Abstract:
An electronic output module, especially an electronic output module for CMOS-LVDS levels (LVDS-low voltage differential signalling), is suitable for analogue and digital high-frequency circuits. The output module has a first and a second transistor which are connected to a current source at a first connection and to input terminals by a control connection. A third and a fourth transistor are connected to a supply voltage potential by a first connection, to a second connection of the first and second transistors, and to an output terminal by a second connection and to converted input signals by a control connection.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
         [0001]    This application is a continuation of copending International Application No. PCT/DE01/01234, filed Mar. 28, 2001, which designated the United States and which was not published in English.  
         BACKGROUND OF THE INVENTION  
       Field of the Invention  
         [0002]    The invention lies in the fields of electronics and electronic circuits. More specifically, the invention relates to an electronic output stage, particularly to an output stage for CMOS LVDS (Low Voltage Differential Signaling) levels for use in analog and digital high-frequency circuits.  
           [0003]    The publication “Makrozellen für serielle Gbit/s Schnitt-stellen in 0.35 μm CMOS” (Tagungsband 8, ITG Fachtagung 3.-4.3.98 Hannover, pp.107-112, ITG Fachbericht 147) describes a circuit for an electronic output stage for digital signals with a large-signal control. The output stage comprises a controlled push-pull stage whereby a high-level signal and a low-level signal can be independently controlled. The control is carried out i) individually for the high and low levels by means of analog control voltages from a reference block and a digital scaling, or ii) only by means of digital scaling. The digital scaling can be set by way of a computer interface. The prior art circuit has the disadvantage that a full CMOS swing (e.g., 3.3. or 5 Volts) and an expensive control for the bias currents of the provided differential amplifier stages are needed for drive purposes. In the prior art circuit, the transistors of the electronic output stages are used as switches, and therefore they can be operated in an unsaturated state.  
         SUMMARY OF THE INVENTION  
         [0004]    It is accordingly an object of the invention to provide an electronic output stage for CMOS LVDS levels which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and wherein the electronic output stage can be driven with small signals, preferably with high-frequency small signals.  
           [0005]    With the foregoing and other objects in view there is provided, in accordance with the invention, an electronic output stage for amplifying differential input signals with a small voltage swing, in particular high-frequency input signals. The novel output stage comprises:  
           [0006]    a first input terminal, a second input terminal, a first output terminal, and a second output terminal;  
           [0007]    a controlled current source;  
           [0008]    a first transistor having a first terminal connected to the controlled current source, a second terminal, and a control terminal connected to the first input terminal;  
           [0009]    a second transistors having a first terminal connected to the controlled current source, a second terminal, and a control terminal connected to the second input terminal;  
           [0010]    a third transistor having a first terminal connected to a first supply voltage potential, a second terminal connected to the second terminal of the first transistor and to the first output terminal;  
           [0011]    a fourth transistor having a first terminal connected to the first supply voltage potential, a second terminal connected to the second terminal of the second transistor and to the second output terminal;  
           [0012]    the control terminals of the third and fourth transistors receiving respective drive signals formed from the input signals by amplifying and providing the input signals with an adjustable offset voltage; and  
           [0013]    wherein a voltage change of the input signals and drive signals at the first and third transistors and at the second and fourth transistors, respectively, behave in opposite ways.  
           [0014]    In accordance with an added feature of the invention, at least one of the output voltages of the output terminal is adjustable with the offset voltage, and a current flowing through the first and second output terminals and an output resistance is adjustable with the current source and a dimensioning of the third and fourth transistors.  
           [0015]    In accordance with an additional feature of the invention, the first and second transistors have equal dimensions, and the third and fourth transistors have equal dimensions. Preferably, also, the first, second, third, and fourth transistors are all MOS transistors.  
           [0016]    In accordance with another feature of the invention, there is provided a differential amplifier circuit for setting the offset voltage such that the third and fourth transistors are at least partly conductive.  
           [0017]    In accordance with a further feature of the invention, the current source is set such that an output current equals approximately 3.5 mA. The third and fourth transistors are preferably dimensioned such that an output resistance between the first and second output terminals equals approximately 50 ohms. That is, with suitable dimensioning of the electronic elements of the output stage, it is possible to realize an output resistance of approx. 50 ohms, which is required by the LVDS standard and strongly recommended for high-frequency drivers.  
           [0018]    In accordance with again an added feature of the invention, the offset voltage is set to result in a voltage offset value between the first and second output terminals of approximately 1.2 V.  
           [0019]    In accordance with again an additional feature of the invention, there is provided a voltage control circuit for regulating the offset voltage. Also, there may be provided a differential amplifier circuit having a voltage supply. In that case, the offset voltage is regulated by controlling the voltage supply to the differential amplifier.  
           [0020]    In accordance with yet an added feature of the invention, there is provided a mirror circuit for setting a current through the first and second output terminals and a voltage between the first and second output terminals, the mirror circuit comprising two input terminals carrying respective reference voltages and the mirror circuit controlling the current source and the offset voltage such that a low level at one of the output terminals and a high level at another of the output terminals correspond to each of the reference voltages, respectively.  
