Abstract:
A variable impedance circuit is provided as an active load between an input line L1 and an output line L2. This circuit has low impedance with respect to a DC electric current signal and has high impedance with respect to an AC electric current signal, structured from a series circuit of resistors R1, R2, and R3 connected between lines L1 and L2; a transistor Q1 having the collector connected to the line L1 and the base connected between the resistors R2 and R3; a resistor R4 connected between the emitter of the transistor Q1 and the line L2; a capacitor C1 with one end connected between the resistors R2 and R3; a resistor R5 connected between the other end of the capacitor C1 and the line L2; a capacitor C2 having one end connected between the resistors R1 and R2; and a resistor R6 connected between the other end of the capacitor C2 and the line L2.

Description:
CROSS-REFERENCE TO PRIOR APPLICATION 
       [0001]    This application claims priority to Japanese Patent Application No. 2011-226985, filed Oct. 14, 2011. This application is incorporated herein by reference in its entirety. 
       FIELD OF TECHNOLOGY 
       [0002]    The present invention relates to a positioner for receiving a supply of a DC electric current signal through a pair of electric wires from a higher-level system to produce its own operating power supply from the DC electric current signal that is supplied, and for controlling the degree of opening of a regulator valve (valve) in accordance with a value of the supplied DC electric current signal. 
       BACKGROUND 
       [0003]    Conventionally this type of positioner is designed so as to operate with an electric current between 4 and 20 mA (a DC electric signal) sent through a pair of electric wires from a higher-level system. For example, if a current of 4 mA is sent from the higher-level system, the opening of the regulator valve is set to 0%, and if a current of 20 mA is sent, then the opening of the regulator valve is set to 100%. 
         [0004]    In this case, the supplied electric current from the higher-level system varies in the range of 4 mA (the lower limit electric current value) through 20 mA (the higher limit electric current value), and thus the internal circuitry within the positioner produces an operating power supply itself from an electric current of no more than the 4 mA that can always be secured as an electric current value that is supplied from the higher-level system. (See, For Example, Japanese Unexamined Patent Application Publication 2004-151941 (“JP &#39;941”).) 
         [0005]    The opening setting value for the regulator valve is inputted into the positioner by the higher-level system. Moreover, the actual opening value for the regulator valve is obtained through the opening sensor. Consequently, the positioner is able to perform regulator valve fault diagnostics, self-diagnostics, and the like, through performing calculations on the relationship between the opening setting value and the actual opening value for the regulator valve. The provision of such fault diagnostic functions in the positioner makes it possible to increase the functionality of the system at a low cost, through eliminating the need for providing a separate fault diagnosing device. (See, for example, JP &#39;941.) 
         [0006]    For reasons such as these, in recent years there have been proposals for positioners that have, in addition to their actual functions of controlling the degree of opening of the regulator valves, also opening degree transmitting functions, regulator valve fault diagnostics, and functions for sending, to the higher-level system, the results of fault self-diagnostics, and the like.  FIG. 8  shows the structure of the critical components of a positioner that has a communication function for the higher-level system. (See, for example, Japanese Unexamined Patent Application Publication H11-304033 (Japanese Patent Application Number 3596293).) 
         [0007]    In  FIG. 8 , the input terminals T 1  and T 2  input DC electric current signals that are between 4 and 20 mA. A zener diode ZD 1  is connected through a resistor RA to the input terminals T 1  and T 2 , to produce a power supply voltage for use by the internal circuitry such as the modem  4  and the CPU  6 , and the like. A capacitor CA is inserted between the transmitting circuit  1  and receiving circuit  2  and the input terminal T 2 , thus providing DC insulation between the power supply voltage V 1  and the digital communication signals. A capacitor CB is a decoupling circuit for the power supply voltage V 1 , to prevent transfer or feedback of energy between the power supply voltage V 1  and the ground GND. 
         [0008]    A transmitting circuit  1  sends, through digital communications, a response signal to the higher-level system. A receiving circuit  2  receives a request signal, from the higher-level system, requesting a transmission. Here the higher-level system is connected to the input terminals T 1  and T 2  through two transmission lines (a pair of electric wires). Moreover, the impedance due to the resistor RA is used effectively in the digital communications in the transmitting and receiving circuits  1  and  2  in order to maintain a communication amplitude above a given voltage level. An electric current detecting circuit  3  is for detecting the value of the electric current signal that is inputted into the input terminals T 1  and T 2 , and sends the detected signal to an A/D converting device  7 . 
