Abstract:
A clock configuration for driving switched op-amp circuits operated in opposite phases is presented in which a common off-phase of variable length is inserted between the on-phases of the individual operational amplifiers. The length of the off-phase can be adapted to the transient response of the operational amplifiers used. The clock configuration according to the invention can be used for further reducing the power consumption of switched op-amp circuits.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to a circuit configuration in switched op-amp technology and to a method for clocking successive operational amplifier stages constructed in switched op-amp technology. 
     The switched op-amp technology has developed from the switched capacitor technology to be able to take into account the requirement for ever-lower supply voltages. The switched op-amp technology is used in the construction of filters and converters and is especially suitable for fields of application in which low power consumption is of importance. The field includes, for example, applications in the mobile radio field in which the load on the battery must be kept as low as possible. 
     Whereas the capacitors are switched on and off by clocked switches in the switched capacitor technology, the operational amplifiers are also switched on and off by a switching clock signal in the switched op-amp technology. Such a configuration results in considerable power saving. 
     In the prior art, two successive operational amplifier stages are operated with opposite clock pulses. A prior art non-overlapping two-phase clock is used for the clocking. Accordingly, the operational amplifiers of the successive stages are never active at the same time. However, the clock configuration is selected such that each operational amplifier is switched on for approximately 50%. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a variable clock configuration for switched op-amp circuits and a method for clocking successive operational amplifier stages constructed in switched op-amp technology, that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type and that achieves a further reduction in the power consumption of switched op-amp circuits. 
     With the foregoing and other objects in view, there is provided, in accordance with the invention, a circuit configuration in switched op-amp technology including at least one switchable operational amplifier having an input and an output, at least one sampling capacitor connected to the input, at least one integrating capacitor connected to the input and to the output, a clock generator producing at least two non-overlapping switching-clock signals each having switching-clock phases including an on-phase and an off-phase; and a phase-variance device varying the switching-clock phases in which the first and second switching-clock signals are in the off-phase. The phase-variance device is connected to the clock generator. The at least two non-overlapping switching-clock signals include a first switching-clock signal and a second switching-clock signal. The clock generator controls charging of the sampling capacitor with the first switching-clock signal and switching the operational amplifier on and off with the second switching-clock signal. 
     The circuit configuration in switched op-amp technology according to the invention includes at least one switchable operational amplifier, at least one sampling capacitor that can be connected to the input of the operational amplifier, and at least one integration capacitor disposed between the input and the output of the operational amplifier. 
     At least two non-overlapping switching-clock signals are generated for controlling the circuit configuration. As long as one of the two switching-clock signals is at 1, the sampling capacitor is charged up by the input signal. During the phase, the other one of the two switching-clock signals is at 0 and the operational amplifier is in its switched-off state. At the end of the sampling phase, all switching-clock signals are in a common off-phase. The other one of the two switching-clock signals then changes to 1 and, thus, switches on the operational amplifier. During the integration-phase that now starts, the operational amplifier, as the active component, transfers the charge of the sampling capacitor connected to its input to the integration capacitor. At the end of the integration-phase, both switching-clock signals are again at 0. 
     The circuit configuration according to the invention has a device or means for varying the switching-clock phases in which all switching-clock signals are in the off-phase. The configuration makes it possible to stretch the switching-clock phases in which both switching-clock signals are at 0, at the cost of the on-phases. It is only necessary to ensure that the sampling capacitor can be completely charged up by the input signal during the on-phase of one switching-clock signal and that the operational amplifier can settle sufficiently for the charge to be transferred to the integration capacitor during the on-phase of the other switching-clock signal. 
     Thus, the common off-phase of the two switching-clock signals can be extended up to the limit predetermined by the transient response, making it possible to use the potential for power saving to its full extent. The invention is, thus, particularly suitable for use in mobile transceivers (mobiles), mobile Internet devices (WAP technology) but also for medical use (hearing aids, pacemakers etc.) in which a long life of the batteries used is of importance. 
     In particular, the invention makes it possible to adapt the length of the on- and off-phases individually to the Q factor of the operational amplifiers used. To take into consideration the influence of process spreads in the manufacture, analog circuits must be dimensioned such that they still meet the required specification even with disadvantageous process effects. In many cases, therefore, the behavior of an analog circuit is better than specified by the manufacturer. If the transient response of the operational amplifier used is better than specified, the potential can be exploited for saving power using the invention. 
