Abstract:
Circuitry is disclosed for controlling the slope of rising and falling edges of a signal. The circuitry includes a ramp signal generator that receives an input signal and that generates a trapezoidal signal based on the input signal, and a circuit array that receives the trapezoidal signal and that generates control signals based on the trapezoidal signal. Output transistors have gates that receive a set of the control signals. The output transistors include a top transistor and a bottom transistor. The top transistor has a source connected to a supply potential and a drain connected to an output. The bottom transistor has a source connected to a reference potential and a drain connected to the output. The top transistor and the bottom transistor are gated by the control signals to control a shape of an edge of an output signal at the output.

Description:
TECHNICAL FIELD  
       [0001]     This application relates to a circuit array for controlling the slope of an edge of an output signal.  
       BACKGROUND  
       [0002]     In integrated circuits, there is normally a need to reduce electromagnetic interference emissions. Steep signal edges lead to relatively large emitted and conducted emissions, especially during operation with high-frequency, digital signals. However, to ensure that electronic systems do not significantly impede the operation of other electronic system, regulatory measures have been imposed, such as the establishment of severity values, which are intended to guarantee the electromagnetic compatibility (EMC) of different systems. Electromagnetic compatibility (EMC) relates to both allowable electromagnetic emissions and required resistance to interference.  
         [0003]     One method of reducing high-frequency signal components within the frequency spectrum of a digital signal includes reducing the slopes of rising and falling edges of the digital signals. Therefore, efforts have been made to reduce the slopes of both rising and falling signal edges of digital signals so as to comply with EMC standards or EMC specifications, and to guarantee reliable operation of the circuits.  
         [0004]     In the rapid digital signal processing used today, such as in digital signal processors, digital signals are designed to be as fast as possible, often in the range of a hundred picoseconds. This leads to the large, high-frequency spectral component of the signals. In addition, integrated circuits used in fast digital signal processing normally occupy a small amount of surface area on a chip and consume low power during operation.  
         [0005]     In U.S. Pat. No. 6,225,844, an output stage is disclosed, which can be operated stably with a relatively small signal slope. The specified output stage comprises two CMOS inverter stages connected in parallel, with respect to their load paths, between a reference potential connection and a supply potential connection. An output signal with reduced edge slope can be obtained from these inverter stages. Whereas the first of the two input stages is directly driven by the input signal, an additional control circuit for influencing signal edge slope is provided to drive the gate electrodes of the transistors of the second transistor inverter stage. This circuit enables the two transistors of the second output stage to be switched on or off in a controlled manner. Because threshold values of the transistors are used to drive the second output stage, the principle described above can only be used to construct a two-stage circuit array.  
         [0006]     A circuit array for reducing the edge slope of an output signal is also described in U.S. Pat. No. 5,140,194. In this case, several CMOS inverter output stages are connected in parallel. Delay times are generated to drive the various inverter stages. According to the patent, the delay times are generated with RC time constants. A disadvantage of this approach, however, is that it involves use of resistors having narrow tolerance ranges and that require a relatively large chip surface area. Alternatively, it is also proposed that the RC elements be replaced with delay elements, which are also difficult to implement. The disadvantage remains of having to adjust the time constant individually for each inverter stage on the output end, and of having to implement individually in terms of circuit technology.  
         [0007]     Another disadvantage shared by the above-described circuit arrays is that the slope of the output signal is dependent on the characteristics of the input signal.  
       SUMMARY  
       [0008]     One object of this invention is to specify a circuit array for providing an output signal with an edge whose slope is adjustable, which makes it possible to use any number of inverter stages to control the edge slope to any degree of precision, and which further provides an output signal whose edge slope can be adjusted independently of the signal characteristics of the input signal.  
         [0009]     According to the invention, the object is achieved by a circuit array for providing an output signal with adjustable edges slopes, comprising: 
        an input for supplying an input signal,     an output for accepting an output signal derived from the input signal, whose edges slope is adjustable,     a ramp signal generator, which is coupled with the input of the circuit array on the input end and provides a trapezoid signal at its output,     a first pair of output transistors, the controlled paths of which are coupled with the output of the circuit array, on the one hand, and with a supply or reference potential connection, on the other, and each having a control connection which is coupled with the output of the ramp signal generator, using two switching thresholds, and     a second pair of output transistors, the controlled paths of which are coupled with the output of the circuit array, on the one hand, and with a supply or reference potential connection, on the other,     and each having a control connection which is coupled with the output of the ramp signal generator, using two additional switching thresholds.        
