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• Op-Amp Accuracy Question
• Certified Electronics
• Q&A TIPS — Batteries
Op-Amp Accuracy Question
QFor years, I have used a variation of the circuit in Figure 1. The output was reliably zero, plus or minus 0.1 millivolts or so, with no input. Lately, almost all LM324s I buy —
regardless of price — are likely to be plus or minus
one millivolt, and I discard most of them as useless.
Can anyone tell me where to get a quad op-amp that
has reliable low offset?
— Milton Lilie
AYou are talking about what is known as output offset voltage which is the value of voltage at the output terminal of an operational amplifier (op-amp), with zero
voltage across the input terminals (shorted input
terminals). The op-amp is a high gain/DC-coupled
differential amplifier with a large bandwidth, high
input impedance (doesn't heavily load the previous
circuit), and low output impedance (dissipates less
energy in output stages) which — in modern times —
is packaged in a single small Integrated Circuit (IC)
device (you can also make op-amps with vacuum
Op-amps act as instrument amplifiers (increase
level of millivolt sensor outputs), comparators,
summing circuits, etc. Op-amps are constructed of
transistors, diodes, resistors, and capacitors which are
"formed" on silicon substrates (kind of like micro
versions of printed circuits). Like discrete
components, these epitaxial components have
tolerances which means the electrical characteristics
of different devices on the same chip vary.
The op-amp input is a differential (difference) amplifier
made up of pairs of transistors which roughly oppose each
other. Herein lies the output offset problem. If the
transistor's characteristics (mostly the transistor gain, hfe)
exactly match, when the input to the + (non-inverting) and
- (non-inverting) terminals is equal, the output voltage will
In the real world, transistor characteristics match
exactly on very rare occasions (maybe influenced by the
lunar-planetary alignment ???).
In order to correct the output offset voltage, we
introduce an input offset voltage to force the op-amp's
output to zero when the + and - terminal voltages are
equal. I am assuming that your circuit uses the LM-324 op-amp IC as an inverting amplifier, so Figure 2 would work
as a offset voltage compensation circuit (for non-inverting
■ FIGURE 1.
■ FIGURE 2.
8 May 2015