Non- Ideal Properties of OpAmps -I -Operational Amplifiers Types Tutorials Series

Non-Ideal Properties of Op-amps:

Output Saturation Voltage:-
•Although we have been assuming the op-amp will saturate at the supply voltages VPOS and VNEG, in actual practice an op-amp circuit will saturate at somewhat lower than VPOS and higher than VNEG, due to internal voltage drops in the design
–Emitter-follower output stage (BJT design) will drop a VBE
–CMOS design will have a similar drop
Input-Offset Voltage (Above Fig):-
•We have been assuming v+ = v- when vOUT = 0. In actual practice, however, there is usually a small input (or output) dc offset voltage in order to force vOUT to 0, under open-loop operation.

–The input-offset voltage (labeled VIO in the figure at the left) can be positive or negative and is usually small (anywhere from 1 uV to 10 mV)

Input-Offset Voltage Effect on Output Voltage (Fig. Left):

•To examine the effect input-offset voltage has on the output voltage, consider the non-inverting op-amp
–The gain of the op-amp is (R1 + R2)/R1 = 100
–Assume the input voltage is modeled adequately by a source VIO = +/- 10 mV
–Then, we can write that the output voltage is given by

vOUT = (vIN + VIO)(R1 + R2)/R1

= 100 vIN +/- 1 volt
–Thus, a 10 mV input-offset causes a 1V offset in vOUT