 
         [0016]     Each of the pairs of output transistors preferably comprises two complementary transistors. These transistors are complementary with respect to their conductivity type, for example, an n-channel and a p-channel field effect transistor is provided in each case.  
         [0017]     The term “pair of output transistors”, in this context, refers to two output transistors which together form an output stage, but are preferably not driven jointly at their control electrodes, but independently from one another.  
         [0018]     Preferably, three or more output stages are provided, each of which comprises two output transistors.  
         [0019]     The controlled paths of the output transistors, which together form a pair, are preferably connected in series. These series connections are connected in parallel with one another. The connection nodes of the controlled paths of the output transistors, which together form a pair, are preferably connected to one another and to the output of the circuit array.  
         [0020]     According to this arrangement, the ramp signal generator controls the input and output behavior of the first and second pair of output transistors via the trapezoid signal. Additional transistors, e.g., third, fourth, etc. may be controlled by the ramp signal generator. A shared trapezoid signal generated by the shared ramp signal generator is used for the first and second pair of output transistors, as well as for any other number of additional pairs of output transistors.  
         [0021]     The output signal of the ramp generator is a trapezoid signal with relatively small slope, both in its rising and in its falling signal edges.  
         [0022]     Characteristics of the trapezoid signal, especially its edge slope, are independent of the input signal, including the edge slope of the input signal.  
         [0023]     An output signal with an adjustable edge slope can be generated with this circuit array. Using the independently adjustable switching thresholds of the transistors of the output signals, which are individually switched on or off in dependence on the signal progression of the ramp signal, a desired switching sequence and desired switching delays can be set precisely and independently of the progression of the input signal of the circuit array. In particular, sequential switching of the output transistors with the trapezoid signal can prevent the occurrence of unwanted transverse currents. Accordingly, the present principle allows for the operation of circuits in integrated circuit technology at high signal speeds with, at the same time, relatively low and, especially, adjustable electromagnetic emissions, because reduction of the edge slope, which is adjustable, results in the reduction of both the high-frequency signal components in the spectrum of the digital signals and the related, high-frequency electromagnetic emissions. To operate the present circuit array with adjustable edge slope, a large number of expensively implemented delay elements are unnecessary.  
         [0024]     According to an advantageous embodiment of the present circuit array, pairs of pre-drivers are provided to supply the desired switching thresholds to couple the ramp signal generators with the first and second pair of output transistors. Advantageously, the switching thresholds can be adjusted differently, both for the output transistors within a pair of output transistors and between the pairs of output transistors, and are activated during operation by the trapezoid signal.  
         [0025]     Preferably, one pre-driver is assigned to each output transistor.  
         [0026]     The pre-drivers are preferably formed as inverters.  
         [0027]     To combine a necessary driver power with a small threshold, as can be provided with some of the output transistors of the circuit array, several inverters can preferably be connected in series to form a pre-driver.  
         [0028]     According to the described principle, the inverters of the pre-drivers are switched on and off sequentially by means of the trapezoid signal with a ramp-shaped progression. The pre-drivers themselves drive the output transistors.  
         [0029]     Alternatively, the pre-drivers can also be formed as Schmitt triggers, for example, or any other circuit element that provides a trapezoidal signal.  
         [0030]     In the described circuit array, controllable switches can be provided to rapidly switch off the output transistors, which, with their controlled paths, connect a control connection of the output transistors to the reference or supply potential connections. The controllable switches allow a rapid shutdown of the correspondingly assigned output transistors and thus an especially short delay time of the output signal of the circuit array relative to the input signal. When augmented in this manner, the circuit array is especially suitable for rapid digital signal processing.  
         [0031]     The controllable switches are preferably connected via their control electrodes to the input of the circuit array, such as the input of the ramp signal generator.  
         [0032]     An especially rapid connection of the input of the circuit array with the output, involving an especially short signal run time in the circuit array, can be achieved, in accordance with a preferred embodiment of the circuit array, in that, in a third pair of output transistors, the controlled paths are coupled with the output of the circuit array, on the one hand, and with the supply or reference potential connection, on the other, while each of the control connections of the output transistors of the third pair has a control connection which is coupled with the input of the ramp signal generator to supply the input signal.  
