Differential amplifiers are known to include a pair of amplifying transistors that receive their respective DC bias currents from a DC power supply via matched biasing circuits and are controlled by a DC current source located between the common node of the pair of amplifying transistors and a return of the DC power supply. AC input signals that are provided to an input terminal of each amplifying transistor are amplified by the differential amplifier such that an output signal proportional to the difference of the AC input signals is provided at the differential amplifier's output. Operation of the differential amplifier is well known; thus, no further discussion will be presented.
Differential amplifier stages are extensively utilized as the input amplifier in various operational amplifiers, but are often also incorporated in analog multipliers. As is known, analog multipliers provided multiplication of an input AC voltage or current. Multipliers generally include a pair of differential amplifier stages which receive their respective DC bias currents via matched current mirrors and are controlled by independent DC current sources. An operational amplifier is typically employed in the analog multiplier to provide simultaneous control of the pair of differential amplifier stages which subsequently results in multiplication of the input current with the DC currents provided by the current sources. To produce voltage multiplication, an input voltage is provided to the input of a current multiplier via a resistor that converts the input voltage to an input current. The input current is multiplied by the current multiplier and delivered to the output of the multiplier. The output current is subsequently reconverted back to a voltage via an output operational amplifier to effectively produce voltage multiplication of the input voltage.
Ideally, differential amplifiers provide no output voltage, or current, when either no input voltages are applied or both input voltages have substantially identical magnitudes. However, since both transistors do not typically have identical DC current versus voltage characteristics, an offset DC voltage appears at the output of the differential amplifier without any differential input voltage applied. The offset voltage typically varies with power supply voltage changes and ambient temperature excursions depending on the characteristics of each transistor. Thus, since operational amplifiers and voltage multipliers employ differential amplifiers for gain control fluctuations in the offset voltage results in corresponding undesired fluctuations in the gain of the operational amplifiers and voltage multipliers that incorporate the differential amplifiers.
To reduce the offset voltage, prior art methodology applies an external secondary offset voltage to the differential amplifier in an attempt to counteract the effects of the internal offset voltage produced by the transistor characteristics. However, this secondary offset voltage must be accordingly adjusted for each pair of transistors and requires temperature compensation and a constant power supply voltage.
Therefore, a need exists for a differential amplifier that provides minimal offset between its differential inputs while being substantially invariant to temperature and power supply voltage variations and eliminates the need for an external secondary offset voltage.