Integrated preamplifier circuit for detecting a signal current from a photodiode

1. An integrated preamplifier circuit for detecting a signal current from a photodiode and converting the signal current into an output voltage, the circuit comprising:

a signal transimpedance amplifier and a dummy transimpedance amplifier, the dummy amplifier being matched to the signal amplifier, each of said amplifiers including an input transistor and an output transistor, the input transistor of the signal amplifier receiving an input signal derived from the signal current and the input transistor of the dummy amplifier receiving no input signal, and the signal and dummy amplifiers providing a differential output signal, wherein the input transistors of the signal and dummy amplifiers each have a biasing current source forced to follow a reference current source implemented within the integrated circuit.

2. The integrated preamplifier circuit according to claim 1 , wherein the reference current source comprises a band-gap reference voltage source and an on-chip resistor.

3. The integrated preamplifier circuit according to claim 1 or claim 2 , wherein the biasing current source comprises a transistor having a current path connected between a supply terminal and a biasing current input for both input transistors, and a control electrode connected to an output of an operational amplifier, the operational amplifier having a first input receiving an input signal derived from the input of the dummy amplifier and a second input receiving an input signal derived from the reference current source.

4. The integrated preamplifier circuit according to claim 3 , wherein the reference current from the reference current source is fed through a diode-connected transistor to ground and the voltage drop across the diode-connected transistor is applied to the second input of the operational amplifier.

5. The integrated amplifier circuit according to claim 2 , wherein the reference current source comprises a resistor through which a reference control current is fed and a control loop controlling the voltage drop across the resistor so as to equal the voltage of the band-gap voltage source.

6. The integrated amplifier circuit of claim 5 , wherein a diode-connected transistor is connected in series in the current path of the resistor and the reference current is taken from a current mirror associated with the diode-connected transistor.

7. The integrated amplifier circuit according to claim 2 , wherein the reference current source comprises a transistor with a control electrode to which a voltage derived from the band-gap voltage source is applied, and with a current path including a resistor through which a reference control current is fed, the reference control current being in turn controlled by the voltage applied to the control electrode of the transistor.

8. The integrated amplifier circuit of claim 7 , wherein a diode-connected transistor is connected in the current path of the transistor and the reference current is taken from a current mirror associated with the diode-connected transistor.

9. The integrated amplifier circuit according to claim 2 , wherein the reference current source comprises

a first reference current source with a first resistor through which a first reference control current is fed and a control loop controlling the voltage drop across the first resistor so as to equal the voltage of a first band-gap voltage source, and

a second reference current source with a transistor that has a control electrode to which a voltage derived from a second band-gap voltage source is applied, and a current path including a second resistor through which a second reference control current is fed, the second reference control current being in turn controlled by the voltage applied to the control electrode of the transistor;

the reference current being combined from the first and second reference current sources.