Abstract:
A differential amplifier circuit comprises: an amplifying section including first and second current branches and an output stage which comprises a current sinking element and a control element. The circuit also includes a current limiting section which comprises a current detecting element connected to detect the current through the current sinking element and arranged to drive the current limiting element when the detected current exceeds a predetermined threshold to inject current at the collector of the transistor in the first current branch.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to a differential amplifier circuit.  
         BACKGROUND OF THE INVENTION  
         [0002]    If a differential voltage is applied on the input of a transconductance amplifier, the amplifier sinks current from an output pin of the circuit, through a current sinking transistor to ground. The output voltage (an amplified version of the differential input) is developed across a load resistor connected to the output pin. When the transconductance amplifier is implemented on a chip, in order not to damage the chip it is advisable to limit the sinking current to a predetermined value.  
         SUMMARY OF THE INVENTION  
         [0003]    According to the present invention there is provided a differential amplifier circuit comprising: an amplifying section for generating at an output node an amplified output dependent on a differential input voltage, said amplifying section including first and second current branches, each including a respective transistor, said transistors having their bases connected together and parallel collector paths an output stage comprising a current sinking element having a control node, and a controllable path connected to the output node, and a control element having a control node connected to the collector path of the second current branch and a controllable path connected to the control node of the current sinking element; and a current limiting section comprising a current detecting element connected to detect the current through the current sinking element and arranged to drive a current limiting element when the detected current exceeds a predetermined threshold to inject current at the collector of the transistor in the first current branch.  
           [0004]    The current limiting section serves to limit the current sunk through the current sinking element by current feedback into its control node. In the described embodiment, the current sinking element, the control element, the current detecting element and the current limiting element are all implemented as bipolar transistors.  
           [0005]    For a better understanding of the present invention and to show how the same may be carried into effect, reference will now be made by way of example to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    [0006]FIG. 1 is a circuit diagram of a differential amplifier including a current limitation stage;  
         [0007]    [0007]FIG. 2 is a graph showing the current through a load resistor against differential input voltage; and  
         [0008]    [0008]FIG. 3 is a graph showing the collector current through a current limiting element in relation to differential input voltage to the amplifier. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0009]    [0009]FIG. 1 is a circuit diagram of a differential amplifier including an amplification stage  2 , an output stage  4  and, in accordance with the described embodiment of the present invention, a current limitation stage  6 .  
         [0010]    The construction of the amplification stage  2  and the output stage  4  is largely conventional, apart from the interconnections with the current limitation stage  6 . Therefore a complete description is not given in the following of the function of the amplification stage  2  and output stage  4 , it being assumed that this will be familiar to a person skilled in the art. Only those aspects of these stages are discussed which are necessary for the understanding of the current limitation stage  6 .  
         [0011]    Nevertheless a brief discussion of the construction of the amplification stage  2  and output stage  4  is now given.  
         [0012]    The amplification stage  2  comprises first and second differential amplifying transistors Q 1 , Q 2  receiving respective differential voltage inputs v m , v p . The emitters of the differential amplifying transistors Q 1 , Q 2  are connected together and to a supply rail V c  via a transistor Q 4 . A diode connected transistor Q 3  is connected in series with a current source CS between the voltage supply rail V c  and ground. The base of the diode connected transistor Q 3  is connected to the base of the transistor Q 4 . The base connection of the diode connected transistor Q 3  drives the base of a transistor Q 5  which is connected in series with a diode connected transistor Q 10  and a resistor R 1  between the voltage supply rail V c  and ground. The base connection of the diode connected transistor Q 3  also drives the base of an output transistor Q 9  which forms part of the output of stage  4  and will be discussed in more detail hereinafter.  
         [0013]    The collectors of the differential amplifying transistors Q 1 , Q 2  are respectively connected into parallel legs of the amplifying circuit, each parallel leg comprising, connected in series between the voltage supply rail V c  and ground, base connected transistors Q 6 , Q 8 , base connected transistors Q 11 , Q 12  and resistors R 2 , R 3 . The collectors of the differential amplifying transistors Q 1 , Q 2 , are connected between the respective base connected transistors Q 11 , Q 12  and their associated resistors R 2 , R 3 . The bases of the transistors Q 11 , Q 12  are connected to the base of the diode connected transistor Q 10  discussed above. A transistor Q 13  has its base connected between the transistor Q 6  and the collector of transistor Q 11  of the first parallel leg  8 . Its emitter is connected to the bases of the transistors Q 6 , Q 8  of the parallel legs  8 ,  10  respectively. Its collector is connected to ground.  
         [0014]    The output stage  4  comprises a control element in the form of a transistor Q 16 , and the output transistor Q 9  already mentioned which has its emitter connected to the supply rail V c  and its collector connected to the collector of a current sinking transistor Q 7 . An output resistor R LOAD  is connected between an output terminal  12  (at the collector of the output transistor Q 9 ) and the supply rail V c . The load resistor can just as well be connected to another voltage source (positive power supply). The only condition is that the op-voltage at the output terminal is low enough to keep Q 9  working in forward active mode and high enough to keep Q 7  working in forward active mode.  
