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The 741 opAmp DC and Small Signal Analysis Jeremy Andrus For Engineering 332 May 15, 2002 Prof. Ribeiro

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Overview: Five Parts of the 741 Biasing Currents Input Stage Second Stage Output Stage Short Circuit Protection

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Overview: 741 Schematic

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Biasing Current Sources Generates the reference bias current through R5

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Biasing Current Sources: DC Analysis The opAmp reference current is given by: For V cc =V ee =15V and V BE11 =V BE12 =0.7V, we have I REF =0.73mA

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Input Stage The differential pair, Q1 and Q2 provide the main input Transistors Q5-Q7 provide an active load for the input

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Input Stage: DC Analysis - 1 Assuming that Q10 and Q11 are matched, we can write the equation from the Widlar current source: Using trial and error, we can solve for I C10, and we get: I C10 =19 A

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Input Stage: DC Analysis -2 From symmetry we see that I C1 =I C2 =I, and if the npn is large, then I E3 =I E4 =I Analysis continues:

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Input Stage: DC Analysis -3 Analysis of the active load:

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Second (Intermediate) Stage Transistor Q16 acts as an emitter-follower giving this stage a high input resistance Capacitor Cc provides frequency compensation using the Miller compensation technique

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Second Stage: DC Analysis Neglecting the base current of Q23, I C17 is equal to the current supplied by Q13b I C13b =0.75I REF where P >> 1 Thus: I C13b =550uA=I C17 Then we can also write:

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Output Stage Provides the opAmp with a low output resistance Class AB output stage provides fairly high current load capabilities without hindering power dissipation in the IC

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Output Stage: DC Analysis Q13a delivers a current of 0.25I REF, so we can say: I C23 =I E23 =0.25I REF =180 A Assuming V BE18 = 0.6V, then I R10 =15 A, I E18 =180-15=165 A and I C18 =I E18 =165 A I C19 =I E19 =I B18 +I R10 =15.8 A

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Short Circuit Protection These transistors are normally off They only conduct in the event that a large current is drawn from the output terminal (i.e. a short circuit)

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DC Analysis Summary

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741 opAmp Simulation: Schematic

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741 opAmp Simulation: Input

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741 opAmp Simulation: Output

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Conclusions The 741 is a versatile opAmp that can be used in a multitude of different ways When you break it down into the different components, it’s operation is actually understandable and comprehendible

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