Abstract:
Described is a high voltage, high current operational amplifier in which galvanic separation of the input and output stages is achieved by means of an optical bridge, consisting of an LED and a photoresistor or phototransistor. The output stage utilizes two current sources, connected with inverse polarity and controlled by the optical bridge, thus allowing the transition between the low-voltage input stage and the high voltage output stage in a single step. Depending on the exact embodiment of the amplifier this invention can be customized to function with the properties of a Class A, B or C operational amplifier and can, in each of these embodiments, be used as an isolation amplifier.

Description:
FIELD OF THE INVENTION 
     This invention relates to a high voltage operational amplifier, and more particularly to a high-output voltage amplifier in which the input stage and output stage are electrically separated. 
     BACKGROUND OF THE INVENTION 
     An typical operational amplifier amplifies a voltage difference on the inputs to generate a desired output voltage. If the desired output voltage exceeds 50 V, the design of the amplifier typically cascades multiple transistors to reach the required control voltage for the output stage. This design leads to increased signal propagation times and results in intermodular distortions. In case of thermal overload or other component failure like the breakdown of a junction, a voltage breakdown between the input stage and output stage can occur, jeopardizing the low-voltage circuitry. 
     Thus the aim of this invention is to present a circuit design for the control of high voltages which will have short response times to the change of an input signal as well as providing effective electrical separation between the input and output circuitry. 
     SHORT DESCRIPTION 
     This invention relates to a high voltage operational amplifier in which the output stage is formed from one or two current sources controlled by opto-electronical means. The linearity of the output stage is achieved by means of a feedback loop. If the target output voltage is independent of the high voltage output, e.g. by means of generating the low-voltage feedback by means of a mechanical sensor, then this high voltage operational amplifier can be used as an isolation amplifier. 
     This invention allows the production of high voltage amplifiers for different voltage and current ranges with largely unchanged internal layout. The circuit as described by this invention is proof against short circuiting and guarantees, by merit of the electrical separation of input and output, the highest possible protection of circuitry connected to the input stage. This invention is suitable for mirror focusing controls, piezo controls, high-voltage power supplies, etc. 
    
    
     SHORT DESCRIPTION OF FIGURES 
     FIG. 1 shows an electrical schematic diagram of a current source as used in a typical embodiment of the current invention, formed with a N-channel PMOSFET transistor and an optical coupling consisting of a LED and a phototransistor (or photoresistor). 
     FIG. 2 shows an electrical schematic diagram of a complementary current source as used in a typical embodiment the current invention, formed with a P-channel PMOSFET transistor and an optical coupling consisting of a LED and a phototransistor (or photoresistor). 
     FIG. 3 shows an electrical schematic diagram of a first embodiment of a high-voltage amplifier (class AI) according to the invention in a typical load configuration. 
     FIG. 4 shows an electrical schematic diagram of a second embodiment of a high-voltage amplifier (class BI) according to the invention in a typical load configuration; 
     FIG. 5 shows an electrical schematic diagram of a third embodiment of a high-voltage amplifier (class CI) according to the invention in a typical load configuration. 
     FIG. 6 shows a more detailed circuit diagram of the first embodiment of a high voltage amplifier (class AI) according to the invention. 
     FIG. 7 shows the voltage response at the amplifier output of FIG. 6 as response to a rectangle signal applied to the amplifier input. 
     FIG. 8 shows the current responses at transistor T 1  and resistor RP of FIG. 6 as response to a rectangle signal applied to the amplifier input. 
     FIG. 9 shows a more detailed circuit diagram of the third embodiment of a high voltage amplifier (class CI) according to the invention. 
     FIG. 10 shows the voltage response at the amplifier output of FIG. 9 as response to a rectangle signal applied to the amplifier input 
     FIG. 11 shows the current responses at transistors T 1  and T 2  of FIG. 9 as response to a rectangle signal applied to the amplifier input 
    
