CHARACTERISTICS OF PRACTICAL OPERATIONAL AMPLIFIERS

The modern operational amplifier is a solid state, high gain, DC voltage amplifier. Practical feedback circuits employing it are based on the circuits that were derived in the preceding section using the ideal operational amplifier model. Substituting a real for an ideal operational amplifier will result in some predictable variation from ideal operation that is negligibly small in many applications. This section is intended to acquaint the reader with the characteristics of the real devices so that they may be utilized to the fullest possible extent in practical circuits.

Open Loop Characteristics- 

In the case of the ideal operational amplifier, circuit operation was seen to be  dependent entirely on the feedback used.  It is possible to use the real operational amplifier open loop, but control and stability problems are encountered due to the high open loop gain (X100000 typically at DC). Random noise from the input circuit and noise generated within the operational amplifier itself plus any variations in amplifier characteristics due to temperature change or aging components are all multiplied by open loop gain. Slight variations in the manufactured unit become noticeable due to this effect; hence open loop specifications are sometimes given conservative “typical” values.

Open loop operational amplifier specifications have a relatively remote connection to closed loop operation of a circuit since they do not as much define circuit operation as they do limit it. The sheer numbers of useful operational amplifier circuits make it impossible for a manufacturer to completely specify closed loop operation. Since each closed loop circuit is, in essence, a special case, it is necessary to understand both open and closed loop characteristics before the intelligent design of circuitry using operational amplifier can begin. Any statements that are to be made about operational amplifier circuits must be qualified by the information “open loop” or “closed loop,” and the character of the feedback should be specified for “closed loop”

information.

                 

The open loop gain, A, is measured by the slope of the curve so it can be seen that the operational amplifier only amplifies between the saturation values of EO.The slope of the amplifying portion of the transfer curve is dependent on the frequency of the input voltage while the saturation voltages remain constant. This relation between input and output holds regardless of the feedback configuration used as long as the amplifier is not in overload.

The “well behaved” aspect of this operational amplifier is the fact that its transfer curve goes through the origin. In practice, all operational amplifiers exhibit offset, a fault that effectively shifts the transfer curve from the origin. To complicate matters further, this offset value will wander, producing drift. Both of these phenomena are of the same order of magnitude as the input voltage necessary to drive the open loop amplifier into saturation and a necessary part of circuit design is to minimize their effect.