         [0033]     Accordingly, the third pair of output transistors is not driven by the trapezoid signal of the ramp signal generator, but directly by the input signal itself.  
         [0034]     The ramp signal generator preferably comprises a capacitive component, which, as a result of charging and discharging effects, determines the signal ramp of the ramp-shaped signal in terms of its time progression, especially its edge slope. To this end, the ramp signal generator can have a flip-flop structure, for example. With a ramp signal generator constructed in this fashion, a ramp progression of the trapezoid signal is preferably achieved which is independent of the progression of the input signal.  
         [0035]     According to a further embodiment of the present circuit array, the capacitance of the capacitive component of the ramp signal generator can be designed to be adjustable, as varactor diodes, for example. Alternatively, a combination of different, fixed capacitances can be provided, which can be switched on and off. They can be implemented as internal or as external capacities.  
         [0036]     As a result, an additional means of adjusting the output signal of the circuit array can be achieved through adjustable progression of the trapezoid signal.  
         [0037]     Additional details and advantageous embodiments of the invention are the subject of the dependent claims.  
         [0038]     In the following, the invention is described in greater detail on the basis of several exemplary embodiments shown in the drawings: 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0039]      FIG. 1   a  shows, using a simplified block diagram, a first exemplary embodiment of a circuit array according to the invention for providing an output signal with an adjustable edge slope.  
         [0040]      FIG. 1   b  shows the progression of the trapezoid signal for driving the output transistors shown in  FIG. 1   a.    
         [0041]      FIG. 2  shows an exemplary embodiment of a ramp signal generator shown in  FIG. 1   a , using a circuit schematic.  
         [0042]      FIG. 3  shows the time progressions of the input signal and output signal of a circuit array shown in  FIG. 1   a.    
         [0043]      FIG. 4  shows the depiction shown in  FIG. 3 , but based on a group of curves with an adjustable edge slope for the input signal relative to the rising signal edge.  
         [0044]      FIG. 5   a  shows a further embodiment of the circuit array shown in  FIG. 1   a , with rapid shutoff.  
         [0045]      FIG. 5   b  shows the progression of a trapezoid signal for driving the output transistors shown in  FIG. 5   a.    
         [0046]      FIG. 6   a  shows a further embodiment of the circuit array shown in  FIG. 5   a  for shorter signal run times.  
         [0047]      FIG. 6   b  shows the progression of a trapezoid signal for driving the output transistors shown in  FIG. 6   a.    
         [0048]      FIG. 7  shows time progressions of the input signal and output signal when the capacitance in the ramp signal generator is changed. 
     
    
     DETAILED DESCRIPTION  
       [0049]      FIG. 1   a  shows a circuit array for providing an output signal B with an adjustable edge slope, comprising an input  1  for supplying an input signal A and an output  2  for measuring an output signal B derived from the input signal A, whose edge slope is adjustable. The circuit array also comprises a ramp signal generator  3 , the input of which is coupled with the input  1  of the circuit array on the input end and which provides, at its output, a trapezoid signal C with a relatively gradually rising and gradually falling signal edge, both exhibiting a ramp-shaped progression.  
         [0050]     The output of the ramp signal generator  3  is connected to each of the inputs of a plurality of pre-drivers  4  formed as inverters. The inverters  4  have different switching thresholds. Each of the outputs of the pre-drivers  4  is connected to a gate connection of a metal oxide semiconductor (MOS) field effect transistor. Pairs of complementary MOS transistors  5 ,  6 ;  7 ,  8 ;  9 ,  10 ;  11 ,  12  form pairs of output transistors, such that each pair of output transistors forms an inverter stage. Each of the inverter stages on the output end comprises a p-channel transistor  5 ,  7 ,  9 ,  11 , the load path of which couples a supply potential connection  13  with the output  2  of the circuit and an N-channel-MOS transistor  6 ,  8 ,  10 ,  12 , the load path of which connects the output  2  with a reference potential connection  14 .  