    
     DETAILED DESCRIPTION 
     Three typical embodiments of the invention will be described to explain the functioning of the invention. In the following description of FIGS. 1,  2 ,  3 ,  4  and  5 , equal abbreviations denote identical elements. 
     (1) Current Sources 
     For the description of these exemplary circuit designs, the current transfer ratio (CTR) between the LED diode and the phototransistor is assumed to be 100%. The physical properties of the circuit elements are idealized, e.g. the forward voltage of the LED diode is assumed to be zero V. 
     (1a) Current Source Using N-channel Power-MOSFET 
     The optically controlled current source shown in FIG. 1 is formed using an N-channel PMOSFET transistor T 1 , a Zener-diode D 1 , a resistor R 1 , an LED D 3  and a phototransistor PH 1 . When the voltage V GS  of the transistor T 1  is approximately constant (about 5 V), the voltage at the phototransistor PH 1 , U CE , is also approximately constant U CE ≈U D1−V   GS . It follows that for CTR=100% the current across PH 1  is equal to the current across the LED diode D 3 . 
     (1b) Complementary Current Source Using P-channel Power-MOSFET 
     The optically controlled current source shown in FIG. 2 is formed using a P-channel PMOSFET transistor T 2 , a Zener diode D 2 , a resistor R 2 , an LED D 4  and a phototransistor PH 2 . When the voltage V GS  of the transistor T 2  is approximately constant (about 5 V), the voltage at the phototransistor, U CE , is also approximately constant U CE ≈U D1 −V GS . It follows that for CTR=100% the current across PH 2  is equal to the current across the LED-diode D 4 . 
     (2) High Voltage Operational Amplifier, Class AI 
     In a first embodiment of the amplifier described by this invention, shown in FIG. 3 in a typical load configuration with resistors RA and RB to define the amplification and a capacitive load CL, the current source described in  1   a  in combination with the resistor RP and the differential amplifier OP 1  form a high voltage amplifier of Class AI (similar to an output stage Class A) with the properties of a high voltage operational amplifier. Due to the linear dependence of photocurrent in PH 1  on the current across the photodiode D 3 , the maximum output current available from the output stage is given by the supply voltage of the operational amplifier OP 1  divided by R 3  and multiplied by CTR. The maximum output voltage is defined by V DS  of the PMOSFET transistor T 1 . 
     Starting with both inputs at equal potential of 0 V, the output voltage of the differential amplifier is also 0 V. The current of the LED-diode D 3  and the current source are equal to:            0   -     (     -   _V     )       R3     =       I   D3     =       I   PH1     =     I   T1                                
     and the output voltage of the high voltage operational amplifier is 
     
