24 Operational Amplifiers with Single-Ended Outputs: Applications

Electrical applications increasingly use a single supply voltage of 5 V or less as portable electrical equipment becomes more popular. The supply voltage for portable systems can be as low as the voltage provided by one battery cell (1.5 V). Reduced supply voltage designs must use the complete power supply span to have a usable dynamic range. Operational amplifiers that use the complete span between negative and positive supply voltage for signal conditioning are generally known as rail-to-rail amplifiers. The usable span is an important value because it influences several parameters such as noise susceptibility, signal-to-noise ratio (SNR), and dynamic range. Signal sources are often connected to the positive or negative supply rail. Operational amplifiers need rail-to-rail input capability to match both signal sources with one device. This report explains the function and the use of rail-to-rail operational amplifiers.

Dynamic Range and SNR in Low Single Supply Systems

Reducing the operating supply voltage from a ±15-V split supply to a single 5-V supply significantly reduces the maximum available dynamic range. The dynamic range at the output is determined by the ratio of the largest output voltage to the smallest output voltage. An industry standard operational amplifier like the TLC271 is specified at 5-V single supply with 3.8 Vpp for the maximum output swing. This means that the whole supply span can not be used for the output swing, resulting in a further reduction of the maximum available dynamic range and SNR. A rail-to-rail operational amplifier like the TLV24xx family can use the full span of the supply range for signal conditioning at the input and output. Operational amplifier disturbance levels are independent of the supply voltage. This results in smaller spacing between usable and noise signals. If the operational amplifier is used with ac signals, by decoupling the signals from dc, then noise forms the determining disturbance signal. For a standard operational amplifier such as the TLC271C, the input noise voltage Vn at a signal bandwidth of 1 MHz equals 68 umV†= 68 nV/(Hz)^1/2*(1 MHz)^1/2. With a 5-V single supply, the reduced output range allows a maximum signal level of 3.8 Vpp. This results in a unity gain configuration in a SNR of 95.4 dB=20 log(4 V/68 uV). In the same configuration, a rail-to-rail amplifier such as the TLV246xI with Vn=11 nV/(Hz) )^1/2 † and a maximum signal level of 5 Vpp at the input and output provides a signal-to-noise ratio of 113 dB=20 log(5 V/11 umV) at BW=1 MHz. In a precision system the operational amplifier must amplify the dc voltage level precisely. Errors in this area result from offset and gain problems. In a 5-V system with a constant common-mode voltage, the TLC271C has an input offset voltage VIO of 1.1 uV†. This alone limits the dynamic range to 71 dB=20 log(3800/1.1) in a unity gain configuration. The TLV245x, however, with VIO = 20 mV† and the rail-to-rail characteristic has a significantly higher dynamic range of 108 dB=20 log(5000/0.02) in the same circuitry.

When signal-to-noise ratio and dynamic range are critical design parameters, rail-to-rail characteristics of the operational amplifier must ensure that these parameters are met.

The Output Stage

If the output swing from a standard operational amplifier is not large enough to fit the system requirement (for example the analog-to-digital-converter input range), then a rail-to-rail operational amplifier must be used. Operational amplifiers with rail-to-rail output stages achieve the maximum output signal swing in systems with low single-supply voltages. They can generate an output signal up to the supply rails. A large output voltage swing results in increased dynamic range. For example, Figure 1 shows the output signal of a TLV2462 with a 5-Vpp input signal. The TLV2462 with a 5-V single supply operates as a voltage follower and drives a load of 1 kW. The low 1-kW load results in a voltage drop of several mV, which is not visible in the diagram.

Construction of a Rail-to-Rail Output Stage

The rail-to-rail characteristic is achieved by altering the output stage construction. Figure 2 shows the basic construction of a rail-to-rail CMOS output stage as used in the TLC227x. A complimentary MOS transistor pair, consisting of a self-locking P-channel and self-locking N-channel, forms the output. Both transistors operate as a common source circuit. A common source circuit functions like a common emitter circuit for bipolar transistors. Along with the current amplification, a voltage amplification also takes place. The voltage loss VDS at the output stage transistors has a disadvantageous effect on the voltage gain. As the current increases through a MOS transistor the resistance between drain and source increases slightly. During high loading of the output, this resistance, together with the increased current, results in a higher voltage drop VDS. The full output range of a rail-to-rail operational amplifier is therefore only
useable with low load. Figure 3 shows this on the output level of the TLV243x and TLV246x.

Reduction of the output signal due to the load also results in a reduction of the open-loop gain AVD. Because the open-loop gain is dependent on the connected load, the load should always be considered during comparison of the open-loop gain of different amplifiers. Figure 4 shows the influence of a resistive load on the amplification of a TLV246x.