           [0021]    In accordance with yet a further feature of the invention, a high level of the output voltage is set with the regulated offset voltage. A low level of the output voltage is set with the current impressed by the current source.  
           [0022]    The primary advantage achieved by the invention compared to the prior art is that the proposed electronic output stage can be driven with high-frequency small signals (&lt;1V). This is advantageous particularly when amplifier circuits are integrated wherein small swings predominate.  
           [0023]    Further advantages over the prior art are a reduced power loss and an ability to forgo the controlling of the current sources.  
           [0024]    Another advantage of the electronic output stage compared to pure digital CMOS circuits is that there are no high-frequency voltage and current peaks which could interfere with the sensitive analog circuits. In the proposed circuit, a substantially constant current is generated in output drivers by the current sources. The transistors are driven in the saturated state.  
           [0025]    Furthermore, interference is prevented by the reduced input swing.  
           [0026]    Other features which are considered as characteristic for the invention are set forth in the appended claims.  
           [0027]    Although the invention is illustrated and described herein as embodied in an electronic output stage, it is nevertheless not intended to be limited to the details shown, since 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.  
           [0028]    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  
       [0029]    [0029]FIG. 1 is a schematic diagram of an LVDS output stage according to the invention; and  
         [0030]    [0030]FIG. 2 is a schematic block diagram of the LVDS output stage according to FIG. 1 with a mirror circuit.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0031]    Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is shown an LVDS output stage. A differential input voltage V IN  drives an output stage comprising n-channel MOS transistors T 6 , T 7  and T 8 . The transistors T 6  and T 7  are identically constructed and connected as a differential amplifier DV 1 ; that is, their source terminals T 6 S, T 7 S are connected to each other and to the transistor T 8 , which is a controlled current source T 8  that impresses a current I 2 . The current I 2  is set with a suitable voltage at the control terminal T 8 G of the transistor T 8 , so that the current I 2  equals somewhat more than 3.5 mA. The non-inverting input V IN+  is connected to the control terminal T 6 G of the transistor T 6 . The inverting input V IN−  is connected to the control terminal T 7 G of the transistor T 7 . An output signal V OUT  is tapped off at the drain terminals T 6 D, T 7 D of the transistors T 6  and T 7 . The inverted output signal V OUT+  is thereby tapped at the drain terminal T 6 D of the transistor T 6 , and the non-inverted output signal V OUT−  is tapped off at the drain terminal T 7 D of the transistor T 7 .  
         [0032]    A source terminal T 4 S of an n-channel MOS transistor T 4  is connected to the drain terminal T 6 D of the transistor T 6 . A source terminal T 5 S of an n-channel MOS transistor T 5  is connected to the drain terminal T 7 D of the transistor T 7 . The drain terminals T 4 D, T 5 D of the likewise identically constructed transistors T 4  and T 5  are connected to a supply voltage potential V DD . A differential signal from a differential amplifier DV 2  is present at the control terminals T 4 G, T 5 G of the transistors T 4  and T 5 , respectively.  
         [0033]    The differential amplifier DV 2  comprises three n-channel MOS transistors T 1 , T 2 , T 3 , two equivalent resistors R 1 , and a voltage regulating circuit V CC . The differential amplifier DV 2  serves for amplifying the input signals V IN+  and V IN− , inverting them, and providing them with a voltage offset. In the differential amplifier DV 2 , the source terminals T 1 S, T 2 S of the transistors T 1  and T 2  are connected to each other and to a current source whose current I 1  is determined with the aid of a control terminal T 3 G of the transistor T 3 . The drain terminal T 1 D of the transistor T 1  is connected to the control terminal T 4 G of the transistor T 4  and a terminal of one of the two equivalent resistors R 1 ; the drain terminal T 2 D of the transistor T 2  is connected to the control terminal T 5 G of the transistor T 5  and to a terminal of the other of the two equivalent resistors R 1 . A voltage V Ghigh  which is determined by the voltage regulating circuit V CC  is connected to the other respective terminal of the two equivalent resistors R 1 . By setting V Ghigh , the identical resistances R 1 , and the current I 1  in the differential amplifier DV 2 , a high and low level of signals VR 1 + and VR 1 − pending at the drain terminals T 1 D, T 2 D of the transistors T 1  and T 2 , respectively, can be precisely set, whereby V Ghigh  represents the high level and V Glow =V Ghigh− −R 1 ·I 1  represents the low level. By means of these high and low levels from the differential amplifier DV 2 , the desired output voltage V OUT  at the output of the output stage can be set (taking into consideration the gate-source voltage V GS  of the transistors T 4  and T 5 ).  
