Abstract:
A switched current source has a first voltage source, a second voltage source, and a third voltage source. A first transistor has a drain terminal coupled to one terminal of a load and a source terminal coupled to the third voltage source. A second transistor has drain, gate and source terminals. The drain terminal of the second transistor is coupled to the gate terminal of the first transistor. The source terminal of the second transistor is coupled to the source terminal of the first transistor. The gate terminal of the second transistor is coupled to the first voltage source. A third transistor has drain, gate and source terminals. The drain terminal of the third transistor is coupled to the gate terminal of the first transistor. The source terminal of the third transistor is coupled to the second voltage source. The gate terminal of the third transistor is coupled to the first voltage source.

Description:
RELATED APPLICATION 
   This application is related to U.S. Provisional Application Ser. No. 60/721,940, filed Sep. 29, 2005, in the name of the same inventors listed above, and entitled, “MOSFET TRANSISTOR AMPLIFIER WITH CONTROLLED OUTPUT CURRENT”. The present patent application claims the benefit under 35 U.S.C. §119(e). 

   FIELD OF THE INVENTION 
   The invention relates to a MOSFET amplifier circuit and, more specifically, to a MOSFET transistor amplifier circuit having a controlled output current. 
   BACKGROUND OF THE INVENTION 
   In some MOSFET amplifier applications it is desirable to control the maximum output current that is sent to the load. This is of particular importance when the load is capacitive and it is desired to have a well controlled rate of change of the load voltage during transitions. A particular application with this requirement is the driving of transducers for ultrasonic imaging systems, where the harmonic content of the waveforms applied to the transducers is of primary importance. In these systems the transducer is typically a piezoelectric ceramic material, and appears predominantly as a capacitive load, with other reactive and resistive characteristics also present. 
   Therefore, it would be desirable to provide an improved MOSFET amplifier circuit. The improved MOSFET amplifier circuit would be able to control the maximum output current that is sent to the load of the MOSFET amplifier circuit. 
   SUMMARY OF THE INVENTION 
   In accordance with one embodiment of the present invention, a switched current source is disclosed. The switched current source has a first voltage source, a second voltage source, and a third voltage source. A first transistor has a drain terminal coupled to one terminal of a load and a source terminal coupled to the third voltage source. A second transistor has drain, gate and source terminals. The drain terminal of the second transistor is coupled to the gate terminal of the first transistor. The source terminal of the second transistor is coupled to the source terminal of the first transistor. The gate terminal of the second transistor is coupled to the first voltage source. A third transistor has drain, gate and source terminals. The drain terminal of the third transistor is coupled to the gate terminal of the first transistor. The source terminal of the third transistor is coupled to the second voltage source. The gate terminal of the third transistor is coupled to the first voltage source. 
   In accordance with another embodiment of the present invention, a switched current source is disclosed. The switched current source has a first voltage source, a second voltage source, and a third voltage source. A first transistor has a drain terminal coupled to one terminal of a load and a source terminal coupled to the third voltage source. A second transistor has drain, gate and source terminals. The drain terminal of the second transistor is coupled to the gate terminal of the first transistor. The source terminal of the second transistor is coupled to the source terminal of the first transistor. The gate terminal of the second transistor is coupled to the first voltage source. A third transistor has drain, gate and source terminals. The drain terminal of the third transistor is coupled to the gate terminal of the first transistor. The source terminal of the third transistor is coupled to the second voltage source. The gate terminal of the third transistor is coupled to the first voltage source. A complimentary mirror switched current source circuit is coupled to the load to act as a current sink and allowing the switched current source to be a bi-directional current source. 
   The foregoing and other objectives, features, and advantages of the invention will be apparent from the following, more particular, description of the preferred embodiment of the invention, as illustrated in the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, as well as a preferred mode of use, and advantages thereof, will best be understood by reference to the following detailed description of illustrated embodiments when read in conjunction with the accompanying drawings, wherein like reference numerals and symbols represent like elements. 
       FIG. 1  is a simplified schematic of a circuit for controlling the drain current of a MOSFET used in an amplifier. 
       FIG. 2  is a simplified schematic of a current controlled bidirectional output stage. 
       FIG. 3  is a simplified schematic of a current controlled bidirectional output stage with diodes. 
       FIG. 4  is a simplified schematic of a voltage regulator used in the current controlled bidirectional output stage of  FIGS. 2 and 3 . 
