Op-amps that interface with digital logic (which sometimes causes significant power supply noise) generally should have good PSRR ratings. Higher operating voltages and lower noise frequencies generally bring better PSRR figures. PSRR depends on the actual power supply voltage as well as the frequency of the noise. So, a 60-dB value means a one-volt change in the power supply will cause a 0.001-V change in the output. This specification is listed in decibels (dB) where every 20 dB rise is a ten-fold increase in the ratio. Power-supply rejection ratio (PSRR) identifies the op-amp sensitivity to variations in the power supply. Tests can establish these parameters, and the most important of them can be found in op-amp specsheets.
There are several key parameters that help determine the suitability of a given op-amp to a task at hand. So negative voltages are not allowed at the inputs with this single-supply specification. Noteworthy as well is that the inputs to the op-amp must stay within the power supply (op-amps dubbed single-supply types allow the input to deviate down to the negative rail, usually ground). This is because the input is assumed to be at ground and ac input signals deviate above and below ground. But some op-amps are specified as requiring positive and negative power supplies. Note also that there is no ground pin on an op-amp, and op-amps can operate from a single supply. Gain and response are then determined largely by the feedback network instead of by opamp qualities.Ī point to note is that there are special types of op-amps, including high frequency (10s to 100s of MHz) units, current-difference, or trans-conductance op amps. By applying a portion of the output to the inverting input, the gain is reduced to a sensible level. In closed-loop configuration, circuit performance is stabilized and becomes far more predictable. Positive feedback can be implemented for regeneration. In open-loop configuration (no negative feedback) the op-amp is a comparator. In the rarely used open-loop configuration, the device is said to be saturated when this difference is greater than the supply voltage. Because the output is fed back to the negative input, the device is non-inverting.ĭue to the enormous gain, a small difference between inputs causes the amplifier output to closely approach the supply voltage.
Bottom, this op-amp has negative feedback. Top, this op-amp is open-loop because it has no feedback circuit. A type of differential amplifier with high gain, the differential inputs and single output enable this small, inexpensive IC to use negative feedback in a wide variety of applications, in which its gain (hundreds of thousands of times higher than the difference between the two inputs), bandwidth and input and output impedances are set by external circuitry. The operational amplifier (op-amp) has been one of the most successful integrated circuits.
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