TheuA741OperationalAmplifier.pptVIP

The uA741 Operational Amplifier Outline Brief History Stages DC Bias Point Analysis Small Signal Analysis Concluding Remarks Brief History 1964 – Bob Widlar designs the first op-amp: the 702. Using only 9 transistors, it attains a gain of over 1000 Highly expensive: $300 per op-amp 1965 – Bob Widlar designs the 709 op-amp which more closely resembles the current uA741 This op-amp achieves an open-loop gain of around 60,000. The 709’s largest flaw was its lack of short circuit protection. Brief History (cont) After Widlar left Fairchild, Dave Fullagar continued op-amp design and came up with the uA741 which is the most popular operational amplifier of all time. This design’s basic architecture is almost identical to Widlar’s 309 op-amp with one major difference: the inclusion of a fixed internal compensation capacitor. This capacitor allows the uA741 to be used without any additional, external circuitry, unlike its predecessors. The other main difference is the addition of extra transistors for short circuit protection. This op-amp has a gain of around 250,000 Schematic Stages Input Differential Stage Intermediate Signal-Ended High-Gain Stage Output Buffering Stage Current Source / Short Circuit Protection Input Differential Stage Intermediate Single-Ended High-Gain Stage Output Buffering Stage Current Source / Short Circuit Protection Transistors Q11 and Q12 form one half of a current mirror that is used to supply current to the entire op-amp. Transistor Q10 is used to supply a bias current to the Input Stage, Q13B supplies the Second Stage, and Q13A supplies the Output Stage. Transistors Q15, Q21, Q24, Q22, and resistors R6, R7, and R11 make up the short circuit protection circuit. For a more detailed description see your text. (Microelectronic Circuits by Sedra / Smith 4th addition, pg 813) DC Analysis Reference Bias Current This current is generated by Q11, Q12 and resistor R5. From these, we can write: From this value of IREF, the curre