Abstract:
A transimpedance amplifier system includes a current source, such as a photodiode, coupled between two transimpedance amplifiers, each having feedback circuits with different impedances. Thus, for example, the cathode of a photodiode is coupled to a first transimpedance amplifier while the anode of the photodiode is coupled to the second transimpedance amplifier. Consequently, two voltage gains can be achieved without the use of conventional switched feedback circuits or the use of additional gain stages. A clamp circuit in parallel with one of the feedback circuits can be used to ensure that the both transimpedance amplifiers operate within their linear regions.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to transimpedance amplifiers and in particular to transimpedance amplifiers with more than one voltage gain output.  
         BACKGROUND  
         [0002]    Transimpedance amplifiers are known in the art. FIG. 1 shows a schematic view of a conventional transimpedance amplifier  10 . As shown in FIG. 1, transimpedance amplifier  10  includes an operational amplifier (“op-amp”)  12  with the noninverting input coupled to ground and the inverting input coupled to the cathode of photodiode  14 . The anode of photodiode  14  is coupled to ground. The photodiode  14  operates as a current source with high output impedance. A feedback circuit  16  is coupled between the output terminal and the inverting input terminal of the op-amp  12 .  
           [0003]    The voltage gain of transimpedance amplifier  10  is determined by the feedback circuit  16 . To achieve more than a single gain in a transimpedance amplifier, the feedback circuit can be switched or additional voltage gain stages may be added. A switching feedback circuit may include multiple resistors of different values that can be switchably coupled between the output terminal and the inverting input terminal of the op-amp  12 . Thus, for example, as shown in FIG. 1, the feedback circuit  16  may include a number of resistors  18  and  20 , a capacitor  19 , and a transistor  22  in parallel with one of the resistors. Thus, by turning on and off the transistor, the impedance of the feedback circuit  16  may be altered. However, placing switching components in series with the feedback circuit affects the accuracy, stability, noise and frequency of the response.  
           [0004]    Alternatively, a separate gain stage  24 , including another op-amp  26  and feedback circuit  28  may be added to the output of transimpedance amplifier  10  through a resistor  30 . However, adding a voltage gain stage to the output terminal of transimpedance amplifier  10  will unfortunately multiply noise and offset.  
           [0005]    Thus, what is needed is a transimpedance amplifier system that provides more than one voltage gain output but that does not suffer from loss of accuracy, stability, or increased noise and offset as found in conventional systems.  
         SUMMARY  
         [0006]    A transimpedance amplifier system, in accordance with the present invention, provides two voltage output signals with different gains without loss of accuracy, stability or increasing noise or offset. The transimpedance amplifier system includes two transimpedance amplifiers with a current source, such as a photodiode, coupled between the inverting input terminals of the two transimpedance amplifiers.  
           [0007]    In accordance with one embodiment of the present invention, the transimpedance amplifier system includes a current source that provides a current signal from a first terminal and a second terminal, a first transimpedance amplifier that receives the current signal from the first terminal and converts the current signal to a first voltage output signal with a first gain; and a second transimpedance amplifier that receives the current signal from a second terminal and converts the current signal to a second voltage output signal with a second gain. The current source may be, e.g., a photodiode. In one embodiment a clamp circuit is provided in parallel with one of the feedback circuits of the transimpedance amplifiers. The clamp circuit holds the transimpedance amplifiers within their linear operating regions inside the full scale range of the circuit.  
           [0008]    In another embodiment of the present invention, a method of converting a current signal to a dual voltage signals with different gains includes providing a current signal from a current source, such as a photodiode. The current signal from a first terminal of the current source is received and transformed to produce a first output voltage signal having a first gain. The current signal from a second terminal of the current source is also received and transformed to produce a second output voltage signal having a second gain. Transforming the current signal to produce a voltage signal includes converting the voltage output signal to a feedback current signal, combining the feedback current signal with the current signal from the current source, and converting the combined current signals into the voltage output signal, for example as performed by a transimpedance amplifier.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 shows a schematic view of a conventional transimpedance amplifier having multiple voltage gain outputs.  
         [0010]    [0010]FIG. 2 is a schematic view of a transimpedance amplifier system having dual voltage gain outputs in accordance with an embodiment of the present invention.  
         [0011]    [0011]FIG. 3 is a schematic diagram of one embodiment of a clamp circuit that may be used with the transimpedance amplifier system shown in FIG. 2. 
     
    
     DETAILED DESCRIPTION  
       [0012]    [0012]FIG. 2 is a schematic view of a transimpedance amplifier system  100  having dual voltage gain outputs in accordance with an embodiment of the present invention. Advantageously, transimpedance amplifier system  100  uses two separate transimpedance amplifiers (which may be within the same package) and feedback circuits to produce the different voltage gains and, thus, eliminates problems with loss of accuracy, stability, or increased noise and offset found in conventional systems. The dual voltage gain transimpedance amplifier system  100  maybe particularly useful in light power measurements, which is in general well known to those skilled in the art.  
         [0013]    As shown in FIG. 2, transimpedance amplifier system  100  includes a current source, such as a photodiode  102 , between a first transimpedance amplifier  110  with a feedback circuit  112  that provides a first gain and a second transimpedance amplifier  120  with a feedback circuit,  122  that provides a second (different) gain. As shown in FIG. 2, a photodiode  102  may be used with the cathode coupled to the first transimpedance amplifier  110  and the anode coupled to the second transimpedance amplifier  120 . The application of a photodiode  102  may be utilized, e.g., for light power measurements. A photodiode, such as a part number FD1500W manufactured by Fermionics, Inc. in Simi Valley, Calif., may be used.  
