Abstract:
A light emitting diode is driven by an input signal and electromagnetically coupled to a photodiode. The photodiode is connected to a high impedance load across which is obtained a signal linearly related to but electrically isolated from the input signal.

Description:
BACKGROUND OF THE INVENTION 
     A light emitting diode driven by an input signal and optically coupled to a photodiode to provide an output signal across the photodiode in response to the input signal is known in the art. The output signal produced by such prior devices is not a linear function of the input signal applied to the light emitting diode. 
     SUMMARY OF THE INVENTION 
     A photodiode is operated into a high impedance load to utilize the photodiode&#39;s resulting logarithmic open circuit voltage versus short circuit photocurrent characteristic. The photodiode is electromagnetically coupled to a photon emitting means having an exponential current versus voltage characteristic to produce an output signal from the photodiode which is a substantially linear function of an input signal applied to the photon emitting means. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of a photon coupled isolator made in accordance with the invention. 
     FIG. 2 is a diagram of another embodiment of a photon coupled isolator made in accordance with the invention. 
     FIG. 3 is a diagram of an additional embodiment of a photon coupled isolator made in accordance with the invention. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, there is shown a photon emitting means 2 which may be, for example, a light emitting diode or the like, hereafter referred to as an LED, electromagnetically coupled to a photodiode 4 driving a high impedance load 5. LED 2 is driven by an input voltage V i  producing a current I i  therethrough. 
     The input current of LED 2 is generally represented as 
     
         I.sub.i = I.sub.o [e.sup.qv.sbsp.i/nkT - 1] 
    
     where I o  is its reverse leakage current, q is the magnitude of the the charge of an electron, n is a number whose value is dependent upon the particular mechanism responsible for current flow through LED 2, k is Boltzmann&#39;s constant and T is the absolute temperature. Over a selected range of I i , n is essentially constant. The values of these constants are well known or readily obtainable by one skilled in the art. 
     Analysis of the Thevenin equivalent circuit of an excited photodiode having open circuit voltage V oc , output impedance R D  and coupled to a load of impedance R L  shows the voltage V o  across the load of impedance R L  to be ##EQU1## where ##EQU2## AND ##EQU3## WHERE I o  * is the dark reverse leakage current of the photodiode 4, n* is a number, less than or equal to 2, whose value is dependent upon the particular mechanism responsible for current flow through the photodiode and I s  is the short circuit photocurrent. For ##EQU4## thereby producing a substantially logarithmic relation. For R L  &lt; R D , V o  ≈ I s  R L  thereby producing a substantially linear relation. 
     In the preferred embodiment the photodiode 4 is operated into the high impedance load 5, high being ##EQU5## The output flux of LED 2 in general varies as a power m of input current I i . 
     
         φ.sub.LED = C.sub.1 I.sub.i.sup.m 
    
     I s  is proportional to the input flux 
     
         I.sub.s = C.sub.2 φ.sub.LED + I.sub.o * 
    
     C 1  and C 2  being constants. Combining the above and assuming I s  &lt; I o  * yields &gt;&gt; ##EQU6## 
     In the preferred embodiment of FIG. 1, n and n* are substantially constant, V i  is greater than the quantity nkT/q 
     and I s  is greater than I o  * thereby producing a substantially linear relationship which is expressed by ##EQU7## 
     Referring to FIG. 2, a second preferred embodiment of a photon coupled isolator is shown wherein an input voltage V i  varies positively and negatively about a zero voltage reference. The output voltage V o  is a substantially linear function of V i . A voltage 18 at the output of a biasing means 7 is equal to ##EQU8## where V 20  is the voltage applied at point 20, R 6  is the resistance of resistor 6, R 8  is the resistance of resistor 8 and R 10  is the resistance of resistor 10. 
     In the preferred embodiment of FIG. 2, V 20  is selected to provide the maximum symmetrical swing in a linear region of the system&#39;s transfer characteristic, and the high impedance load 5 is an amplifier 16. 
     The embodiment shown in FIG. 2 exhibits a gain of approximately 3 and a bandwidth of approximately 1 KHz for a range of V i  or ± 180 mV. Amplifier 14 may be, for example, a National type LM307, or the like, and amplifier 16 may be, for example, a National type LH0022CD, or the like. Voltage 20 is approximately -1.3 volts. The resistor 12 supplies input bias current to the positive terminal of amplifier 14. 
     Referring to FIG. 3, a third preferred embodiment of a photon coupled isolator is shown. The input voltage V i  varies positively and negatively about a zero voltage reference. The output voltage V o  is a substantially linear function of V i . A voltage 48 at the output of bias means 7 is ##EQU9## where V 30  is the voltage applied at point 30, R 36  is the resistance of resistor 36, R 38  is the resistance of resistor 38 and R 40  is the resistance of resistor 40. Voltage 48 is applied to photon emitting means 2. 
     Photon emitting means 2 in FIG. 3 comprises an exponential means 50 coupled to receive the output of bias means 7 and to an input of an amplifier 24. Exponential means 50 has a current-voltage characteristic wherein its current is exponentially related to the voltage appearing across its terminals. In the preferred embodiment the exponential means 50 is for example a transistor type 2N3053 having its base and collector connected together and to point 48 and its emitter connected to point 21, a solid-state diode or the like. The current which flows through the exponential means 50 also flows through photon emission means 26 having photon emission proportional to the magnitude of its current raised to a power. In the preferred embodiment a second photon emitting means 26 is a light-emitting diode or any photon emitting means having photon emission proportional to the magnitude of a signal applied thereto raised to a power. 
     The embodiment shown in FIG. 3 exhibits a gain of approximately 0.5 and a bandwidth of approximately 2 KHz for a range of V i  of ± 150 mV. Amplifiers 44 and 24 may be, for example, National type LM307, or the like. Voltage V30 is approximately -0.6 volts. Resistor 22 supplies input bias current to the positive terminal of amplifier 24, and resistor 42 supplies input bias current to the positive terminal of amplifier 44.