   

   DESCRIPTION OF PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , a circuit  10  for controlling the drain current of a MOSFET used in an amplifier is shown. The transistor M 1  is the signal amplifying transistor, with its power being provided by voltage source V 3  and the signal voltage being developed across the terminals of load  5 . The drain current I 1  through the load  5  produces the signal voltage. Gate drive for M 1  is provided by a CMOS inverter realized with devices M 2  and M 3 . Power for the CMOS inverter is provided by supply V 2 . 
   When the input signal V 1  at node  2  is greater than or equal to V 2  at node  1 , transistor M 2  is in a conducting state and transistor M 3  is not conducting. As a result the inverter output voltage at node  3  will be at the source potential of transistor M 1 . In this state, the output transistor M 1  is off, and the drain current I 1  through the load is zero. When the input signal V 1  at node  2  is near the source potential of transistor M 2 , the inverter state is reversed. Transistor M 3  is in a conducting state, and transistor M 2  is not conducting. The resulting inverter output voltage at node  3  is then essentially the same as node  1 . In this state, the output transistor M 1  is in its conducting state, with the maximum value of drain current set by the value of the gate-source voltage derived from node  3  and node  11 . Therefore variation of the voltage of the supply V 2  can be used to control the maximum drain current of transistor M 1 , while the on or off state of M 1  is controlled in turn by the independent input signal V 1 . The circuit will function correctly as long as the peak value of V 1  is equal to or greater than V 2 . 
   The importance of this design is that the circuit  10  permits the maximum output current of the output transistor M 1  to be determined by a signal or voltage V 2  which is separate from the input signal V 1 . In turn, V 1  determines whether the output is conducting or not. Therefore a voltage regulator may be used as a source of V 2  to set the conduction properties of the output transistor M 1  without knowledge of the actual signal waveform that is being amplified by M 1 . In these systems, the transistor M 1  is typically either in a fully conducting state or a non-conducting state. M 1  is not required to provide linear amplification or partial conduction. 
   In applications where the load contains a resistive or DC conductive component, the circuit  10  of  FIG. 1  will operate correctly. However, if the load is capacitive or will not conduct a DC current, a separate means must be provided for conducting the average current drawn by the drain of M 1 . 
   Referring to  FIG. 2 , a bi-directional amplifier stage  20  that overcomes the above problem is shown. The circuit  20  will thus conduct an average current drawn by the drain of M 1  if the load is capacitive or will not conduct a DC current. In the circuit  20 , a second amplifier  10 A that is complementary to the amplifier  10  of  FIG. 1  is added using M 4  as its output transistor, and M 5  and M 6  as the source of gate drive voltage for M 4 . By this method when M 1  conducts, its drain current I 1  sinks current out of the load  5 , and when M 4  conducts, its drain sources current to the load  5 . Thus, the composite current I 3  to the load, which is I 2  minus I 1 , becomes a bi-directional current capable of both charging and discharging a capacitive or capacitively coupled load which will not pass significant DC current. 
   In  FIG. 2 , the transistors M 1 , M 2 , and M 3  and sources V 1 , V 2 , and V 3  operate in a manner similar to that described previously for  FIG. 1 . The power source V 3  is located at the source side of M 1  for convenience. The circuit portion containing M 4 , M 5 , M 6 , V 4 , V 5 , and V 6  is symmetrically constructed to be able to source current I 2  into the load for bi-directional operation. When V 4  is greater than V 5 , the output of the CMOS inverter formed by M 5  and M 6  at node  8  is at the potential V 6  of node  12 , causing transistor M 4  to be in a non-conducting state. When V 4  is near zero, the output of the CMOS inverter formed by M 5  and M 6  at node  8  is at the potential of (V 6 -V 5 ), applying the voltage V 5  to the gate-to-source terminals of transistor M 4 . Thus the maximum drain current in this case is set by the conduction properties of M 4  with the voltage V 5  applied to its gate-to-source terminals. 
   The overall performance of  FIG. 2  is that when input V 1  is zero, transistor M 1  turns on with a drain current limited by the gate drive from the voltage V 2 , and when input V 4  is zero, transistor M 4  turns on with a drain current limited by the gate drive from the voltage V 5 . In both cases, if the other transistor M 1  or M 4  is not conducting, and the load is capacitive or draws a sufficiently small current, the output voltage at  9  will ultimately go to a value near the available supply voltages. When M 1  conducts, the limiting voltage is set by V 3 , and M 1  in that case functions in its substantially resistive operating mode. Conversely, when M 4  conducts, the limiting voltage is set by V 6 , and M 4  in that case functions in its substantially resistive operating mode. If both M 1  and M 4  are conducting, then the voltage and current applied to the load terminal is in an in-between state. 
