The present invention relates generally to the field of differential amplifiers and operational amplifiers and more particularly to an amplifier of this kind fabricated in integrated circuit technology and having a rail-to-rail common mode range at the amplifier input and output.
There are times when an operational amplifier is required which can linearly amplify an input signal over nearly the full supply voltage range, especially in the case of a supply voltage as low as 1.5 volts. Operational amplifiers to be used with low supply voltages should utilize as much as possible of the available supply voltage range in order to obtain the best signal-to-noise ratio. This requirement is especially severe in the case of low power applications or when the supply voltage is low as in battery powered systems.
U.S. Pat. No. 4,555,673 issued Nov. 26, 1985 in the name of J.H. Huijsing et al and an article by J.H. Huijsing et al in the IEEE JSSC, vol. SC-20, No. 6, 12/85, entitled "Low Voltage Operational Amplifier with Rail-to-Rail Input and Output Ranges", describe circuits which generally work well to amplify an input signal while using a large part of the supply voltage range. The Huijsing et al article describes an operational amplifier having rail-to-rail input, a so-called folded cascode stage and a rail-to-rail class A-B output stage. The input stage with a rail-to-rail common-mode input voltage range provides a constant transconductance over the full common-mode range. However, this operational amplifier also requires an intermediate stage to increase the gain and a completely separate class A-B quiescent bias network. Another disadvantage of the Huijsing et al operational amplifier is that it requires a fairly large number of integrated circuit components.
In the Huijsing et al patent and article the rail-to-rail common-mode voltage swing at the amplifier input is obtained by using two differential amplifier input stages in parallel and operating same such that the common mode (CM) voltage of one stage can reach the positive supply voltage rail and that of the other the negative supply voltage rail. Three common-mode input voltage ranges are present in these circuits. In a first range, which extends from the negative supply voltage to an intermediate range, only one differential amplifier input stage is operative. In a second range extending from the positive supply voltage to the intermediate range, the other differential amplifier input stage is operative. In the intermediate range, both differential amplifier stages are in operation.
The Huijsing et al patent describes an improved differential amplifier with a rail-to-rail input capability which attempts to correct the problem of a variation in the amplifier transconductance as the input common-mode voltage varies across the supply voltage range. The patent describes a current steering control circuit that regulates the operating currents for the two differential amplifiers such that the overall amplifier transconductance is controlled to be substantially constant as the common-mode voltage of the amplifier input signal varies across the supply voltage range. A summing circuit selectively combines internal currents supplied by the two differential amplifiers so as to derive at least one output signal representative of the input signal.
Another prior art operational amplifier is described in an article by D.M. Monticelli entitled "A Quad CMOS Single-Supply Op Amp with Rail-to-Rail Output Swing, in the IEEE JSSC, Vol. SC-21, No. 6, December 1986. This article describes a Quad CMOS amplifier which includes a class A-B output stage with quiescent current biasing. This op-amp circuit does not have a rail-to-rail input and does not use a folded cascode arrangement. It also requires two independent matched internal current sources for class AB quiescent current biasing so that any mismatch therebetween will result in a contribution to the input offset voltage. Furthermore, the Monticelli circuit does not eliminate the dependence of the quiescent bias current in the output transistors of the class A-B output stage on the voltage supply.
Further background information can be obtained from a review of U.S. Pat. Nos. 4,893,091 (Jan. 9, 1990); 4,958,933 (Sep. 18, 1990) and 4,797,631 (Jan. 10, 1989) U.S. Pat. No. 4,797,631 describes a rail-to-rail input stage and folded cascode stage. The circuit does not include a class A-B output stage or floating current source. Although this patent shows an input stage and a folded cascode stage, it does not include the current steering transistor of the Huijsing et al patent in the differential input stage and which transfers current from one current source transistor to the other as the input signal traverses the input common mode range. Thus, it does not maintain a constant current gain in the input stage, as is accomplished in the op-amp described in Huijsing et al patent discussed above.
U.S. Pat. No. 4,958,133 also describes a rail-to-rail input stage and a folded cascode stage, but again does not include a class A-B output stage, floating current source or current steering transistors in the input stage. A disadvantage of this circuit is that it does not maintain the bias currents constant.
U.S. Pat. No. 4,893,091 describes an amplifier with a class A-B output stage. However, this patent uses an intermediate gain stage between the input and output and wherein the output is a common collector driver. The requirement of an intermediate amplifier stage is a major disadvantage of this patent.