     In accordance with another feature of the invention, it is an advantage if each of the switching-clock phases in which all switching-clock signals are in the off-phase is variable. Both the common off-phase that follows the on-phase of the first switching-clock signal and the common off-phase that follows the on-phase of the second switching-clock signal are extended. The process results in a uniform switching configuration. However, it is also possible to make the off-phase following the on-phase of the first switching-clock signal and the off-phase following the on-phase of the second switching-clock signal be different lengths. 
     As an alternative, in accordance with another feature of the invention, it is possible to vary only every second one of the switching-clock phases in which all switching-clock signals are in the off-phase. In such a solution, in each case only the off-phase following the on-phase of the first switching-clock signal is extended or in each case only the off-phase following the on-phase of the second switching-clock signal is extended. Thus, every second common off-phase is in each case extended. 
     It is an advantage, in accordance with an added feature of the invention, if the duration of the switching-clock phases in which all switching-clock signals are in the off-phase can be varied in dependence on the transient response of the operational amplifier. To achieve maximum power saving, the on-phase must be selected to be just long enough for the operational amplifier to be able to settle. Once the transient has ended, the operational amplifier can be switched off. 
     In accordance with an additional feature of the invention, the duration of the switching-clock phases in which all switching-clock signals are in the off-phase can be varied in dependence on the switching speed of the transistors. Because an operational amplifier is made of individual transistors, the transient response of the operational amplifier is essentially determined by the switching speed of the transistors. Thus, the transistor switching speed is an easily detectable measure of the transient response of the operational amplifier. The switching speed of the transistors can be used directly for determining the duration of the common off-phases: The faster the switching of the transistors, the longer the common off-phase can be selected to be and the greater the power savings. 
     It is an advantage, in accordance with yet another feature of the invention, if the circuit configuration includes a device or means for detecting the transistor switching speed connected to the operational amplifier. With such a device, the transistor switching speed can be detected individually on the substrate so that the length of the common off-phases can be matched to the Q factor of the operational amplifier used. 
     It is then an advantage, in accordance with yet a further feature of the invention, if the switching speed of n-channel FETs and/or of p-channel FETs can be detected separately. The process for producing n-type FETs and p-type FETs includes completely different process steps. As such, it is possible that n-type FETs and p-type FETs located on the same substrate differ considerably with regard to their switching speeds. 
     Depending on the internal circuitry of an operational amplifier, its transient response may be mainly determined by the behavior of the n-type FETs or of the p-type FETs. In such a case, it is recommended to detect the switching speed of the significant device type separately. 
     It is an advantage, in accordance with yet an added feature of the invention, if the device for detecting the transistor switching speed includes an XOR gate. An undelayed edge signal and an edge signal delayed through an inverter chain are applied to the inputs of the XOR gate. Using the simple circuit, the signal delay effected by the inverter chain can be converted into a pulse, the duration of which corresponds exactly to the signal delay caused by the inverter chain. Because the inverter chain is built up of individual FETs, the circuit can be used for detecting the transistor switching speed. In particular, it is possible to construct the inverter chain such that the delay is caused either mainly by n-type FETs or mainly by p-type FETs. As such, the circuit can also be used for separately detecting the switching speed of n-channel FETs and/or of p-channel FETs. 
     In accordance with yet an additional feature of the invention, there is provided an inverter chain and the detector has one of an XOR gate with XOR inputs, one of the XOR inputs receiving an undelayed edge signal, and another of the XOR inputs receiving an edge signal delayed through the inverter chain, and an XNOR gate with XNOR inputs, one of the XNOR inputs receiving an undelayed edge signal and another of the XNOR inputs receiving an edge signal delayed through the inverter chain. 
     Quite generally, it is an advantage, in accordance with yet an additional feature of the invention, if the device for detecting the transistor switching speed generates pulses, the duration of which characterizes the switching speed of the transistors. The duration of such pulses can be detected accurately with the aid of counter and timer chips and can be used as the basis for digital closed-loop control. 
     It is of advantage, in accordance with again another feature of the invention, to adjust the duration of the switching-clock phases in which all switching-clock signals are in the off-phase in dependence on the duration of the measuring circuit pulses. As the switching of the transistors becomes faster, the pulses occurring at the output of the measuring circuit become shorter and the on-phases that can be selected also become shorter. The common off-phases of the switching-clock signals can be correspondingly extended. 