         [0051]     The mode of operation of the circuit shown in  FIG. 1   a  is explained in the following on the basis of a signal progression of the voltage of the ramp-shaped signal C, as shown in  FIG. 1   b , which is supplied at the output end of the ramp signal generator  3 , and the time progression of which, especially its edge slope, is dependent on the input signal A. The trapezoid signal C sequentially switches the plurality of inverter stages  4  used as pre-drivers on and off, depending on the signal voltage and the switching thresholds of the inverters. The pre-drivers  4 , in turn, drive the output transistors  5  to  12 . The signal progression of the trapezoid signal C exhibits reversing thresholds of the inverters  4 , which, for this purpose, are differently configured based on the reference symbols of the transistors  5  to  12  shown in  FIG. 1   a , which are assigned to the inverters  4 , at various times t1 to t16.  
         [0052]     In the circuit schematic shown in  FIG. 1   a , the area ratios of the output transistors  5  to  11  as well as  6  to  12  increase from left to right, which is synonymous with the output transistors  11 ,  12  having the highest current driver capacitance, and the output transistors  5 ,  6  having the lowest current driver capacitance. Thus, the effective transistor areas of the output transistors are designed to be sloped in pairs relative to one another, the objective being to achieve the smoothest possible transition in the output signal B from a low to a high signal level, and vice-versa.  
         [0053]     The benefit of driving the output transistors  5  to  12  via pre-drivers  4  with different switching thresholds by a shared trapezoid signal C is that, on the one hand, a plurality of output stages, i.e., output transistors, can be provided, depending on the desired accuracy of adjustment of the output signal B relative to its edge slope, and that, on the other hand, a structure with a small chip area is possible and that, finally, the edge progression of the output signal B is independent of that of the input signal A, because signal progression of the trapezoid signal C is always constant and adjustable.  
         [0054]     The pre-driver inverters  4 , which drive the output transistors  5  to  12 , are designed to have different threshold voltages. Each inverter  4  is switched on when the voltage level of the trapezoid signal  3  reaches its preset voltage threshold value, and is switched off again when the voltage level of the trapezoid signal C falls below this preset threshold voltage. The threshold voltages of the inverters, which drive the P-channel transistors  5 ,  7 ,  9 ,  11 , are set to different values than the threshold voltages of those inverters  4  that drive the N-output transistors  6 ,  8 ,  10 ,  12 , the purpose being to avoid transverse currents. As explained earlier, instead of one inverter  4 , a plurality of inverters connected in series can be provided, such as three inverters connected in series, the input-end inverter being designed to generate the desired threshold voltage and the output-end inverter to drive the output transistor that is connected to it.  
         [0055]     Another advantage can be achieved by designing the output-end inverter  4  to be especially small relative to its transistor area. This results in an additional reduction in the edge slope of the output signal B, because the gate connection of the corresponding output transistor  5  to  12  is charged relatively slowly. This forces the output transistor  5  to  12  to allow an extended period of time to pass until the output transistor has been completely connected.  
         [0056]     The size, that is, the transistor areas and, therefore, the driver capacitance of the output transistor pairs  5 ,  6 ;  7 ,  8 ;  9 ,  10 ;  11 ,  12  relative to one another are configured differently. The output transistor pair  5 ,  6  has the smallest effective transistor area. The adjacent output transistors  7  have a larger transistor area than the transistors  5 ,  6 , and, in turn, the output transistors  9 ,  10  have a larger effective area than the output transistors  7 ,  8 , and so on. When the output signal is switched on, i.e., when there is a transition from low to high, the output transistors  12 ,  10 ,  8 ,  6  are switched off consecutively, beginning with the largest. Then the output transistors  5 ,  7 ,  9 ,  11  are switched off consecutively, beginning with the smallest output transistor. The result of connecting a smaller output transistor  5 ,  6  is that a smaller transistor requires more time to charge and discharge an external load capacitance than a large one. In each output stage  5 ,  6 ;  7 ,  8 ;  9 ,  10 ;  11 ,  12 , an external load is charged or discharged a little further, until the output voltage at the output  2  reaches the desired voltage level assigned to the corresponding logic level, high or low.  
         [0057]     As mentioned earlier, the advantage of sequentially switching the output transistors  5  to  12  on and off is that transverse currents are reduced to a minimum. In addition to reducing the electromagnetic emission of the circuit, this also reduces the power consumption of the circuit.  