   During the transitions of the voltage at node  9  from M 1  to M 4  conducting or the reverse, the voltage rate of change (slew rate) at node  9  is determined primarily by the load capacitance component and the drain currents of M 1  and M 4 . Therefore control of the voltages V 2  and V 5  will in turn control the slew rate at node  9 . The timing of the voltage changes at node  9  will be determined by the input control signals V 1  and V 4 , independent of the slew rates. 
     FIG. 3  shows a variation of the circuit of  FIG. 2  in which a pair of diodes D 1  and D 2  is added to the circuit. The diodes D 1  and D 2  permit the load voltage at node  10  to exceed the supply voltages V 3  and V 6 . In this case, it is possible to connect several drive circuits similar to  FIG. 3  with their outputs at node  10  in parallel. The driver with the highest supply voltage and a conducting output transistor will have a conducting diode connecting it to the load, and the other diodes will be non-conducting. A system configuration of this type is capable of generating an output pulse train containing several voltage levels, including zero. To get an approximately zero output voltage level, the supplies V 3  and V 6  of the associated driver section are set to zero. The jumper JP 1  may be used to short the nodes  9  and  13  together for improved dynamic response in the highest output voltage driver section. 
   Since the conduction properties of transistors M 1  and M 4  are a function of the fabrication process used and its variations, as well as device size and temperature, means may be employed to remove the effects of these variations from the output waveform. This is conveniently done in  FIG. 2  by providing voltage regulators to function as the sources for V 2  and V 5 . Circuitry may then be built into these regulators as known in the state of the art to cause the regulator output voltage to vary with temperature. Additionally, the regulator outputs may also be made to vary in a manner similar to the properties of transistors M 1  and M 4 . For example, if a transistor similar to M 1  is incorporated into the regulator for V 2 , then when the conduction properties of M 1  vary, the regulator output voltage V 2  will vary in a way to cancel the effect. Use could additionally be made of the monolithic matching of similar transistors fabricated in the same integrated circuit to achieve this effect. The regulator does not have to be fabricated in the same circuit as the transistor M 1  as long as it is able to properly respond to the environmental influences that cause M 1  to change. 
   As an example, consider the regulator structure of  FIG. 4 . In this circuit, a reference current I 4 , generated by a circuit well known in the state of the art, is used to power a series string composed of resistor R 1  and transistor M 7 . The reference current may incorporate a non-zero temperature dependency if desired, or be stable with temperature. Likewise, resistor R 1  may be either constant or a function of temperature. The voltage at node  14  will contain variations which are chosen to mimic the variations present in the output transistors M 1  or M 4  as a function of temperature and fabrication process variations. In addition, the voltage drop across resistor R 1  adds to the voltage at node  14  to determine a voltage at node  15  such that the drain current of transistor M 1  or M 4  will have the desired characteristics. Normally resistor R 1 , current  14 , and transistor M 7  will all vary with temperature. Proper choice of each of their characteristics will enable the drain current of M 1  or M 4  to be made stable with temperature and process variations, so that the overall system performance will not vary. 
   Additionally, a means may be provided for adjustment of the value of the resistor R 1  or transistor M 7  after the circuit is fabricated. In this way, calibration may be done to match the regulator and bipolar driver performance to the desired overall system requirements. 
   As shown in  FIG. 4 , in order to provide a voltage source to the driver circuits in  FIGS. 2 and 3 , an amplifier  16  with a gain of A and a low output impedance is added to the voltage source made with I 4 , R 1  and M 7 . The low output impedance of the amplifier  16  provides the current needed to drive the capacitance of the gate of transistors M 1  and M 4  at the signal switching frequency. Voltage gain may also be provided to scale the voltage at node  15  to the value needed at node  17  for V 2  or V 5 . For V 5 , the circuit may be built in complementary form with a PMOS transistor for M 7 , and the polarity of I 4  reversed. 
   Any other voltage source, regulator, or amplifier means as is known in the state of the art may be alternatively employed to produce the voltages V 2  and V 5 . 
   For the circuits of  FIGS. 2 and 3 , a pair of voltage regulators of function similar to  FIG. 4  is required. These are then used as the voltage sources for V 2  and V 5 . The two voltage regulator circuits may have different characteristics as needed to match the corresponding output transistors M 1  and M 4 . The overall system result is that the drain currents of M 1  or M 4  are made stable with temperature and process variations as desired. It is also possible to make the voltage sources V 2  and V 5  vary so that the operation of the output transistors M 1  and M 4  is not stable, but varies in a desired and predictable way for some other system purpose. 
   While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.