Understanding of Operational Amplifier (2)

Editor : ElectRobot

1) Input Bias Current

 One of the practical op-amp limitations is that the input current is not exactly zero as we assume in the current rule. There is a tiny input bias current for an op-amp which is about 80 nA for 741 type op-amps. For FET- input op-amps it may be a few picoamps. The superbeta Darlington LM11 may have an input current of 25 picoamps and the MOSFET ICH8500 is one of the very lowest at 0.01 picoamp. Judgements have to be made because those with the lowest input bias currents cannot operate at high speed. For high speed one may choose an op-amp with higher bias current at the cost of seeing some voltage drop across the resistors of the feedback network, bias network or source impedance. This may restrict you to smaller resistors and place practical limits on gain, or may produce some variations in output voltage.

(Figure 1) Input Bias Current Effect

2) Input Offset Voltage

 An ideal op-amp amplifies the differential input; if this input is 0 volts (i.e. both inputs are at the same voltage with respect to ground), the output should be zero. However, due to manufacturing process, the differential input transistors of real op-amps may not be exactly matched. This causes the output to be zero at a non-zero value of differential input, called the input offset voltage.

Typical values for Vos are around 1 to 0.1V for cheap commercial-grade op-amp integrated circuits. This can be reduced to several microvolts if nulled using the integrated cirduits’ offset null pins or using higher-quality or laser-trimmed devices.

(Figure 2) Noninverting Amplifier’s Output Error

3) Input & Output Range

 We often receive applications questions relating to the power supply, input and output voltage range capabilities of our op amps. It can be confusing so here is an attempt to sort it out. First, common op amps don’t have ground terminals. A standard op amp does not “know” where ground is so it cannot know whether it is operating from a dual supply (±) or from a single power supply. As long as the power supply, input and output voltages are within their operating ranges, all is good

(Figure 3) Input & Output Voltage Specification

(Figure 4) OP-Amp Current Flow Chart

4) Slew Rate

 The ideal op-amp has an infinite frequency response. This means that no matter how fast the input changes, the output will be able to keep up. In real op-amps, this is not the case, and when the input changes too fast the output is not able to keep up.

(Figure 5) Slew Rate Characteristic & Signal

5) Common Mode Rejection Ratio

 The common-mode rejection ratio (CMRR) of a differential amplifier (or other device) measures the ability of the device to reject common-mode signals, those that appear simultaneously and in-phase on both amplifier inputs. An ideal differential amplifier would have infinite CMRR; this is not achievable in practice. A high CMRR is required when a differential signal must be amplified in the presence of a possibly large common-mode input. An example is audio transmission over balanced lines.

(Figure 6) CMRR Formula

 CMRR can affect a current sensing solution’s accuracy. It is a measure of a device’s ability to reject common-mode signals. This is important because common-mode signals can manifest themselves inside a device as differential signals, thereby decreasing the accuracy of a solution.

(Figure 7) CMRR effect