The 741 opAmp

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The 741 opAmp. DC and Small Signal Analysis. Jeremy Andrus For Engineering 332 May 15, 2002 Prof. Ribeiro. Overview: Five Parts of the 741. Biasing Currents Input Stage Second Stage Output Stage Short Circuit Protection. Overview: 741 Schematic. Biasing Current Sources.

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The 741 opAmp

DC and Small Signal Analysis

Jeremy Andrus

For Engineering 332

May 15, 2002

Prof. Ribeiro

Overview: Five Parts of the 741
• Biasing Currents
• Input Stage
• Second Stage
• Output Stage
• Short Circuit Protection
Biasing Current Sources
• Generates the reference bias current through R5
Biasing Current Sources:DC Analysis
• The opAmp reference current is given by:
• For Vcc=Vee=15V and VBE11=VBE12=0.7V, we have IREF=0.73mA
Input Stage
• The differential pair, Q1 and Q2 provide the main input
• Transistors Q5-Q7 provide an active load for the input
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 IC10, and we get: IC10=19A
Input Stage:DC Analysis -2
• From symmetry we see that IC1=IC2=I, and if the npn is large, then IE3=IE4=I
• Analysis continues:
Input Stage:DC Analysis -3
• Analysis of the active load:
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
Second Stage:DC Analysis
• Neglecting the base current of Q23, IC17 is equal to the current supplied by Q13b
• IC13b=0.75IREF where bP >> 1
• Thus: IC13b=550uA=IC17
• Then we can also write:
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
Output Stage:DC Analysis
• Q13a delivers a current of 0.25IREF, so we can say: IC23=IE23=0.25IREF=180mA
• Assuming VBE18 = 0.6V, then IR10=15mA, IE18=180-15=165mA and IC18=IE18=165mA
• IC19=IE19=IB18+IR10=15.8mA
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)
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