         [0058]     For example, if the output signal B is to increase from a low to a high level, the trapezoid signal C, which drives the output transistors  5  to  12  via the inverters  4 , also moves from low to high. This signal ramp increases gradually over time, finally reaching the threshold voltages of the inverters  4 . The threshold voltage of the inverter  4  assigned to the largest N-channel output transistor  12  is reached first.  
         [0059]     This output transistor  12  switches off. Then the trapezoid signal C reaches the threshold voltage of the next smaller N-channel transistor  10 , and transistor  10  also switches off. In the same manner, output transistors  8  and  6  are switched off consecutively, until all N-channel output transistors are switched off. As the trapezoid signal C increases further, the threshold voltage of the inverter  4  assigned to the smallest P-channel transistor  5  is reached and output transistor  5  is switched on. Subsequently, the threshold voltage of the next larger P-channel transistor  7  is reached, and it too switches on. Then the P-channel output transistors  9  and  11  are switched on, and the process continues until all P-channel output transistors are switched on. The total driver capacitance that is capable of converting a load connected to an output  2  to a high level is defined as the sum of the transistor areas of all P-channel output transistors of the circuit, and the driver capacitance in the low state is defined as the sum of the transistor areas of all N-channel transistors.  
         [0060]     Driving the output signal B from high to low occurs analogously. In this process, first the P-channel output transistors are connected in sequence and then the N-channel output transistors are connected in sequence. The sequential switching on of transistors with increasing transistor area leads to a gradually rising or falling edge of the output signal B. A significant advantage over known output drivers is that, in the present principle, the peak current for charging and discharging external loads is divided into a plurality of current branches, which drive a relatively low current. In addition to the relatively low load current of the individual output transistors, another advantage is that the longer amount of time needed to switch the output  2  leads to a lower current gradient di/dt, thereby reducing the emission of electromagnetic energy from integrated semiconductor circuits. Another advantage lies in the reduced overshooting of the output signal by the drive shown.  
         [0061]      FIG. 2  shows the circuit schematic of a possible structure of the ramp signal generator  3  from  FIG. 1   a . The input signal A is supplied to the ramp signal generator  3  at the input  1 . The ramp signal generator  3  supplies the trapezoid signal C at its output  15 . The ramp signal generator  3  comprises two transistors  16 ,  17  cross-coupled with one another, which are formed as p-channel-MOS transistors, one end of their load paths being coupled with the supply potential connection  13 . The transistor  17  is connected to the reference potential connection  14  through a capacitor  19 . The control connection of the gate connection of the transistor  16  is connected to a load connection of the transistor  17 , while the control connection of the transistor  17  is connected to a load connection of the transistor  16 . In parallel to the capacitor  19 , a switching transistor  21  is connected to its load path, its controlled path being coupled with the input  1 . In this process, the input signal A is supplied to the control connection of the transistor  20  in unchanged form, whereas the input signal A is supplied to the control connection of the transistor  21  in inverted form. To this end, an inverter  22  is connected between the input  1  and the control connection of the transistor  21 .  
         [0062]     The underlying principle of the circuit of the ramp signal generator is to charge the load capacitor  19  with a constant current. The capacitor  19  does not necessarily have to be designed as external capacitors, but can preferably be designed as a gate capacitor of MOS transistors. The capacitor  19  is charged by means of the PMOS transistor  17  and discharged by means of the NMOS transistor  21 , which is connected in parallel to the capacitor  19 . Thus, the circuit shown in  FIG. 2  corresponds to a flip-flop principle. Advantageously, the ramp signal generator  3  exhibits no or only very little quiescent current. Current only flows when the capacitor  19  is charged or discharged. In addition, the ramp signal generator  3  advantageously exhibits low current consumption.  
         [0063]      FIG. 3  shows, based on the progressions of the signal voltage over time, the output signal A, with a rising and a falling edge, and the output signal B, also on the basis of a rising and falling edges. It is evident that the desired effect is achieved with the present circuit, namely a relatively slow rise in the signal edge and a relatively slow fall in the signal edge, with soft transitions and little overshoot, as well as a relatively short delay time.  
         [0064]      FIG. 4  shows, also based on the progressions of the signal voltage over time, a diagram of the rising edge of the input signal A and, below it, at very high chronological resolution, the rising edge of the output signal B according to the present principle. The signal progressions of the input and output signal A, B are shown as a group of curves, wherein the pitch of the rising edge of the input signal A is selected as the grouping parameter. It is clearly evident that, regardless of the edge of the input signal A, the progression of the rising edge of the output signal B is equal in each case and, in particular, has the same edge slope. Only the delay time is slightly dependent on the edge slope of the input signal A.  
