Electrical supply circuit, in particular for APDs

Abstract: Supply circuit, especially for Avalanche Photo Diodes. The supply voltage is generated in a temperature-independent voltage source (VS1) and a temperature-dependent voltage source (VS2), both connected to a voltage amplifier (A1, A2), the output voltages being added to one another. The last-mentioned voltage supply comprises a temperature-independent voltage source (A3) whose output voltage is subtracted from the output voltage of a partially temperature-dependent voltage source in the form of a temperature-sensitive impedance, such as a diode (D) in the forward direction, connected to a current source (CS).

Description: A. BACKGROUND OF THE INVENTION 1. Field of the invention The invention relates to an electrical supply circuit, in particular for supplying an 'Avalanche Photo-Diode' (APD), but which can also be used more generally. 2. Prior Art 'Avalanche Photodiodes' (APDs) are diodes which are used as electro-optical converters especially in optical signal transmission engineering. Diodes of this type only give an optimum optical-electrical conversion ('gain') of an optical input signal if the applied diode voltage approaches the breakdown voltage very closely. One problem in this connection is that the breakdown voltage is temperature-dependent, as a result of which, if the applied voltage were to be kept constant, on the one hand, said voltage would not approach the breakdown voltage closely enough, as a result of which the output signal of the diode would be inadequate, or on the other hand, the diode would breakdown in the event of any - unavoidable - increase in the (ambient) temperature. To eliminate the problem posed, supply circuits for APDs are known whose output voltage has a specified temperature dependence which is matched as well as possible to the temperature dependence of the APD used. Thus, the Japanese patent application JP 60-180,347 discloses such as circuit. However, said circuit has the disadvantage that the level of output voltage and the degree of temperature dependence cannot be controlled independently of one another, which does not, however, have to be a drawback if the circuit is intended solely to supply a specified type of APD; the values of the components can then adjusted by making allowance for said type of APD. At the same time, however, no allowance can be made for specimen variations within the same type of APD. As a consequence thereof, the 'gain' of the APD cannot be optimised, and in practice this nevertheless signifies a loss which cannot be neglected. A APD bias voltage generating circuit using two differential amplifiers is known from JP-A-60094511. B. Summary of the invention The invention provides a supply circuit which can be used, in particular, to supply APDs which, in contrast to known similar circuits, provide means for the optimum adjustment of both the temperature-independent part of the supply voltage and of the temperature-dependent part thereof, in particular the degree of temperature dependence. The invention is characterised by a first, temperature-independent voltage source (VS1) whose output voltage (U1) is constant and which is connected to the input of a first voltage amplifier (A1) having a controllable gain (a1), and in addition, by a second, temperature-dependent voltage source (VS2) whose output voltage (U2) is essentially equal to the mathematical product of a constant (C) and the temperature (t) measured at a measuring point and which is connected to the input of a second voltage amplifier (A2) having a controllable gain (a2), the output voltage of the first voltage amplifier and the output voltage of the second voltage amplifier being added to one another. According to the invention, the temperature-independent component and the temperature-dependent component of the supply voltage (quiescent voltage) to be applied to an APD are generated independently of one another by the voltage sources mentioned and the output voltages of said voltage sources are, in addition, adjustable independently of one another by means of the two amplifiers mentioned; the level of the voltage is regulated by means of the first amplifier and the voltage/temperature ratio by means of the second amplifier. These measures make it possible for any APD to be adjusted extremely delicately and, consequently, optimally without, at any temperature whatsoever, either the difference between the supply voltage and the breakdown voltage becoming too large (resulting in a lower 'gain') or the supply voltage being able to exceed the breakdown voltage (with the APD becoming inoperative as a result). A preferred embodiment of the invention is characterised in that the second, temperature-dependent voltage source mentioned is formed by a third, temperature-independent voltage source (VS3) with a third, constant output voltage (U3) and by a series circuit of a current source and a temperature-dependent impedance(D), the third output voltage mentioned being equal to the voltage (U4) generated by the current source (CS) mentioned across the impedance and being subtracted therefrom, the impedance mentioned preferably being formed by a semiconductor diode connected in the forward direction. C. Exemplary embodiment The figure shows an exemplary embodiment of the invention. The temperature-independent voltage source VS1 mentioned above is formed by a commercially available component such as the type µA723 whose (very stable) output voltage U1 is approximately 7 volts. This voltage source VS1 is connected to an input terminal of a controllable voltage amplifier A1 whose other input terminal is connected to earth. A voltage equal to a1*7 volt can therefore be tapped off at the output, a1 being the (variable) gain. The temperature-dependent voltage source VS2 is formed by a temperature-independent voltage source VS3, for example also the component µA723 mentioned, with a fixed voltage divider, in combination with a temperature-dependent voltage source which is formed by a diode D incorporated in a current circuit supplied by a current source CS; said current source is formed in turn by a voltage source VS4 and a relatively high-resistance resistor R. The fixed voltage divider connected to the temperature-independent voltage source VS3 is adjusted in such a way that its output voltage is equal to the voltage across the diode D, for example 0.7 volts. The APD to be supplied and the diode are sited in such a way that they are exposed to the same ambient temperature, for example mounted on the same printed circuit board; the fixed voltage divider is then also adjusted at a temperature which is equal to the normal operating temperature at which the APD to be supplied operates, normally room temperature. In a subtraction circuit '-', the diode voltage is subtracted from the output voltage of the voltage divider and the resultant voltage difference is presented to the input of an amplifier A2, whose other input is at earth. The output voltage of the voltage divider is, for example, 0.7 volts. The voltage across the diode is also 0.7 volts, increased or decreased by the mathematical product of a temperature constant and the difference between the current temperature and room temperature; the temperature constant is, for example, 2 millivolts per degree Celsius. In that case, the voltage presented to the input of the amplifier A2 is therefore equal to (0.7 + 2 mV/deg) - 0.7 = 2 mV/deg. This voltage is therefore solely dependent on the temperature. It can be amplified by the amplifier A2 by a gain of a 2, with the result that the voltage at the output of the amplifier A2 is equal to a2*2mV/deg. In a voltage adder '+', the output voltages of the first amplifier A1 and that of the second amplifier A2 are added to one another resulting in an output voltage (a1*7 + a2*mV/deg), it therefore being possible to adjust a1 and a2 independently of one another.