     In accordance with again a further feature of the invention, the duration of the switching-clock phases in which all switching-clock signals are in the off-phase can be adjusted in a number of predetermined steps. The embodiment is based on the concept that a large proportion of the power saving can be achieved already with a relatively coarse adjustment of the duration of the on-phases and of the common off-phase. As such, it makes sense to adapt the duration of the switching-clock phases in which all switching-clock signals are in the off-phase only in steps. The adaptation can be achieved with little circuit expenditure. 
     In accordance with again an added feature of the invention, the clock generator or means for clock generation and the device for varying the switching-clock phases in which all switching-clock signals are in the off-phase are preferably implemented by a programmable clock generator. In a programmable clock generator, the lengths of the individual switching-clock phases are represented digitally and are converted into corresponding switching-clock signals by counter and timer circuits. The duration of the individual switching-clock phases can be reprogrammed in a simple manner. 
     As an alternative, it is an advantage, in accordance with again an additional feature of the invention, if an external squarewave generator and a divider circuit implement the clock generator and the device for varying the switching-clock phases in which all switching-clock signals are in the off-phase. In such a configuration, the divider circuit generates the at least two switching-clock signals from the squarewave signal. The generation makes it possible to find out with little external circuit expenditure what the magnitude of the spread of the transient response is with a certain filter or converter circuit and whether or not there is still potential for power saving. The duty ratio of the squarewave signal can be adjusted at the external squarewave generator. The common off-phase of the switching-clock signals can be varied by the duty ratio. 
     It is an advantage, in accordance with still another feature of the invention, if the circuit configuration is implemented in fully differential circuit technology. In mobile radio technology applications in particular, differential construction of the signal lines makes it possible to eliminate interference effectively. 
     With the objects of the invention in view, there is also provided a method for clocking successive operational amplifier stages constructed in switched op-amp technology, including the steps of generating at least two non-overlapping switching-clock signals, switching a first operational amplifier on and off with a first signal of the two switching-clock signals, switching a second operational amplifier on and off with a second signal of the switching-clock signals, and varying switching-clock phases in which the operational amplifiers are switched off. 
     In the method according to the invention for clocking successive operational amplifier stages constructed in switched op-amp technology, at least two non-overlapping switching-clock signals are generated in a first step, the first switching-clock signal switching a first operational amplifier on and off, and the second switching-clock signal switching a second operational amplifier on and off. In a second step, the phases of the switching-clock signals in which all operational amplifiers are switched off are varied. 
     Introducing a controllable common off-phase makes it possible to reduce the on times of the operational amplifier to the necessary degree so that the power consumption of the circuit can be restricted to a minimum. 
     In accordance with still a further mode of the invention, each of the switching-clock phases in which the operational amplifiers are switched off is varied. 
     In accordance with still an added mode of the invention, each second one of the switching-clock phases in which the operational amplifiers are switched off is varied. 
     In accordance with still an additional mode of the invention, a duration of the switching-clock phases in which the operational amplifiers are switched off dependent on a transient response of the operational amplifiers is varied. 
     In accordance with another mode of the invention, a duration of the switching-clock phases in which the operational amplifiers are switched off dependent on a switching speed of transistors of the operational amplifiers is varied. 
     In accordance with a further mode of the invention, at least one of a switching speed of n-channel FETs and a switching speed of p-channel FETs are separately detected. 
     In accordance with an added mode of the invention, at least one of a switching speed of n-channel FETs and a switching speed of p-channel FETs are separately detected. 
     In accordance with an additional mode of the invention, a duration of the switching-clock phases in which the operational amplifiers are switched off is adjusted in a number of predetermined steps. 
     In accordance with yet another mode of the invention, the non-overlapping switching-clock signals are generated with a programmable clock generator. 
     In accordance with yet a further mode of the invention, the non-overlapping switching-clock signals are generated with an external squarewave generator and a divider circuit. 
     In accordance with a concomitant mode of the invention, the switching-clock phases in which the operational amplifiers are switched off are varied by adjusting a duty ratio of a squarewave signal from the squarewave generator. 