       
           U   out =+ —   HV−RP*I   T1 =0 
       
     
     Upon change of the input voltage, the differential amplifier OP 1  is initially saturated and its output voltage will reach −_V or +_V. In the former case the current source is turned off and the capacitive load CL is charged across RP until the desired output voltage is reached. In the latter case the capacitive load CL is discharged by the current source until the desired output voltage is reached. The output voltage defined by the feedback loop is:          U   out     =       -     U   in       ·     RB   RA                              
     In the output stage the following equations hold:          I     T   1       =           HV   -     U   out       RP     -         U   out     RB                   or                   U   out         =       RB     RP   +   RB       ·     (     HV   -     RP   ·     I     T   1           )                                
     For capacitive loads the output voltage of the high voltage operational amplifier changes exponentially. There are, however, applications where this is not a problem and this is a cost effective way of creating a high-voltage operational amplifier. To achieve higher maximum current, the phototransistor can be replaced with a Darlington phototransistor, e.g. with a CTR of 1200%. 
     The connection of the N-channel PMOSFET transistor, T 1 , in this circuit is realized as a common gate connection, which is extremely fast. Thus the properties of the high voltage operational amplifier are largely dependent on the choice of the differential amplifier OP 1 . The high-voltage operational amplifier is then free of intermodular distortions and there is no danger of an internal high-voltage breakdown. If the target output voltage is independent of the high voltage output, e.g. due to generating the low-voltage feedback by means of a mechanical sensor, then this high voltage operational amplifier can be used as an isolation amplifier. 
     (3) High Voltage Operational Amplifier, Class BI 
     In a second embodiment of the amplifier described by this invention, shown in FIG. 4 in a typical load configuration with resistors RA and RB to define the amplification and a capacitive load CL, the resistor RP of a class AI high voltage operational amplifier as described above is replaced by an arbitrary current source to form a class BI output stage (similar to output stage, Class B). In a possible embodiment this could be achieved using a bipolar transistor to deliver a constant current of, e.g. 5 mA. As a result of this approach, the output voltage of the high voltage operational amplifier will change linearly with the change of input voltage. In this embodiment the output stage is less noisy than a Class AI output stage. At the current state of the art, a class BI high-voltage operational amplifier constructed using the principles described by this invention is well suited for voltages up to ca. 400 V and high currents. If the target output voltage is independent of the high voltage output, e.g. due to generating the low-voltage feedback by means of a mechanical sensor, then this high voltage operational amplifier can be used as an isolation amplifier. 
     (4) High Voltage Operational Amplifier, Class CI 
     In a third embodiment of the amplifier described by this invention, shown in FIG. 5, in a typical load configuration with resistors RA and RB to define the amplification and a capacitive load CL, a high voltage operational amplifier of Class CI is formed, in which two complementary current sources, as described in (1) above, are connected with opposite polarities (similar to the push-pull stage of a Class C output stage). The two current sources form, together with the differential amplifier, OP 1 , a high voltage amplifier of Class CI, with the properties of a high voltage operational amplifier. 
     Due to the linear dependence of photocurrent in PH 1 (PH 2 ) on the current across the photodiode D 3  (D 4 ), the maximum negative (positive) output current available from the output stage is given by the supply voltage of the operational amplifier OP 1  divided by R 3  (R 4 ) and multiplied by CTR. The maximum output voltage is defined by V DS  of the PMOSFET transistor T 1  (T 2 ). 
     Starting with both inputs at equal potential of 0 V, the output voltage of the differential amplifier is 0 V. The currents of the LED diodes, D 3  and D 4 , and the current sources are given by:                (     +   _V     )     -   0     R4     =       I   D4     =       I   PH2     =       I   T2                   and                                  0   -     (     -   _V     )       R3     =       I   D3     =       I   PH1     =     I   T1                                
     Upon change of the input voltage, the differential amplifier is initially saturated and its output voltage reaches −_V or +_V. In both cases one of the current sources is shut down and the other produces its maximum current. The capacitive load CL is charged or discharged at the constant maximum current of the output stage until the desired voltage is reached. The output voltage defined by the feedback loop is:          U   out     =       -     U   in       ·     RB   RA                              
     In the output stage the following equation holds:            U   out     RB     =       (         +   _V     R4     +       U   out       2      RB         )     -     (         -   _V     R3     -       U   out       2      RB         )                              
     In this embodiment the output of the high voltage operational amplifier is short-circuit-proof and, using a capacitive load CL, the output voltage changes linearly with a change of input voltage. 
     As opposed to a classical Class C output stage design, which has a small, constant quiescent current, the quiescent current in a class CI output stage as described by this invention is dynamically dependent on the output voltage. The highest quiescent current is observed at Uout=0V. A reduction of the supply voltage for the diodes D 3  and D 4  make is possible to customize the quiescent current as needed. It should be noted that for the quiescent current Iq the following relation applies:          I   q     ≥     HV   RB                            
     To achieve higher maximum currents the phototransistors PH 1  and PH 2  can be replaced by Darlington phototransistors, e.g. with a CTR of 1200%. 
     As mentioned in (2) above, the current sources are extremely fast. Thus the properties of the high-voltage operational amplifier are largely dependent on the choice of the differential amplifier OP 1 . The high voltage operational amplifier is free of intermodular distortions and there is no danger of an internal high-voltage breakdown. If the target output voltage is independent of the high voltage output, e.g. due to generating the low-voltage feedback by means of a mechanical sensor, then this high voltage operational amplifier can be used as an isolation amplifier. 
     (5) Best Mode Description, Class AI 
     FIG. 6 shows a typical embodiment of the high voltage operational amplifier (Class AI) as described by this invention. In this embodiment the amplifier is formed using four resistors, one Zener diode, one capacitor, one optocoupler and one operational amplifier AD825, preferentially latchup-free. 
     An example of FIG.  6 : 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Max. current of OP1 
                 10 
                 mA 
               