         [0009]    A modem  4  is for performing modulation and demodulation of the digital signals of the transmitting and receiving circuits  1  and  2 , and exchanges the contents of those signals with a CPU  6 . The CPU  6  performs the digital communications and the positional control of a regulator valve  14 , and has a communication processing program, for request signals, response signals, and the like, and a controlling program, such as PID, control, or the like, stored in a memory  5 . Because the control output of the CPU  6  is a digital signal, it is converted into an analog signal by the D/A converting device  8 . 
         [0010]    A driving circuit  9  amplifies, and adjusts the impedance of, the analog signal that is sent from the D/A converting device  8 , and sends the result to an electropneumatic converting module  11 . A sensor interface circuit  10  processes a signal of a position sensor  13 , and sends it to the A/D converting device  7 . The A/D converting device  7  digitizes the inputted electric current signal from the electric current detecting circuit  3  and the position signal for the regulator valve, sent from the sensor interface circuit  10 , and sends the result to the CPU  6 . 
         [0011]    The electropneumatic converting module  11  is for converting the inputted driving current into a pneumatic signal, and controls the pneumatic pressure of a nozzle through a torque motor. A control relay  12  is for amplifying the pneumatic signal, where the opening and closing of the regulator valve  14  is driven by the amplified pneumatic signal. The opening/closing control of the regulator valve  14  is performed through the position signal of the position sensor  13  being sent to the CPU  6  through the sensor interface circuit  10  and the A/D converting device  7 , through the CPU  6  performing controlling calculations, and through the control output being sent to the driving circuit  9  through the D/A converting circuit  8 . As a result, the regulator valve  14  is driven to control the degree of opening to the target value through the following path: driving circuit  9 →electropneumatic converting module  11 →control relay  12 →regulator valve  14 . 
         [0012]    In the positioner  100 , an AC electric current signal is superimposed on the DC electric current signal that is between 4 and 20 mA, to enable communication between the system on the higher-level side and the positioner  100 . As the content of this communication there are exchanged control parameters for the control calculations pertaining to the regulator valve  14 , amounts of adjustment of the zero/span point, the signal outputs of the position sensor, and self-diagnostic results, as the content of the communications. The communication data is read in through the following path: the receiving circuit  2 →the modem  4 →the CPU  6 ; and the transmission of the communication data is through the following path: the CPU  6 →the modem  4 →the transmitting circuit  1 . The DC electric current signal inputted into the positioner  100  is recognized through the following path: the electric current detecting circuit  3 →the A/D converting device  7 →the CPU  6 . 
         [0013]    In the positioner  100 , in order to perform the digital communications, the resistor RA must be above about 250 ohms, so the voltage drop will be more than 5 V with an inputted electric current of 20 mA, causing the voltage V 1  that is produced by the zener diode ZD 1  to become smaller. Given this, a variable impedance circuit Z 1 , as illustrated in  FIG. 9 , is used as an active load instead of the resistor RA. 
         [0014]    In the variable impedance circuit Z 1 , the transistor Q 2  has the collector connected to a line L 1 , and the emitter connected to a line L 2  through a resistor R 7 . The transistor Q 1  has its collector connected to the line L 1  and connected through a resistor R 2  to the base thereof, and the emitter is connected to the base of the transistor Q 2 , and also connected to the line L 2  through a resistor R 4 . The base of the transistor Q 1  is connected to the line L 2  through a parallel circuit of the resistor R 3  with a capacitor C 1  and a resistor R 5 . Note that the line L 1  is a line connected to the zener diode ZD 1 , and the line L 2  is a line connected to the terminal T 2 . 
         [0015]      FIG. 10  is an impedance characteristic diagram for the variable impedance circuit Z 1 . As can be understood from the impedance characteristic diagram, the variable impedance circuit Z 1  has characteristics wherein the impedance (|Z|) is low in the low-frequency domain and the impedance (|Z|) is high in the high-frequency domain. That is, it has characteristics wherein the impedance is low for a DC electric current signal, and the impedance for an AC electric current signal is higher than the impedance for the DC electric current signal. The use of such a variable impedance circuit Z 1  makes it possible to reduce the voltage drop in the variable impedance circuit Z 1 , to thereby increase the voltage V 1  that is produced by the zener diode ZD 1 . 