         [0015]    The emitter of the current sinking transistor Q 7  is connected to ground. Its base is connected to its collector via a capacitor C 1 , to ground via a resistor R 4  and to the emitter of the transistor Q 16 . The base of the control transistor is connected to the collector of the current sinking transistor Q 7  via a capacitor C 2 .  
         [0016]    With the current limitation stage in place (described later), current is injected through the transistor Q 11 . This increases the base-emitter voltage at Q 11  and (since the bases of Q 11  and Q 12  are connected together) the base-emitter voltage at the transistor Q 12  as well. This decreases the base current of the transistor Q 16 . Since the value of R 4  is quite high, it can be said that the transistors Q 16  and Q 7  form a Darlington pair (i.e. almost the full amount of the emitter current of Q 16  goes into the base of Q 7 ).  
         [0017]    The current limitation stage  6  will now be described. It comprises a current detecting transistor having its collector connected via a resistor R 5  to the supply rail V c  and its emitter connected via a resistor R 6  to ground. Its base is connected to the base of the current sinking transistor Q 7 . These transistors have different areas. When the collector current through Q 7  reaches some certain value the base-emitter voltage reaches a certain corresponding value. Depending on the area ratio of the transistors and on the size of R 6  the transistor Q 15  will be switched on when its base-emitter voltage is large enough.  
         [0018]    A current limiting transistor Q 14  has its base connected at a node  14  between the resistor R 5  and the collector of the transistor Q 15 . The emitter of the current limiting transistor Q 14  is connected to the supply rail V c . The collector of the current limiting transistor Q 14  is connected to the collector of the transistor Q 11  in the first parallel leg  8 .  
         [0019]    Operation of the current limitation stage  6  will now be described. If the current through the output drive transistor Q 7  becomes too large, the first current limiting transistor Q 15  is turned on by current injected into its base as described above.  
         [0020]    When the current detecting transistor Q 15  is turned on, current flows through the resistor R 5  which causes the current limiting transistor Q 14  to be turned on by base current drive. The collector current of the transistor Q 14  is accordingly increased and is injected into the collector of the transistor Q 11  in the first parallel leg  8 . This negative feedback leads to a reduction in the base current for the transistor Q 16  and thus a reduction in the base current of the output drive transistor Q 7 . This therefore limits the maximum value of the collector current of the output drive transistor Q 7 , and thus limits the current through the load transistor R LOAD .  
         [0021]    Assuming that the transistors are working in forward active mode, the following equations can be used to describe the relation between current I and voltage U for the relevant transistors, where the following denotation applies:  
         [0022]    I E =emitter current  
         [0023]    I s =saturation current  
         [0024]    U BE =base emitter voltage  
         [0025]    U T =threshold voltage  
         [0026]    I E,Q7  is the emitter current for the transistor Q 7 , other numbers and denotations applying accordingly. 
         Equation 1 —I   E,Q7   =I   s,Q7   exp  ( U   BE,Q7   /U   T ) 
         Equation 2 —I   E,Q15   =I   s,Q15   exp ( U   BE,Q15   /U   T ) 
         Equation 3 —U   BE,Q7   =U   BE,Q15   +I   E,Q15   R   6   
         Equation 4 —I   E,Q15   /I   E,Q7   =I   s,Q15   /I   s,Q7   exp (( U   BE,Q15   −U   BE,Q7 )/ U   T ) 
         Equation 5 —U   BE,Q15   −U   BE,Q7   =−I   E,Q15   R   6  (from Equation 3) 
         Equation 6 —I   E,Q7   /I   E,Q15   =I   sE,Q7   /I   s,Q15   exp (( I   E,Q15   R   6 )/ U   T ) 
         [0027]    Thus, equation 6 gives the relationship between the emitter currents for transistors Q 7  and Q 15  in terms of their saturation currents, the value of the resistor R 6  and the transistor threshold voltage.  
         [0028]    It is thus possible to determine from this equation a value of the resistor R 6  to limit the output current at a desired level. If transistors Q 7  and Q 15  are chosen to be of the same type, their saturation currents I s,Q7, I s,Q15  will be directly proportional to the transistor areas A Q15 ,A Q7  respectively. If a limiting value for the output current is selected for both current branches, denoted I Q15,limit ,I Q7,limit  then the appropriate resistor value for R 6  can be written as: 
         Equation 7 —R   41   =U   T   /I   Q15,limit   In (( I   Q7,limit   /I   Q15,limit ) ( A   Q40   /A   Q7 )). 
         [0029]    Note that R 41 →R 6  and AQ 40 →AQ 15 .  
         [0030]    The value for the resistor R 5  is chosen to give the correct voltage drop to turn the second current limiting transistor Q 14  on, that is: 
           R 5= U   BE,Q14   /I   E,Q15.   
         [0031]    [0031]FIG. 2 illustrates the effect of the current limiting circuit on the output current through the load resistor R LOAD . Initially, the current tends to sink fairly rapidly, with increasing differential input voltage (v m −v p ), but flattens out as can be seen around a differential input voltage of around 0.001V due to the effect of the current limitation circuit as illustrated in FIG. 3. FIG. 3 illustrates the collector current through the current limiting transistor Q 15  as a function of differential input voltage. The current increases until the differential input voltage is around 0.001V, and then flattens out due to the negative feedback described above.  
         [0032]    From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.