     Other features that are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in a variable clock configuration for switched op-amp circuits, it is, nevertheless, not intended to be limited to the details shown because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic circuit diagram of a prior art circuit in switched op-amp technology including a number of operational amplifier stages; 
         FIG. 2A  is a block and schematic circuit diagram of a prior art circuit for generating a non-overlapping two-phase clock; 
         FIG. 2B  is a timing diagram illustrating the input clock signal and the even and odd switching-clock signals generated by the circuit according to  FIG. 2A ; 
         FIG. 3  is a block circuit diagram of a clock generating unit according to the invention including a circuit for determining the transistor switching speed; 
         FIG. 4  is a block and schematic circuit diagram of an example circuit for determining the gate delay and the transistor switching speed for use in the clock generating unit according to  FIG. 3 ; 
         FIG. 5A  is a timing diagram illustrating the input clock signal and the output signal of the circuit of  FIG. 4  for a case of short gate delays; 
         FIG. 5B  is a timing diagram illustrating a variation with time of the even and odd switching-clock signals generated by the clock generating unit according to  FIG. 3  for the case of short gate delays; 
         FIG. 6A  is a timing diagram illustrating a representation of the input clock signal and of the output signal of the circuit shown in  FIG. 4  for a case of long gate delays; 
         FIG. 6B  is a timing diagram illustrating a variation with time of the even and odd switching-clock signals generated by the clock generating unit according to  FIG. 3  for the case of long gate delays; 
         FIG. 7  is a schematic circuit diagram of a circuit providing for the separate determination of the switching speed of n-channel MOSFETs according to the invention; and 
         FIG. 8  is a timing diagram illustrating an overview of the clock signals of an external clock generating unit according to the invention in which the even clock signal and the odd clock signal are generated from a squarewave signal by a divider circuit. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In all the figures of the drawing, sub-features and integral parts that correspond to one another bear the same reference symbol in each case. 
     Referring now to the figures of the drawings in detail and first, particularly to  FIG. 1  thereof, there is shown a prior art switched op-amp circuit that includes two operational amplifier stages. The operational amplifier  1 , the sampling capacitor  2 , the integration capacitor  3 , and the capacitor  4  form the first operational amplifier stage. The second operational amplifier stage includes the operational amplifier  5 , the sampling capacitor  6 , the integration capacitor  7 , and the capacitor  8 . The various switches shown in  FIG. 1  are switched on and off by two non-overlapping switching-clock signals that will be called even and odd switching-clock signals in the text that follows. Before discussing the operation of the circuit shown in  FIG. 1  in greater detail, the generation of these two switching-clock signals will be explained with reference to  FIGS. 2A and 2B . 
       FIG. 2A  illustrates a prior art clock generator for generating a non-overlapping two-phase clock. A rectangular input clock signal  21  having the frequency f clk  is applied to the input of the circuit. The variation with time of the input clock signal  21  is shown in  FIG. 2B . 
     The input clock signal  21  is present, on one hand, at the input of the inverter  22  and also at an input of the second NOR gate  24 . The output of the inverter  22  is connected to an input of the first NOR gate  23 . At the output of the NOR gate  23 , the output signal  25  is present that is delayed by the two inverters  26 . At the output of the inverter chain, the even switching-clock signal  27  can be picked up, the variation with time of which is shown in  FIG. 2B . The even switching-clock signal  27  is connected to the second input of the second NOR gate  24 , at the output of which the output signal  28  appears. The output signal  28  is delayed by the two inverters  29  and, at the output of the inverter chain, the odd switching-clock signal  30  can be picked up, the variation of time of which is also shown in  FIG. 2B . The odd switching-clock signal  30  is supplied to the second input of the first NOR gate  23 . 
     The comparison of the variation of the even switching-clock signal  27  and of the odd switching-clock signal  30  by referring to  FIG. 2B  shows that the odd switching-clock signal  30  is in each case switched off during the on-phase of the even switching-clock signal  27 . In addition, both switching-clock signals are in a common off-phase between the on-phase of the even switching-clock signal  27  and the on-phase of the odd switching-clock signal  30  during the period δ. It is, therefore, called a “non-overlapping two-phase clock”. 
     Each of the switches shown in  FIG. 1  is now switched on and off by the even switching-clock signal or by the odd switching-clock signal. Next to each switch, the switching clock by which it is clocked is noted. 