         [0016]    However, in a positioner that uses this variable impedance circuit Z 1 , as can be understood also from the impedance characteristic diagram illustrated in  FIG. 10 , the characteristics are gradual at the transition point from the low frequencies wherein the impedance is low to the high frequencies wherein the impedance is high, and thus there is a problem in that it is susceptible to the effects of low-frequency noise. 
         [0017]    The present invention was created to solve such a problem, and the object thereof is to provide a positioner that is robust to the effects of low-frequency noise. 
       SUMMARY 
       [0018]    In order to achieve the aforementioned object, the example of the present invention is a positioner for receiving a DC electric current through a pair of electric wires from a higher-level system, generating a local operating power supply from the DC electric current signal, along with controlling a degree of opening of a regulator valve in accordance with a value of the DC electric current signal, and receiving an AC electric current signal that is superimposed on the DC electric signal current, including a variable impedance circuit wherein the impedance relative to the DC electric current signal is low and the impedance relative to the AC electric current signal is higher than the impedance relative to the DC electric current signal; wherein: the variable impedance circuit has an input line for a DC electric current signal and an AC electric current signal that is superimposed on the DC electric current signal; an output line for a DC electric current signal and an AC electric current signal that is superimposed on the DC electric current signal; a series circuit of a first resistor, a second resistor, and a third resistor, connected between the input line and the output line; a transistor having the collector thereof connected to the input line and the base thereof connected to connecting point between the second resistor and the third resistor; a fourth resistor connected between the emitter of the transistor and the output line; a first capacitor having one end thereof connected to connecting point between the second resistor and the third resistor; a fifth resistor connected between the other end of the first capacitor and the output line; a second capacitor having one end thereof connected to the connecting point between the first resistor and the second resistor; a sixth resistor connected between the other end of the second capacitor and the output line; a second transistor having the collector thereof connected to the input line and having the base thereof connected to the connecting point between the emitter of the first transistor and the fourth resistor; and a seventh resistor connected between the emitter of the second transistor and the output line. 
         [0019]    Given the present invention, the structure is one wherein a low-pass filter, wherein the time constant is determined primarily by the first resistor, the second capacitor, and the sixth resistor, is added to the low-pass filter, wherein the time constant is determined primarily by the second resistor, the first capacitor, and the fifth resistor, thus causing the characteristics of the transition point from the low frequencies wherein the impedance is low to the high frequencies wherein the impedance is high to be sharp, and thus robust to the effects of low-frequency noise. 
         [0020]    In the present invention, the variable impedance circuit may be structured omitting the second transistor and the seventh resistor. That is, the variable impedance circuit may be structured from: an input line for a DC electric current signal and an AC electric current signal that is superimposed on the DC electric current signal; an output line for a DC electric current signal and an AC electric current signal that is superimposed on the DC electric current signal; a series circuit of a first resistor, a second resistor, and a third resistor, connected between the input line and the output line; a transistor having the collector thereof connected to the input line and the base thereof connected to connecting point between the second resistor and the third resistor; a fourth resistor connected between the emitter of the transistor and the output line; a first capacitor having one end thereof connected to connecting point between the second resistor and the third resistor; a fifth resistor connected between the other end of the first capacitor and the output line; a second capacitor having one end thereof connected to the connecting point between the first resistor and the second resistor; and a sixth resistor connected between the other end of the second capacitor and the output line. Given this, the variable impedance circuit operates using only a single transistor, thus reducing the impedance on the DC frequency side, and making it possible to reduce the voltage drop in the variable impedance circuit. 
         [0021]    In the example of the present invention, one end of a second capacitor is connected to the connecting point between a first resistor and a second resistor, and a sixth resistor is connected between the other end of the second capacitor and the output line, and thus the structure is one wherein a low-pass filter wherein the time constant is determined primarily by the first resistor, the second capacitor, and the sixth resistor is added to the low-pass filter wherein the time constant is determined primarily by the second resistor, the first capacitor, and the fifth resistor, so that the characteristics of the transition point from the low frequencies wherein the impedance is low to the high frequencies wherein the impedance is high will be sharp, producing the effect of being robust to the effects of low-frequency noise. 