         [0014]    Transimpedance amplifiers  110  and  120  each include op-amps  111  and  121 , respectively with their noninverting input terminals coupled a low impedance voltage source. For example, as shown in FIG. 2, the noninverting input terminals are coupled to ground. The inverting input terminal of op-amp  111  is coupled to the cathode of photodiode  102 , while the inverting input terminal of op-amp  121  is coupled to the anode of photodiode  102 . While FIG. 2 shows transimpedance amplifier system  100  using two separate op-amps, it should be understood that the op-amps may be within the same package, e.g., as a dual amplifier such as that manufactured by Texas Instruments, as part number OPA2132U.  
         [0015]    The feedback circuit  112  includes, e.g., a resistor  114  and a capacitor  116  in parallel between the output Out 1  of op-amp  111  and the inverting input terminal of op-amp  111 . Similarly, the feedback circuit  122  includes, e.g., a resistor  124  and a capacitor  126  in parallel between the output Out 2  of op-amp  121  and the inverting input terminal of op-amp  121 .  
         [0016]    The specific values of the components in feedback circuits  112  and  122  are chosen for the desired response, i.e., gain and frequency, for the device. By way of example, in feedback circuit  112 , resistor  114  may have a resistance of 50 KΩ and capacitor  116  has a capacitance of 220 pF, while in feedback circuit  122 , resistor  124  may have a resistance of 500 KΩ and capacitor  126  has a capacitance of 33 pF Of course, any appropriate RC values may be used to achieve the desired gain and frequency. It should be understood that feedback circuits  112  and  122  are exemplary and that other feedback circuits may be used to achieve a desired transimpedance gain.  
         [0017]    In operation, the current signal from the cathode of photodiode  102  is received by transimpedance amplifier  110 , while the current signal from the anode of photodiode  102  is received by transimpedance amplifier  120 . The transimpedance amplifiers  110  and  120  transform the received current signal to produce the voltage output signal at the output terminals Out 1  and Out 2 , respectively. The voltage output signals are received by feedback circuits  112  and  122 , which convert the output voltage signals to feedback current signals. The feedback current signals are combined with the current signal from the photodiode  102 , i.e,. the feedback loop is closed, and the combined feedback current signal and current signals from the photodiode  102  are transformed to the voltage output signals by op-amps  111  and  121 .  
         [0018]    The voltage V Out1  at the output of the output terminal Out 1  of transimpedance amplifier  110  is V Out1 =I×Z 112 , where I is the current through the current source, i.e., photodiode  102 , and Z 112  is the impedance of the feedback circuit  112 . Similarly, the voltage V Out2  at the output of the output terminal Out 2  of transimpedance amplifier  120  is V Out   2 =−(I×Z 122 ), where again I is the current through the current source, i.e., photodiode  102 , and Z 122  is the impedance of the feedback circuit  112 . In the case where the impedance of feedback circuit  122  is greater than the impedance of feedback circuit  112 , i.e., Z 122 &gt;Z 112 , the op-amp  121  will reach saturation earlier than op-amp  111 . Consequently, the linear mode of operation for the transimpedance amplifier system  100  will be disturbed when the following equation is not valid:  
                 V   Out1       Z   112       =     -       V   Out2       Z   122                 eq.  1                               
 
         [0019]    To avoid the disturbance of the linear mode of operation, a clamp circuit  130  is coupled in parallel with the feedback circuit  122 , i.e., between the output Out 2  of op-amp  121  and the inverting input terminal of op-amp  121 . The clamp circuit  130  receives a reference voltage (Ref V), which sets the clamping level. The clamping level is chosen to be in the linear range of operation of the op-amp  121 .  
         [0020]    [0020]FIG. 3 is a schematic diagram of one embodiment of clamp circuit  130 . Clamp circuit  130  includes a clamping element  132 , e.g., an n-channel JFET transistor, such part number SST4119 manufactured by Vishay Siliconix, located in Malvern, Pa., coupled between the output terminal Out 2  of op-amp  121  (shown in FIG. 2) and the inverting input terminal of the op-amp  121 , shown in FIG. 3 as “−”. The control terminal of clamping element  132  is coupled to a control circuit. Thus, for example, the control circuit may be an op-amp  134 , such as that manufactured by Texas Instruments as part number OPA2227, which has the output terminal connected to the gate of the clamping element  132 . The noninverting input terminal of op-amp  134  is connected to the output terminal Out 2  of op-amp  121  and the inverting input terminal of op-amp  134  is connected to the output terminal through a feedback circuit  136 , including, e.g., a resistor  138  and capacitor  140 . The inverting input of op-amp  134  is also connected to a reference voltage (Ref V) through a resistor  142 . Op-amp  134  receives +Vcc and −V EE , which may be, e.g., +15V and −15V, while the reference voltage (Ref V) is, e.g,. 10.5V. In feedback circuit  136 , the resistor  138  may have a resistance, e.g., of 500 KΩ and the capacitor may have a capacitance of 100 pF, while the resistor  142  may have a resistance of 5KΩ. Of course, these values may vary depending on the desired operation of the clamp circuit  130 .  
         [0021]    Although the invention has been described with reference to particular embodiments and particular components, the description is only an example of the invention&#39;s application and should not be taken as a limitation. Various adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as defined by the following claims.