Characteristics Of Operational Amplifier

Friday, October 12th, 2018 - Analog Electronics

Characteristics Of Operational Amplifier

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 Of Operational Amplifier

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.

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The open loop input-output relationship for a rather “well-behaved” practical operational amplifier is shown in figure 31. 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 E0.

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.

Open Loop Operation

As an example of open loop operation, consider the Texas Instruments THS4001 used as an open loop DC amplifier. DC open loop gain is 10000 and output saturation occurs at ±13.5 volts (for a supply voltage of ±15V). Hence, for linear operation, the input voltage cannot exceed 10,000 X 2.7 millivolts. The open loop amplifier is also subject to the full effect of random noise, offset, and drift, which may be greater than 2.7 millivolts. Therefore, the open loop amplifier is not useful for linear operation, because any circuit that is so close to saturation may cause the output to latch-up.

Output Limiting

Texas Instruments specifications for operational amplifiers give a voltage and a current output rating, plus output short circuit current. Output saturation voltages are commonly slightly greater than the rated output value when the nominally specified power supply voltage is used. Texas Instruments operational amplifiers will supply full output voltage to a load drawing full rated output current for an indefinite period.

In addition, for lower output voltages, slightly higher output current is available up to the short circuit conditions. Though the current ratings are conservative, exceeding the rated current should be attempted only after some calculation, unless the output voltage is extremely low. Output voltage is self-limiting, and voltage levels above the saturation voltage cannot be achieved. It is not recommended to operate an operational amplifier saturated for an indefinite period of time. Because it does not fit the characteristics of operational amplifier.

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