. (Second Embodiment) Fig. 9 shows the second embodiment of the present invention, in which negative feedback is performed from the input of an operational amplifier output push-pull stage 9, i.e., the output of a cascode current mirror constituting the second operational amplifier, to the inverting input terminal of an operational amplifier. In the arrangement shown in Fig. 7, negative feedback is performed from the output terminal 4 to the other input terminal of the differential input stage 7. In this embodiment, however, negative feedback is performed from a point A to the other input terminal of the differential input stage 7. In this case, since the switching unit 9 is outside the feedback circuit, an input signal is always input to the input terminal of the switching unit 9 after negative feedback processing by the differential input stage 7 and a second cascode circuit 8, although the second embodiment is inferior in precision to the first embodiment. For this reason, the operation speed is controlled within the time during which each of transistors Q8 to Q11 of the output stage/switching unit 9 shifts from the cutoff state to the operative state, and the operation speed of the second embodiment can be basically set to be higher than that of the first embodiment. The second embodiment is the same as the first embodiment in terms of hold steps. The waveforms at the respective portions described with reference to Figs. 8A to 8E are the same as in this embodiment. In the above embodiments, a hold potential corresponding to an input analog signal is held in a hold capacitor C H . However, a buffer circuit having a high-impedance input stage may be arranged on the output stage of this capacitor to reduce a change in output voltage after it is held, i.e., a hold loop. This hold potential is then quantized into a digital value, thus realizing a high-precision, high-speed A/D conversion circuit. In each embodiment described above, a hybrid semiconductor device including bipolar transistors and MOS transistors together is described. However, a device may be constituted by only MOS transistors in the analog operation region to attain a reduction in cost. The above circuits can be variously modified within the spirit and scope of the present invention. In addition, each circuit described above can be formed in a semiconductor device by a general technique. As has been described above, according to the present invention, by using a switching unit constituting a sample hold circuit as the push-pull output stage of an operational amplifier, a high-speed, high-precision sample hold circuit can be formed with the minimum number of elements.

Claims: A sample hold circuit comprising an operational amplifier including: a differential input stage (7) in which main electrodes are commonly connected; a cascode current mirror circuit (82) for receiving a differential output from said differential input stage; and a push-pull output stage (9) having a diamond circuit connected to said cascode current mirror circuit, said diamond circuit comprising an input terminal connected to the base of a first NPN transistor (Q8) and to the base of a first PNP transistor (Q9), the emitter of said first NPN transistor being connected to a first current source (I3) and to the base of a second PNP transistor (Q11), the emitter of said first PNP transistor being connected to a second current source (I4) and to the base of a second NPN transistor (Q10), and the emitters of said second PNP transistor and said second NPN transistor being commonly connected to an output terminal (4), wherein: a hold capacitor (CH) is connected to an output of said operational amplifier; said sample hold circuit further comprises switching means (6,M3,M4) for switching said push-pull output stage between a buffer operation mode and a high-impedance output operation mode in accordance with a logic signal; and either an output from said operational amplifier or an output from said cascode current mirror circuit is negatively fed back to one input terminal of said differential input stage. A circuit according to claim 1, wherein said main electrodes are source electrodes. A circuit according to claim 1, wherein said hold capacitor (CH) is connected to said push-pull output stage (9). A circuit according to claim 1, wherein NPN and PNP transistors (Q10, Q11) constituting said push-pull output stage have the same input capacitance. A circuit according to claim 1, wherein an output from said cascode current mirror circuit (82) constituting a second stage of said operational amplifier is input to an inverting input terminal of said differential input stage. A semiconductor device comprising a sample hold circuit according to any preceding claim.

Inventors: NAKAMURA HIROYUKI, NAKAMURA, HIROYUKI, NAKUMURA, HIROYUKI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1997
Main classification: G11C27
IPC codes: G11C27, H03M1, G01R19