     Firstly, the first operational amplifier stage will now be considered during the on-phase of the even switching clock. The switches  9  and  10  are closed, therefore, whereas the switches  11 ,  12 ,  13  and  14  are open. The operational amplifier  1  is, therefore, inactive in such a phase. The input signal IN is present at one terminal of the sampling capacitor  2  and the other terminal is connected to VSS. The sampling capacitor  2  is, therefore, charged up by the input signal. The capacitor  4  is connected to VSS and VDD through the switches  9  and  10  and is, therefore, charged up by the supply voltage. The on-phase of the even switching-clock signal is followed—after a short common off-phase of both switching-clock signals—by the on-phase of the odd switching-clock signal. During such a phase, the switches  9  and  10  are open whereas the switches  11 ,  12 ,  13  and  14  are closed. Therefore, the operational amplifier  1  is switched on in the phase. One terminal of the sampling capacitor  2  is connected to VDD through the switch  12 . The other terminal of the capacitor  2  is connected to the inverting input of the operational amplifier  1  through the switch  13 . The capacitor  4  that is connected to VSS through the switch  14  in the phase additionally couples in a constant charge that produces a type of DC shift. The injected charge makes it possible to achieve an approximate potential VSS at the inverting input. The operational amplifier  1 , as the active component, now attempts to correct its output to such an extent that the difference between the input voltages becomes zero. Therefore, the operational amplifier  1  attempts to bring the inverting input to VSS potential. As a result, precisely the charge quantity that has been sampled at the sampling capacitor  2  is transferred to the integration capacitor  3 . 
     The second operational amplifier stage is operated in the opposite phase to the first one. Still being considered is the on-phase of the odd switching clock in which the operational amplifier  1  is active. The switches  15  and  16  of the second op-amp stage are closed and that is why the output of the operational amplifier  1  charges up the sampling capacitor  6  belonging to the second operational amplifier stage. Thus, the integration-phase of the first operational amplifier stage and the sampling phase of the second operational amplifier stage are taking place at the same time. 
     In the subsequent switching-clock phase, the charge quantity sampled at the sampling capacitor  6  is transferred to the integration capacitor  7 . During such integration-phase of the second operational amplifier stage, the first operational amplifier stage is already back in the sampling phase. 
     The switching clock configuration shown in  FIG. 2B  is modified by the invention such that the on times of the operational amplifiers are shortened and, thus, a power saving is achieved. The hardware according to the invention is illustrated in  FIG. 3 . A programmable clock generator  31  is supplied with a squarewave input clock signal  32  having the frequency f clk . A circuit  33  for determining the transistor switching speed determines the switching speed of the transistors that is significant for the transient response of the operational amplifiers. A pulse signal  34  characteristic of the switching speed is supplied to the programmable clock generator  31  and taken into consideration in the generation of the even switching-clock signal  35  and of the odd switching-clock signal  36 . The faster the switching of the devices are, the shorter the on-phases of the operational amplifiers can be. 
       FIG. 4  illustrates an example for a circuit  33  for determining the transistor switching speed. The input clock signal  37  is present at the first input of the XOR gate  40 . At the second input of the XOR gate  40 , the delayed and inverted clock signal  39  is present that is obtained from the input clock signal  37  by an odd number of inversions ( FIG. 4  shows three inverters  38 ). If the input clock signal  37  is at 0, the signal  39  assumes the value 1 and the output signal  41  of the XOR gate  40  assumes the value 1. If the input clock signal  37  changes from 0 to 1, the new value 1 is immediately available at the first input of the XOR gate  40 . The signal  39  only changes to the new value 0 after a certain time delay that is determined by the gate delay of the three inverters  38 . During a period that is characteristic of the gate delay, the output signal  41  is, therefore, at 0 and then it assumes the value 1. 
     The duration of the pulses in the output signal  41  represents a measure of the switching speed of the transistors of the substrate. The measurement makes it possible to detect the effect of process spreads on the transistor switching speed directly on the chip and to take it into consideration during the clock generation. Instead of the XOR gate, an XNOR gate can also be used for determining the switching speed of the transistors. 
       FIG. 5A  illustrates the variation with time of the input clock signal  37  and of the output signal  41  of the XOR gate  40 . When the input clock signal  37  changes from 1 to 0, a falling signal edge  42  is obtained that triggers a pulse  43  with a pulse width t D  in the output signal  41 . During the pulse period t D , the output signal  41  assumes the value 0. 
     When the input clock signal  37  changes from 0 to 1, a rising signal edge  44  is obtained that also triggers a pulse  45  of length t D . The pulses  43 ,  45  shown in  FIG. 5A  are short and the corresponding values of t D  are low. Accordingly, the inverters  38  only produce a slight signal delay, which allows a high switching speed of the transistors and a short transient response of the operational amplifiers to be inferred. 
     The pulse signal  41  is supplied to the programmable clock generator that digitizes the period of the pulses  43 ,  45  and uses them for calculating the switching clock configuration. For the case of a short pulse duration t D  shown in  FIG. 5A , the switching clock signals generated by the programmable clock generator, the even switching-clock signal  46 , and the odd switching-clock signal  47  are shown in  FIG. 5B . Because of the fast transient response of the operational amplifiers, only short on-phases  48 ,  49  are required. 
