166-op-amp-current-offset-op-amp-voltage-offset-and-op-amp-slew-rate

Op-amp current offset, op-amp voltage offset and op-amp slew rate are important parameters of any operational amplifier. Every op-amp has high, but finite input impedance. So input biasing currents in op-amp have some small but non-zero value. Because transistors in input stage of the op-amp are not perfectly matched, there is some difference between biasing currents. This bias current difference is modeled with parameter known as op-amp current offset. We also need another parameter to describe op-amp imperfection, and this parameter is called op-amp voltage offset. What is the simplest way to detect offsets in the op-amp? Let’s have a look at the scheme bellow.

offset_scheme01

Input voltage in inverting op-amp is 0V, but output is different from 0V. How is it possible, if we know that output should be

offset_eq01

and VIN=0V? Op-amp current offset and op-amp voltage offset are the parameters that describes that phenomena. Real op-amp can be modeled as ideal op-amp with offset current and voltage, given in figure bellow.

offset_scheme02

In upper inverting op-amp amplifier, current offset can be compensated by adding the resistor in non-inverting input branch. Value of that compensating resistor should be R=R1||R2. Op-amp voltage offset is more serious problem, particularly when we use op-amp as an analog integrator. In that case, voltage offset is integrated together wit the input voltage, and output can be easily saturated for that reason. One way to compensate both current and voltage offset is given in the scheme bellow.

offset_scheme03

Problem with the offset is that is changing in time. Typical hold of given offset value is one to two minutes. Offset changes both value and polarity during time. Typical value of voltage offset for many commercial op-amps is few mV, and offset current is typically few tens of nA. If transistors that op-amp is made of are JFET transistors, op-amp has better slew rate but larger voltage offset as well. On the other hand, if transistors that op-amp is made of are bipolar transistors, voltage offset can be very low, but slew rate is smaller as well. So combination of one fast op-amp with large slew rate and large voltage offset with one op-amp with small slew rate and small voltage offset and low pass filter can be used for amplification input AC signal on high frequencies without problem that amplifier can be saturated due to voltage offset presence. Such combination is presented in figure bellow.

offset_scheme04

In the upper scheme, OP1 is an op-amp with high voltage offset and high slew rate, while OP2 is an op-amp with low voltage offset and low slew rate. OP1 amplifies it’s large offset and invert it’s polarity. Low pass filter extracts this amplified and inverted offset as DC signal, and via low offset buffer OP2, inverted DC signal is returned back to OP1 and automatically cancel voltage offset.
Slew rate is the parameter that describes highest rate at which output voltage can swing. For example, if input voltage is square-wave, effect on the output voltage due to small slew rate is given in figure bellow.

slew_rate

Because slew rate is small, output voltage can not follow desired rate of change and waveform of the output voltage is deformed to trapezoidal waveform. Minimal slew rate for given signal frequency f and maximal voltage output supposed to be
Slew Rate = 2·π·f·VMAX
If for particular purpose is chosen op-amp with smaller slew rate, output voltage waveform will be deformed.

External links:

Op-amp Current Offset on Ecircuitcenter
Op-amp Voltage Offset on Wikipedia
Op-amp Slew Rate on Hyperphysics

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