Claims: Electrical supply circuit comprising a first, temperature-independent voltage source (VS1) whose output voltage U1 is constant and which is connected to the input of a first voltage amplifier (A1) having a controllable gain (a1), and a second temperature-dependent voltage source (VS2) whose output voltage U2 is essentially equal to the mathematical product of a constant C and the temperature t measured at a measuring point and which is connected to the input of a second voltage amplifier (A2) having a controllable gain (a2), the output voltage of the first voltage amplifier and the output voltage of the second voltage amplifier being added to one another. Electrical supply circuit according to Claim 1, wherein the second, temperature-dependent voltage source (VS2) is formed by a third, temperature-independent voltage source (VS3) having a third, constant output voltage U3 and by a series circuit of a current source (CS) and a temperature-dependent impedance (D), the fourth output voltage U4 generated by said current source (CS) across said temperature-dependent impedance having a temperature independent component equal to the third constant output voltage U3 and a temperature dependent component, the output voltage U2 of said second temperature dependent voltage source (VS2) being obtained by substracting said third constant output voltage from said fourth output voltage. Electrical supply circuit according to Claim 2, wherein said temperature-dependent impedance is formed by a semiconductor diode (D) connected in the forward direction with respect to said current source (CS).

Inventors: TAN HAN HIONG, TAN, HAN HIONG
Applicants: NEDERLAND PTT, KONINKLIJKE PTT NEDERLAND N.V.
Application year: 1992
Main classification: H01L31
IPC codes: H01L31, H04B10, G05F1, H03F3