         [0065]      FIG. 5   a  shows a further embodiment of the circuit shown in  FIG. 1   a . In terms of its structure and advantageous mode of operation, it essentially corresponds to that shown in  FIG. 1   a . In addition to the components shown in  FIG. 1   a , however, it has transistors for rapidly switching off  23 ,  24  the output transistors  5  to  12 . Each PMOS transistor  23  for rapidly switching off the PMOS output transistors  5 ,  7 ,  9 ,  11  is connected to the input  1  through its control connection and, through its load path, to the supply potential connection  13 , on the one hand, and to the control connection of each corresponding output transistor  5 ,  7 ,  9 ,  11 . In addition, an NMOS cutoff transistor  24  is assigned to each NMOS output transistor  6 ,  8 ,  10 ,  12 , its controlled paths being connected to the reference potential connection  14 , on the one hand, and to one control input of each of the corresponding N-channel output transistors  6 ,  8 ,  10 ,  12 , on the other, and its control connections being connected to the input  1  of the circuit.  
         [0066]     An advantage offered by the further embodiment according to  FIG. 5   a , with its cutoff transistors  23 ,  24 , is that an especially low delay time of the output signal B relative to the input signal A is achieved with relatively little effort.  
         [0067]      FIG. 5   b  shows the progression of the trapezoid signal C for driving the circuit shown in  FIG. 5   a  essentially corresponding to the signal progression of  FIG. 1   b . Only the reversing thresholds of the inverters  4  assigned to the transistors  5  to  12  and the times t1 to t8 are adjusted to the further embodiment with the cutoff transistors  23 ,  24  according to  FIG. 5   a.    
         [0068]      FIG. 6   a  shows a further embodiment of the circuit according to  FIG. 5   a , essentially corresponding to it in terms of structure and function. Only in terms of the driving of the output transistors  5 ,  6 , which have the smallest transistor areas of all output transistors  5  to  12 , does  FIG. 6   a  differ, advantageously, from the circuit according to  FIG. 5   a . In this case, the inputs of the inverters  4 , which are assigned to the output transistors  5 ,  6  for the purpose of driving them, are not connected to the output of the ramp signal generator  3 , but with the input of the ramp signal generator  3  and thus with the input  1  of the circuit. In the circuit according to  FIG. 6   a , the inverters  4  assigned to the remaining output transistors  7  to  12  are, as explained above, also connected to the output of the ramp signal generator  3 . This allows for an even greater reduction in signal run times and phase delays of the output signal B relative to the input signal A.  
         [0069]     The diagram in  FIG. 6   b  shows the progression of the trapezoid signal C over time t based on the signal voltage, which essentially corresponds to the depiction of the trapezoid signal C in  FIGS. 1   b  and  5   b . Only the reversing thresholds of the inverters  4  assigned to the transistors  7  to  12  and the times t1 to t6 are adjusted to the further embodiment with cutoff transistors  23 ,  24  shown in  FIG. 6   a , as well as to the third output transistors  5 ,  6  being directly driven by the input signal A.  
         [0070]     In the circuits shown in  FIGS. 5   a  and  6   a , the output transistors  5  to  12  are switched on and off sequentially, as in the circuit shown in  FIG. 1   a . The further embodiment with the rapid cutoff transistors  23 ,  24  shown in  FIGS. 5   a  and  6   a  allows for rapid cutoff, so that, advantageously, a more rapid succession of the switching actions of the output transistors can occur, because transverse currents cannot flow because of the rapid cutoff.  
         [0071]      FIG. 7  shows the signal progression of the voltage of the output signal B, using a group of curves, as a factor of the signal progression of the voltage of the input signal A in a circuit according to  FIGS. 1   a ,  5   a  or  6   a , based on a simulation. However, the capacitance, as the grouping parameter, of the capacitive component  19  of the ramp signal generator shown in  FIG. 2  is varied. The output signal B with the steepest edge slope is accepted at the smallest capacitance and that with the smallest edge slope at the largest capacitance. Between these extremes, the capacitance was increased in discrete steps.