Photodetector amplifier circuit for controlling an on state or an off state of an output transistor

1. A photodetector amplifier circuit that converts a photodetector signal photoelectrically into a photocurrent and controls an on state or an off state of an output transistor according to the photocurrent, the photodetector amplifier circuit comprising:

a first control circuit that is connected to a control terminal of the output transistor and controls the on state or the off state of the output transistor according to the photocurrent; and

a switch that is connected between the control terminal of the output transistor and a ground voltage terminal and if the output transistor is turned off, discharges the control terminal of the output transistor by making the control terminal of the output transistor conductive with the ground voltage terminal,

a first resistance device connected between a second control circuit and a control terminal of a second transistor,

wherein the first control circuit comprises the second control circuit and a third control circuit, and the switch is made up of the second transistor.

2. The photodetector amplifier circuit according to claim 1 , wherein the output transistor is an open collector.

3. The photodetector amplifier circuit according to claim 1 , wherein

the second control circuit converts the photocurrent into a voltage, and

the third control circuit controls the on state or the off state of the output transistor according to an output from the first control circuit.

4. The photodetector amplifier circuit according to claim 3 , wherein the switch is controlled according to an output from the second control circuit.

5. The photodetector amplifier circuit according to claim 3 , wherein

if the output transistor is in the on state, the third control circuit connects a power supply voltage terminal to the control terminal of the output transistor, and

if the output transistor is in the off state, the third control circuit blocks the power supply voltage terminal and the control terminal of the output transistor to control the on state or the off state of the output transistor.

6. The photodetector amplifier circuit according to claim 5 , wherein

the third control circuit comprises a first transistor and a second resistance device that are connected in series between the power supply voltage terminal and the ground voltage terminal,

an intermediate node between the first transistor and the second resistance device is connected to the control terminal of the output transistor,

if the output transistor is in the on state, the third control circuit turns on the first transistor so as to connect the second power supply voltage terminal to the control terminal of the output transistor, and

if the output transistor is in the off state, the third control circuit turns off the first transistor so as to block the power supply voltage terminal and the control terminal of the output transistor.

7. The photodetector amplifier circuit according to claim 3 , wherein

the control terminal of the second transistor is connected to the second control circuit.

8. The photodetector amplifier circuit according to claim 1 , wherein the photodetector amplifier circuit is used in a photocoupler.

9. The photodetector amplifier circuit according to claim 1 , wherein the second control circuit comprises an amplifier connected between an output of the second control circuit and a photodiode, the amplifier controlling an on state and an off state of the third control circuit.

10. The photodetector amplifier circuit according to claim 1 , wherein the second transistor and the output transistor are npn transistors.

11. The photodetector amplifier circuit according to claim 1 , wherein the second transistor and the output transistor are pnp transistors.