Double balanced mixer

A double balanced mixer comprising:

• a first differential amplifier including a first pair of transistors having their emitters connected to each other;
• a second differential amplifier including a second pair of transistors having their emitters connected to each other;
• a third differential amplifier, connected to the first and second differential amplifiers, including a third pair of transistors having their emitters connected to each other via a resistor;
• first and second constant-current sources connected to the emitters of the third pair of transistors, respectively;
• and
• a gate circuit, connected to one of the first and second constant-current sources, for selectively activating the connected one of first and second constant-current sources in response to a control signal;
• wherein each of the first and second constant-current sources includes:
• a constant-current transistor having a base that receives a bias current and a collector connected to the third differential amplifier;
• and
• a resistor connected between the emitter of the constant-current transistor and a ground;
• and
• wherein the gate circuit is connected between the resistor and the constant-current transistor of the connected one of the first and second constant-current sources.

An orthogonal modulator comprising:

• a frequency multiplier for receiving a carrier signal and generating a multiplied signal in which the frequency of the carrier signal is multiplied by a predetermined factor;
• a phase shifter, connected to the frequency multiplier, for receiving the multiplied signal and generating a first carrier signal which is an in-phase component of the carrier signal, and a second carrier signal which is an orthogonal component of the carrier signal, by dividing the multiplied signal;
• a first mixer, connected to the phase shifter, for receiving the first carrier signal and a first base band signal and generating a first modulation signal by mixing the first carrier signal and the first base band signal;
• a second mixer, connected to the phase shifter, for receiving the second carrier signal and a second base band signal and generating a second modulation signal by mixing the second carrier signal and the second base band signal;
• and
• an adder, connected to the first and second mixers, for receiving the first and second modulation signals and generating an output signal by adding the first and second modulation signals;
• wherein the first mixer includes,
• a first differential amplifier including a first pair of transistors having their emitters connected to each other and their bases receiving the first carrier signal,
• a second differential amplifier including a second pair of transistors having their emitters connected to each other and their bases receiving the first carrier signal,
• a third differential amplifier, connected to the first and second differential amplifiers, including a third pair of transistors having their emitters connected to each other via a resistor and their bases receiving the first base band signal,
• first and second constant-current sources connected to the emitters of the third pair of transistors, respectively, and
• a first gate circuit, connected to one of the first and second constant-current sources, for selectively activating the connected one of the first and second constant-current sources in response to a control signal, and
• the second mixer includes,
• a fourth differential amplifier including a fourth pair of transistors having their emitters connected to each other and their bases receiving the second carrier signal,
• a fifth differential amplifier including a fifth pair of transistors having their emitters connected to each other and their bases receiving the second carrier signal,
• a sixth differential amplifier, connected to fourth and fifth differential amplifiers, including a six pair of transistors having their emitters connected to each other via a resistor and their bases receiving the second base band signal,
• third and fourth constant-current sources connected to the emitters of the sixth pair of transistors, respectively, and
• a second gate circuit, connected to one of the third and fourth constant-current sources, for selectively activating the connected one of the third and fourth constant-current sources in response to the control signal.