It is important to note that the gain specification is taken at a very low frequency and that gain drops off with increasing frequency. Error term values dependent on the open loop gain and non-inverting closed loop gain of the operational amplifier. As the open-loop gain of the amplifier decreases the affect of the changing open-loop gain becomes more pronounced. If a change of load resistance causes a 10dB drop in open loop gain, the gain of 1000 circuit drops from 999 to 996.8. For an amplifier with an open-loop gain of 120dB (gain ratio of 1,000,000V/V) and a closed-loop gain of unity the response is almost perfect, however, that same amplifier configured for a gain of 1000 will show a 0.1% error. To evaluate the impact of this variable gain on a design, consider the error term values shown in Table I. Notice the error term that contains the open loop gain variable is the same for both circuits and the gain error is a function of the equivalent closed-loop, non-inverting gain. Gain relationships are given for both inverting and non-inverting configuration operational amplifiers below. Open loop gain is dependent upon the load resistance as is shown in this typical curve for the TLV2731.Ĭhanging open loop gain with load impedance may be significant in high accuracy designs. As the load impedance decreases the gain decreases.įigure 3. The significance of this dependency is seen as characteristic curves include gain vs. Output stages that swing very close to the supply rails are known as rail-to-rail outputs although the output voltage does not actually reach the rail voltage. This can affect two primary parameters, the open loop gain under load and the output impedance.įigure 2. The gain of this stage is now dependent on the load resistance. While this reduces the voltage drop from the output to the supply rail, it makes the output a gain stage. The voltage drop from the supply rail is just the VCE-SAT of a single transistor. As the output stage is a unity gain circuit, there is a minimal gain change with change in load resistance.Ī rail-to-rail output stage is generally composed of complementary common-emitter or common-source stages as shown in Figure 2. This stage does have the advantage of a low output resistance. Classical operational amplifier output stage. This total voltage drop would be excessive for the operational amplifier to be considered rail-to-rail output in most cases.įigure 1. The minimum voltage drop from the supply rail to the output is the VCE-SAT of Q2 added to the VBE of Q1.
Transistor Q2 acts as a current source feeding the base of Q1. Designing an operational amplifier with output voltage swing to both supply rails requires some deviations from classical circuit structures.Ī classical output stage would be structured with complementary common emitter stages as shown in Figure 1.
Classical operational amplifiers were designed to operate from 15V supplies with performance specified for 10V signals. Operational amplifier designs now push the signal operating range as close as possible to the supplies on both input and output to achieve the greatest dynamic range possible for the given supply voltage. For best performance, recognize the limitations and performance tradeoffs and be aware of those application circuits that minimize the effects on overall system operation.
The inclusion of this new data can give rise to questions concerning the impact on performance. The data sheets that describe these devices contain specifications and typical performance curves that never appeared in data sheets for classical operational amplifiers. To further these designs, analog circuit manufacturers provide many operational amplifiers designated for “single supply” and “rail-to-rail” operation.
Effects of op amp offset voltage portable#
Portable units rely on batteries for their power and battery power is most efficient when the circuitry is restricted to a single supply voltage. The electronics industry continues to see a significant growth in the number of portable devices being produced.