APD bias circuit with dual analog feedback loop control

1. A bias circuit of an avalanche photodiode, APD, comprising:

a voltage conversion module connected to the APD, wherein the voltage conversion module is configured to convert an input supply voltage to a bias voltage for the APD;

an error amplifier connected to the voltage conversion module for implementing a feedback control;

a voltage feedback loop configured to provide the error amplifier a first analog signal related to the bias voltage; and

a current feedback loop configured to provide the error amplifier a second analog signal related to an APD current;

wherein the first analog signal and the second analog signal are provided to the error amplifier via a feedback node and the error amplifier is configured to control the voltage conversion module based on the first and the second analog signals.

2. The bias circuit according to claim 1 , wherein the error amplifier comprises an operational amplifier configured to compare a feedback signal according to either the first analog signal or the second analog signal against a reference, and wherein the comparison result is integrated and used to control the voltage conversion module.

3. The bias circuit according to claim 1 , wherein the first analog signal is captured via a voltage divider connected to an output node of the voltage conversion module.

4. The bias circuit according to claim 1 , wherein the second analog signal is captured via a current sensing resistor Rs connected to the APD in series.

5. The bias circuit according to claim 4 , wherein a capacitor is connected in parallel to the resistor Rs to bypass high frequency signals, thus creating an AC ground.

6. The bias circuit according to claim 1 , wherein the first analog signal is connected to the feedback node via a diode.

7. The bias circuit according to claim 1 , wherein the second analog signal is connected to the feedback node via an emitter follower.

8. The bias circuit according to claim 1 , the bias circuit further comprising a temperature sensing element for compensating a temperature dependence of a required bias voltage for achieving a certain gain level of the APD.

9. The bias circuit according to claim 8 , wherein the temperature sensing element is a NTC thermistor, which is configured to form a series circuit with a tuning resistor, wherein the series circuit is connected to the feedback node of the error amplifier.

10. The bias circuit according to claim 1 , the bias circuit further comprising an RC filter connected between the voltage conversion module and the APD.

11. The bias circuit according to claim 10 , the bias circuit further configured to provide a bias to more than one APD, wherein the voltage conversion module is connected to each APD via an RC filter, and the more than one RC filter is constructed by:

connecting an output of the voltage conversion module to the more than one APD via more than one cathode resistor, with each APD connected to an individual cathode resistor;

connecting a first capacitor between ground and a first node between a first cathode resistor and a corresponding first APD; and

connecting a further capacitor between the first node and a further node between a further cathode resistor and a corresponding further APD.

12. The bias circuit according to claim 1 , wherein the voltage conversion module comprises more than one up-conversion stage, and the voltage feedback loop is obtained from an intermediate up-conversion stage.

13. The bias circuit according to claim 1 , wherein the voltage conversion module comprises a boost converter as a first stage and a voltage multiplier connected to the boost converter as a further stage, and the first analog signal is captured via a voltage divider connected to an output node of the boost converter instead of the output node of the voltage conversion module.