         [0034]    According to the LVDS specifications, a voltage offset or swing at the output V OUT  of the output stage should equal 1.2 V. A voltage swing of 350 mV results from an output current I OUT  of approximately 3.5 mA such as is required for LVDS stages and a load resistance R OUT  of 100 ohms which is common for LVDS. In the case of a fully blocking transistor T 4  or T 5 , the lower level of the output signal is determined by the voltage drop across the current source T 8 . Since one of the transistors T 4  or T 5  is currentless in this case, the output resistance R OUT  remains high. If the current I 2  is selected higher than V OUT /R OUT  (3.5 mA) with the aid of a suitable voltage at the control terminal T 8 G of the transistor T 8 , the respective transistor T 4  or T 5  (the one with the lower voltage level) remains partially conductive as long as a voltage V contr =V Ghigh −R 1 ·I 1  is set. A corresponding output resistance R OUT  can be set by the allocation, to the current source T 8 , of a value of the current I 2  which ensures that the current I R =I 2 −I A  flows through the respective transistor T 4  or T 5  at which the lower control signal is present. The output resistance R OUT  is thus defined by the voltage drops across the source and drain terminals T 4 S, T 5 S, T 4 D, T 5 D of the transistors T 4  and T 5  and the partial currents I R , I OUT  flowing through them. The output resistance R OUT  of approx. 50 ohms that is needed for LVDS signals can be realized by means of dimensioning the transistors T 4  and T 5  and setting the current I 2 .  
         [0035]    [0035]FIG. 2 also represents a mirror circuit in addition to the output stage represented in FIG. 1. The mirror circuit simulates the output stage and delivers a suitable V Ghigh  and a suitable control signal for setting the current I 2  to the control terminal T 8 G of the current source T 8 . The mirror circuit comprises a transistor T 4 ′, which is the same type as the transistors T 4  or T 5 ; a transistor T 8 ′ which is the same type as the transistor T 8  forming the current source; a resistor R F  of 100 ohms which is equal to the load resistance R OUT ; and two operational amplifiers OP 1  and OP 2 .  
         [0036]    The drain terminal T 4 ′D of the transistor T 4 ′ is connected to a supply voltage potential V DD . The source terminal T 4 ′S of the transistor T 4 ′ is connected to a terminal of the resistor R F . The other terminal of the resistor R F  is connected to the drain terminal T 8 ′D of the transistor T 8 ′. The source terminal T 8 ′S of the transistor T 8 ′ is applied to ground. A reference voltage input for the high level V VRefHigh  is connected to a non-inverting input of the operational amplifier OP 1 . The inverting input of the operational amplifier OP 1  is connected to the source terminal T 4 ′S of the transistor T 4 ′. The output of the operational amplifier OP 1  is adjacent the control input T 4 ′G of the transistor T 4 ′ and, as V Ghigh , the two equivalent resistors R 1  of the differential amplifier DV 2 . The drain terminal T 8 ′D of the transistor T 8 ′ is connected to the non-inverting input of the operational amplifier OP 2  at whose inverting input the reference voltage for the low level V RefLow  is applied. The output of this operational amplifier OP 2  is connected to the control input T 8 G of the transistor T 8  which forms the current source I 2 , and to the control input T 8 ′G of the transistor T 8 ′ in the mirror circuit.  
         [0037]    The mirror circuit works in such a way that the reference voltages are present at the resistor R F ; with V RefHigh  pending at an R F1  terminal of the resistor R F ; and V RefLow  pending at an R F2  terminal of the resistor R F . The operational amplifier OP 1  with the transistor T 4 ′ represents a regulating element which sets the control voltage at the terminal R F1  of the resistor R F  such that the voltage potential at the terminal R F1  corresponds exactly to V RefHigh− . Because the two transistors T 4  and T 4 ′ are identical and connected parallel to one another, the level V RefHigh−  is also present at the output resistance R OUT . Likewise, the operational amplifier OP 2  and the transistor T 8 ′ represent a regulating element, which holds the voltage potential at a terminal R F2  of the resistor R F  exactly at V RefLow  by the impressing of a defined current via the control terminal T 8 ′G of the transistor T 8 ′, which current brings about a defined voltage drop across the resistor R F . The voltage at the control terminal T 8 ′G of the transistor T 8 ′ is likewise applied to the transistor T 8  of the current source, with the result that the same current flows through the transistor T 8  as flows through the transistor T 8 ′.  
         [0038]    The tables 1 and 2 represent states and levels of the transistors and signals (FIG. 1):  
                                           TABLE 1                       Signal/                                   Transistor   V IN+     V IN−     T1   T2   VR1+   VR1−   T4*                   State/   high   low   conduc-   block-   low   high   block-       Level           tive   ing           ing*       State/   low   high   block-   conduc-   high   low   conduc-       Level           ing   tive           tive                          
 
         [0039]    [0039]                                   TABLE 2                       Signal/                           Transistor   T5   T6   T7   V OUT+     V OUT−                     State/Level   conductive   conductive   blocking   low   high       State/Level   blocking   blocking   conductive   high   low                    
         [0040]    By virtue of the mostly integrated design, identical transistors can be fabricated on a substrate; however, the parameters can be set only with large tolerances owing to process fluctuations.  
         [0041]    With the aid of the described mirror circuit, V Ghigh  and the control voltage of the transistor T 8 , and therefore VG low , can be set without knowledge of the precise parameters of the transistors.  
         [0042]    In order to minimize the power consumption of the mirror circuit, it is possible to correspondingly scale the transistors and the resistor to a scaling factor.