         [0022]    In the example of the present invention, the variable impedance circuit may be structured omitting the second transistor and the seventh resistor so that the variable impedance circuit operates using only a single transistor, reducing the impedance on the DC frequency side. This enables a further reduction in the voltage drop in the variable impedance circuit, which, in addition to the effect of being robust to low-frequency noise, adds also the effect of the ability to achieve double connections between the transmission path of the two transmission lines, or connection of another load. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is a diagram illustrating the configuration of an example of a positioner according to the present invention. 
           [0024]      FIG. 2  is a diagram illustrating the configuration of a variable impedance circuit used as an active load in the positioner in the above example. 
           [0025]      FIG. 3  is an impedance characteristic diagram of the variable impedance circuit used in the positioner in the above example. 
           [0026]      FIG. 4  is a diagram to which is added the flow of the electric current to assist in explaining the operation of the variable impedance circuit illustrated in  FIG. 9 . 
           [0027]      FIG. 5  is a diagram to which is added the flow of the electric current to assist in explaining the operation of the variable impedance circuit illustrated in  FIG. 2 . 
           [0028]      FIG. 6  is a diagram illustrating the configuration of a variable impedance circuit used as an active load in the positioner in another example. 
           [0029]      FIG. 7  is an impedance characteristic diagram illustrating the variable impedance circuit used in the positioner in the above example. 
           [0030]      FIG. 8  is a diagram illustrating components in a conventional positioner that can communicate with a higher-level system. 
           [0031]      FIG. 9  is a diagram illustrating the configuration of a variable impedance circuit used as an active load in the conventional positioner. 
           [0032]      FIG. 10  is an impedance characteristic diagram illustrating the variable impedance circuit used in the conventional positioner. 
       
    
    
     DETAILED DESCRIPTION 
       [0033]    Examples according to the present invention can be explained below in detail, based on the drawings.  FIG. 1  is a diagram illustrating the configuration of examples of a positioner according to the present invention. In this figure, codes that are the same as those in  FIG. 8  indicate identical or equivalent structural elements as the structural elements explained in reference to  FIG. 8 , and explanations thereof are omitted. 
         [0034]    In the below, an explanation is given regarding a positioner according to an example, and then regarding a positioner according to another example, but  FIG. 1 , which illustrates the overall configuration, is the same for both examples. 
         [0035]    In the positioner  200  according to this example, the variable impedance circuit ZA illustrated in  FIG. 2  is used as the active load Z. 
         [0036]    That is, with the line L 1  as the input line for the DC electric current signal and for the AC electric current signal that is superimposed on the DC electric current signal, and the line L 2  as the output line for the DC electric current signal and the AC electric current signal that is superimposed on the DC electric current signal, the variable impedance circuit ZA, as illustrated in  FIG. 2 , is connected as the active load Z between the input line L 1  and the output line L 2 . 
         [0037]    The variable impedance circuit ZA is structured from: a series circuit of a first resistor R 1 , a second resistor R 2 , and a third resistor R 3 , connected between the input line L 1  and the output line L 2 ; a first transistor (an NPN transistor) Q 1  with the collector thereof connected to the input line L 1  and the base thereof connected to the connecting point between the resistors R 2  and R 3 ; a fourth resistor R 4  that is connected between the emitter of the transistor Q 1  and the output line L 2 ; a first capacitor C 1  having one end thereof connected to the connecting point between the resistors R 2  and R 3 ; a fifth resistor R 5  connected between the other end of the capacitor C 1  and the output line L 2 ; a second capacitor C 2  having one end thereof connected to the connecting point between the resistors R 1  and R 2 ; a sixth resistor R 6  that is connected between the other end of the capacitor C 2  and the output line L 2 ; a second transistor (an NPN transistor) Q 2  having the base thereof connected to the connecting point between the emitter of the transistor Q 1  and the resistor R 4 , and a seventh resistor R 7  that is connected between the emitter of the transistor Q 2  and the output line L 2 . 
