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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

The 741 opAmp

DC and Small Signal Analysis

Jeremy Andrus

For Engineering 332

May 15, 2002

Prof. Ribeiro

overview five parts of the 741
Overview: Five Parts of the 741
  • Biasing Currents
  • Input Stage
  • Second Stage
  • Output Stage
  • Short Circuit Protection
biasing current sources
Biasing Current Sources
  • Generates the reference bias current through R5
biasing current sources dc analysis
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
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
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
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
Input Stage:DC Analysis -3
  • Analysis of the active load:
second intermediate stage
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
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
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
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
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
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