     The switching clock phases  50 ,  51 , in which both switching-clock signals  46  and  47  are in the off-phase, can be correspondingly extended. In the prior art clock configuration shown in  FIG. 2B , the common off-phases had the period δ. In the clock configuration shown in  FIG. 5B , however, the duration of the common off-phases has been increased to δ+t a . The operational amplifiers are only switched on until the transient is finished. During the common off-phases, all operational amplifiers are inactive. 
       FIG. 6A  shows the input clock signal  52  and the output signal  53  of the XOR gate  40  for the case of transistors switching slowly or for long gate delays. The falling signal edge  54  causes a pulse  55  of duration t D  in the output signal  53  and the rising signal edge  56  correspondingly causes a pulse  57  of duration t D . In the example shown in  FIG. 6A , the transistors only have a low switching speed. The inverters  38 , therefore, delay the signal considerably and the delay leads to a long pulse duration t D , making it possible to infer a slow transient response of the operational amplifiers. 
       FIG. 6B  shows the variation with time of the associated switching-clock signals, the even switching-clock signal  58  and the odd switching-clock signal  59 . Because of the slow transient response of the operational amplifiers, the on-phases  60 ,  61  of the two switching-clock signals must be selected to be long. Accordingly, the common off-phase  62  of the switching-clock signals must be reduced to the minimum period δ. Accordingly, t a  is set to be =0. 
     The programmable clock generator maps the pulse duration t D  onto the duration of the common off-phase δ+t a , a small value of t D  being mapped onto a large value of δ+t a  and a large value of t D  being mapped onto a small value of t a . As such, the switching clock configuration can be adapted to the switching speed of the transistors such that the power saving is at a maximum. 
       FIG. 7  illustrates an embodiment of the circuit  33  for determining the transistor switching speed that selectively detects the switching characteristic of n-type MOSFETs. The use of such a circuit is recommended if the transient response of the operational amplifiers used is mainly determined by the characteristics of the transistors of the n-type. The circuit includes the p-type MOSFETs  65 ,  66 ,  67  and the n-type MOSFETs  68 ,  69 ,  70 ,  71 ,  72 ,  73 . The current through the FETs depends on the width/length ratio (W/L) of the respective FET. In the example illustrated in  FIG. 7 , the p-type FETs  65 ,  66 ,  67  and the n-type FETs  68 ,  69 ,  70  have a large W/L. The n-type FETs  71 ,  72 ,  73 , the W/L ratio of which is much lower than that of the other devices, therefore, have a current-limiting effect. 
     When the input signal  63  changes to VSS, the p-type FET  65  is gated on. The gate of the n-type MOSFET  69  is then at VDD and, if VBIAS has been suitably selected, the n-type FET  72  is also conducting. The potential VSS can then be switched through to the gate of the p-type FET  67 . The p-type FET  67  places an input of the XOR gate  64  at VDD. Because of the low value of W/L in the case of the n-type FET  72 , in comparison with the W/L values of the FETs  65 ,  67 ,  69 , the total delay is essentially determined by the n-type FET  72 . When the input signal  63  changes to VDD, in contrast, the total delay essentially depends on the switching speed of the n-type FETs  71  and  73 . In every case, the total delay is, therefore, mainly determined by the n-type FETs having a small W/L. 
       FIG. 8  illustrates a method of how the clock configuration according to the invention can be generated externally by a squarewave generator and a divider circuit. The squarewave generator supplies a squarewave signal  74  having the frequency (2 f clk ). From the squarewave signal  74 , the even switching-clock signal  75  and the odd switching-clock signal  76 , which each have a period of 
               1     f   clk       ,         
are derived by a divider circuit.
 
     The duration of the common off-phase in which both switching-clock signals are equal to 0 can be adjusted by varying the duty ratio of the squarewave signal. The duty ratio of the squarewave signal  74  is 1/2 whereas the duty ratio of the squarewave signal  77  is 1/4. The squarewave signal  78  has a duty ratio of 3/4. The values of δ+t a  that belong to the individual duty ratios can be seen in the clock configuration illustrated in  FIG. 8 . The greater the selected duty ratio, the shorter the duration of the common off-phase δ+t a . Conversely, a small duty ratio produces a distinct extension of the common off-phase. The external circuit shown in  FIG. 8  makes it possible to find out the magnitude of the spread of the transient response with a certain switched op-amp circuit and whether or not there is still potential for saving power.