         [0038]    In this variable impedance circuit ZA, the voltage that is produced at the connecting point between the resistors R 2  and R 3 , that is, the voltage that is applied to the parallel circuit of the resistor R 3  with the series circuit of the capacitor C 1  and the resistor R 5 , is applied to the base of the transistor Q 1 , and the voltage that is produced at the connecting point between the emitter of the transistor Q 1  and the resistor R 4  is applied to the base of the transistor Q 2 . As a result, as the characteristic of the variable impedance circuit ZA, the impedance is low for a DC electric current signal and the impedance is height for an AC electric current signal. 
         [0039]    Moreover, in the variable impedance circuit ZA, one end of a capacitor C 2  is connected to a connecting point between resistors R 1  and R 2 , and a resistor R 6  is connected between the other end of the capacitor C 2  and the output line L 2 , so that the structure is one wherein a low-pass filter (LPF 2 ) wherein the time constant is determined primarily by the resistor R 1 , the capacitor C 2 , and the resistor R 6  is added to a low-pass filter (LPF 1 ) wherein the time constant is determined primarily by the resistor R 2 , the capacitor C 1 , and the resistor R 5 , causing the characteristics at the transition point between the low frequencies wherein the impedance is low and the high frequencies wherein the impedance is high to be sharp, as illustrated in the impedance characteristic diagram for the variable impedance circuit ZA in  FIG. 3 , thus increasing the robustness to the effect of low-frequency noise. 
         [0040]      FIG. 4  and  FIG. 5  are used to explain in specifics the characteristics wherein the impedance is high being sharp at the transition point between the low frequencies wherein the impedance is low and the high frequencies in the variable impedance circuit ZA.  FIG. 4  is a diagram to which is added the flow of the electric current to assist in explaining the operation of the conventional variable impedance circuit Z 1  illustrated in  FIG. 9 .  FIG. 5  is a diagram to which is added the flow of the electric current to assist in explaining the operation of the variable impedance circuit ZA of the example illustrated in  FIG. 2 . 
         [0041]    First, the operation will be explained based on the conventional variable impedance circuit Z 1 . The input electric current Iin_ac is branched into Iac_R 2 , Iac_C 1 , and Iac_Q 1 b depending on the frequency characteristics of the LPF 1 , which are determined primarily by the time constant of the resistor R 2 , the capacitor C 1 , and the resistor R 5 . The frequency characteristics are such that the Iac_C 1  is large and Iac_Q 1 b is small from the LPF cutoff frequency fc 1  point, with the result that with an input electric current of a frequency that is higher than fc 1 , the collector current of the transistor Q 1  (the AC component) can be small and the base current of the transistor Q 2  (the AC component) can be small, and the collector current of the transistor Q 2  (the AC component) can be small. This is equivalent to having a high AC impedance between the positive and negative terminals. 
         [0042]    In contrast, in the variable impedance circuit ZA in the present example, the structure is one wherein the LPF 2 , which is determined primarily by the time constant of the resistor R 1 , the capacitor C 2 , and the resistor R 6 , is added to the LPF 1  which is determined primarily by the time constant of the resistor R 2 , the capacitor C 1 , and the resistor R 5 . Here, when viewed from the point of the base of the transistor Q 1  wherein Iac_Q 1 b flows, the LPF 2  is connected in series with the LPF 1 , to structure a two-stage LPF from LPF 2  and LPF 1 . As a result, when compared to the single-stage LPF of the LPF 1  that is the conventional variable impedance circuit Z 1 , the characteristics can be those of the two-stage LPF wherein Iac_Q 1 b can be small at a high frequency above the cutoff frequency fc 1  that determines the amplification of Iac_Q 1 b, where this characteristic can be sharp. This is equivalent to the frequency characteristics wherein the AC impedance between the positive and negative terminals is high being sharp at a frequency above fc 1 . 
         [0043]    In the positioner  200  illustrated in  FIG. 1 , if, for example, the supply voltage from the double-wire transmission path is 15 V, and two of these positioners are connected in series, then the voltage between the input terminals T 1  and T 2  (the terminal voltage) will be 7.5 V. In this case, a minimum of 5 V is required for the power supply voltage for the internal circuitry, and thus only a voltage drop of up to 2.5 V is allowed for the active load Z. 
         [0044]    However, in the variable impedance circuit ZA illustrated in  FIG. 2 , the transistors Q 1  and Q 2  are connected in a Darlington structure, so that the impedance for the DC electric current signal (the impedance on the DC frequency side) can be large, thus making it impossible to have the voltage drop in the active load Z be less than 2.5 V. Because of this, it is not possible to have the terminal voltage that enables the operation of the positioner  200  (the minimum operating terminal voltage) be less than 7.5 V, preventing two positioners  200  from being doubly connected between the two-wire transmission lines. 