               
                   
                 Supply voltage for OP1 
                 +/−15 
                 V 
               
               
                   
                 max. voltage for T1 
                 600 
                 V 
               
               
                   
                 HV supply voltage 
                 +/−300 
                 V 
               
               
                   
                 Resistance R3 = 30V/0.01A 
                 3 
                 kOhm 
               
               
                   
                 Resistance RP = 600V/0.01A 
                 60 
                 kOhm 
               
               
                   
                   
               
             
          
         
       
     
     The resistor R 11  serves to protect the optocoupler from damage due to overvoltage. The supply voltage VM of the optocoupler in this embodiment is −15 V, connected to the supply voltage of the operational amplifier AD825. Instead of the Zener diode shown in the schematic FIG. 3, this embodiment utilizes the base-emitter connection of a low power NPN transistor (Avalanche Effect). The resistor RL prevents parasitic oscillations between the inductivity of the feed lines and the load capacitor CL. 
     FIG. 7 shows the characteristic exponetionel response of the output voltage of the high-voltage operational amplifier to a square wave input signal of 100 mVpp. 
     For the same conditions FIG. 8 shows the currents across the transistor T 1  and the resistor RP. As a result of increasing input voltage the transistor T 1  is switched off and the capacitive load CL is charged across the resistor RL. As a result of decreasing input voltage the current source delivers its maximum current of 10 mA. At equilibrium the current in the current source is described by          I     T   1       =         HV   -     U   out       RP     -       U   out     RB                              
     (6) Best Mode Description, Class CI 
     FIG. 9 shows a typical embodiment of the high voltage operational amplifier (Class CI) as described by this invention. In this embodiment the amplifier is formed using six resistors, two Zener diodes, two capacitors, two optocouplers and one operational amplifier AD825, preferentially latchup-free. Instead of the Zener diode shown in the schematic, this embodiment utilizes the base-emitter connection of a low power NPN transistor (Avalanche Effect). 
     The resistors R 11  and R 12  protect the optocouplers from damage due to overvoltage. The supply voltages VP and VM of the optocouplers in this embodiment are +15 V and −15 V, respectively, connected to the supply voltages of the operational amplifier AD825. The resistor RL prevents parasitic oscillations between the inductivity of the feed lines and the load capacitor CL. 
     Example parameters of the circuit in FIG.  9 : 
     
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Max current of OP1 
                 10 
                 mA 
               
               
                   
                 Supply Voltage for OP1 
                 +/−15 
                 V 
               
               
                   
                 max. V DS  Voltage for T1 and T2 
                 600 
                 V 
               
               
                   
                 HV supply voltage 
                 +/−300 
                 V 
               
               
                   
                 Resistances R3 = R4 = 30V/0.01A 
                 3 
                 kOhm 
               
               
                   
                   
               
             
          
         
       
     
     FIG. 10 shows the characteristic linear increase of the output voltage as a result of potential step in the input signal. The voltage ramp for CL=10 nF is as expected:              I   out     *   Δ                 t     C     =           (     10   *     10     -   3         )     *     (     1   *     10     -   6         )         10   *     10     -   9           =       1      V                 and                 thus                     Δ                 U       Δ                 t         =       1      V       1                 µ                 s                                  
     For the same conditions FIG. 11 shows the currents across transistors T 1  and T 2 . As a result of increasing input voltage the current across T 1 =0 mA and the current across T 2 =10 mA. As a result of decreasing input voltage the situation is reversed and T 1 =10 mA and T 2 =0 mA. When both inputs are at the same voltage, both currents are 5 mA.