Contrast control circuit for display apparatus

A display apparatus having a contrast control circuit which receives a first voltage of positive polarity adjusted by a user, and outputs a second voltage of negative polarity for controlling the contrast of a display panel, wherein the contrast control circuit comprises:

• an input terminal for receiving the first voltage and an output terminal at which the second voltage is produced by the contrast control circuit;
• first and second resistors having respective first ends connected to a positive supply voltage;
• a first transistor having an emitter connected to a second end of the first resistor, a base connected to the input terminal, and a collector,
• a second transistor having an emitter connected to the second end of the first resistor, a base connected to a second end of the second resistor, and a collector connected to a negative supply voltage;
• third and fourth serially connected resistors, the serially connected resistors having a first end connected to the base of the second transistor and a second end connected to the output terminal;
• and
• a Darlington circuit having an output connected to the output terminal and an input connected to the collector of the first transistor, the Darlington circuit being connected to the negative supply voltage, whereby the contrast control circuit operates such that absolute value of the second voltage is inversely proportional to absolute value of the first voltage.

A liquid crystal display apparatus having a contrast control circuit which receives a first voltage of positive polarity adjusted by a user, and outputs a second voltage of negative polarity for controlling the contrast of a liquid crystal display panel, wherein the contrast control circuit comprises:

• an input terminal for receiving the first voltage and an output terminal at which the second voltage is produced by the contrast control circuit;
• first and second resistors having respective first ends connected to a positive supply voltage;
• a first transistor having an emitter connected to a second end of the first resistor, a base connected to the input terminal, and a collector;
• a second transistor having an emitter connected to the second end of the first resistor, a base connected to a second end of the second resistor, and a collector connected to a negative supply voltage;
• third and fourth serially connected resistors, the serially connected resistors having a first end connected to the base of the second transistor and a second end connected to the output terminal;
• and
• a Darlington circuit having an output connected to the output terminal and an input connected to the collector of the first transistor, the Darlington circuit being connected to the negative supply voltage, whereby the contrast control circuit operates such that absolute value of the second voltage is inversely proportional to absolute value of the first voltage.