Start signal detection circuit

1. A start signal detection circuit comprising:

a wave-detection circuit that outputs a voltage in accordance with an envelope of a radio signal from an output point, the radio signal being input from a detection point;

a reference voltage generation circuit that outputs a voltage at the output point at a non-signal state as a reference voltage to a reference point; and

a differential amplification circuit that amplifies and outputs a voltage difference between the output point and the reference point;

wherein the wave-detection circuit comprises:

a first diode having an anode connected to a power supply through a first current source and a cathode connected to the detection point;

a second diode connected between the detection point and a ground; and

a first capacitor connected between the anode of the first diode and an alternating ground;

wherein the cathode of the first diode serves as the output point; and

wherein the differential amplification circuit includes a differential pair composed of transistors that use charges as a carrier.

2. The start signal detection circuit according to claim 1 , wherein the transistors constituting the differential pair are NPN-type bipolar transistors or N-channel-type field effect transistors.

3. The start signal detection circuit according to claim 1 , wherein the reference voltage generation circuit comprises a third diode having an anode connected to the power supply through a second current source and a cathode connected to the reference point and, a fourth diode connected between the reference point and a ground.

4. The start signal detection circuit according to claim 3 , wherein a first resistor and a second resistor are provided in place of the first current source and the second current source respectively.

5. The start signal detection circuit according to claim 3 , wherein the reference voltage generation circuit further comprises a second capacitor connected between the anode of the third diode and the alternating ground.

6. The start signal detection circuit according to claim 1 , wherein the differential amplification circuit comprises first and second transistors connected to each other at a connection point of their ground side terminals, a third current source connected between the connection point and a ground, and a load connected between the power-supply side terminals of the first and second transistors and the power supply.

7. The start signal detection circuit according to claim 6 , wherein the load is a resistive load or an active load.