Optional
Amplifiers
May 04
The term operational amplifier or "op-amp" refers to a class of high-gain DC
coupled amplifiers with two inputs and a single output. The modern integrated
circuit version is typified by the famous 741 op-amp. Some of the general
characteristics of the IC version are:
• High gain, on the order of a million
• High input impedance, low output impedance
• Used with split supply, usually +/- 15V
• Used with feedback, with gain determined by the feedback network.
Their characteristics often approach that of the ideal op-amp and can be
understood with the help of the golden rules.
The Ideal Op-amp
The IC Op-amp comes so close to ideal performance that it is useful to state the
characteristics of an ideal amplifier without regard to what is inside the
package.
Infinite voltage gain
Infinite input impedance
Zero output impedance
Infinite bandwidth Zero input offset voltage
(i.e., exactly zero out if zero in).
Compare to real op-amps
Practical departures from ideal op-amp
These characteristics lead to the golden rules for op-amps. They allow you to
logically deduce the operation of any op-amp circuit.
For an op-amp with external feedback
(From Horowitz & Hill*)
I. The output attempts to do whatever is necessary to make the voltage
difference between the inputs zero. The Voltage Rule.
II. The inputs draw no current. The Current Rule.
Comments on 1: The voltage gain of a real op-amp is so high that a fraction of a
millivolt input will swing the output over its full range. Practically, that
amounts to rule 1.
Comments on 2: The input current is so low (0.08 microamps for the 741, picoamps
for an FET-input op-amp) that rule 2 is practically correct.
*Horowitz, Paul and Hill, Winfred,The Art of Electronics, Cambridge
University Press, 1980