Open Loop Operation & Output Limiting

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 vots(fora 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 forlinear operation, because any circuit that is so close to saturation maycause 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.