         [0045]    Given this, in the positioner  200  according to this example, the variable impedance circuit ZB that is illustrated in  FIG. 6  is used as the active load Z. 
         [0046]    The variable impedance circuit ZA is structured from: a series circuit of a first resistor R 1 , a second resistor R 2 , and a third resistor R 3 , connected between the input line L 1  and the output line L 2 ; a first transistor (an NPN transistor) Q 1  with the collector thereof connected to the input line L 1  and the base thereof connected to the connecting point between the resistors R 2  and R 3 ; a fourth resistor that is connected between the emitter of the transistor Q 1  and the output line R 4 ; a first capacitor C 1  having one end thereof connected to the connecting point between the resistors R 2  and R 3 ; a fifth resistor R 5  connected between the other end of the capacitor C 1  and the output line L 2 ; a second capacitor C 2  having one end thereof connected to the connecting point between the resistors R 1  and R 2 ; and a sixth resistor R 6  that is connected between the other end of the capacitor C 2  and the output line L 2 . 
         [0047]    In this variable impedance circuit ZB, as with the variable impedance circuit ZA illustrated in  FIG. 2 , one end of a capacitor C 2  is connected to the connecting point between the resistors R 1  and R 2 , and the resistor R 6  is connected between the other end of the capacitor C 2  and the output line L 2 . Because of this, structure is one wherein a low-pass filter (LPF 2 ), wherein the time constant is determined primarily by the resistor R 1 , the capacitor C 2 , and the resistor R 6 , is added to the low-pass filter (LPF 1 ), wherein the time constant is determined primarily by the resistor R 2 , the capacitor C 1 , and the resistor R 5 , thus causing the characteristics of the transition point from the low frequencies wherein the impedance is low to the high frequencies wherein the impedance is high to be sharp, as illustrated in the impedance characters diagram for the variable impedance circuit ZB, in  FIG. 7 , and thus robust to the effects of low-frequency noise. 
         [0048]    Moreover, in the variable impedance circuit ZB, the voltage that is produced at the connecting point between the resistors R 1  and R 2 , that is, the voltage that is applied to the parallel circuit of the resistor R 3  and the series circuit of the capacitor C 1  and the resistor R 5 , is applied to the base of the transistor Q 1 . As a result, as the characteristic of the variable impedance circuit ZB, the impedance is low for a DC electric current signal and the impedance is high for an AC electric current signal. 
         [0049]    While in the variable impedance circuit ZA illustrated in  FIG. 2 , the operation is through the transistors Q 1  and Q 2  that are connected in a Darlington configuration, in the variable impedance circuit ZB, the operation is through the single transistor Q 1 , alone, reducing the impedance on the DC frequency side (referencing  FIG. 7 ), further reducing the voltage drop in the variable impedance circuit ZB. 
         [0050]    For example, in the present example the voltage drop in the variable impedance circuit ZB is 1.3 V. Consequently, if, in the positioner  200 , the power supply voltage in the internal circuitry is 5 V, that is, if the voltage V 1  generated by the zener diode ZD 1  is 5 V, then the minimum operating terminal voltage for the positioner  200  would be 6.3 V. 
         [0051]    As a result, if, for example, the supply voltage from the double-wire transmission line is 15 V, then when two positioners  200  are doubly connected, the terminal voltage for the positioner  200  will be 7.5 V, but here the positioner  200  can operate even on the 7.5 V terminal voltage. The same is true when another load is connected in series with the positioner  200 , where this can be supported until the terminal voltage drops below 6.3 V. 
         [0052]    Moreover, in this variable impedance circuit ZB, as can be understood from a comparison with the variable impedance circuit ZA, illustrated in  FIG. 2 , the circuit structure is simplified, in a form wherein the transistor Q 2  and the resistor R 7  are eliminated, reducing the number of components, and thus achieving a reduction in costs. 
         [0053]    Note that while NPN transistors were used as the transistors Q 1  and Q 2  in the examples set forth above, the variable impedance circuits ZA and ZB may be structured instead using PNP transistors.