1 The 741 Operational-Amplifier

Reference Bias Current : The 741 op-amp circuit. Reference Bias Current

DC Analysis of the 741 Reference Bias Current V CC -V EE

Bias for input stage The 741 op-amp circuit. Bias for input stage I C10

Input Stage Bias I C10 can be determined by knowing I REF and R V BE11 V BE I C10

Biasing Input Stage : The 741 op-amp circuit. I C3 I C4 I C1 I C2

The dc analysis of the 741 input stage. npn β very high Base current I E /1+β P Relationship I REF & I O Base currents add together

8 A Simple BJT Current Source

Negative Feed-back Loop : 741 input stage. Negative Feed back Loop For some reason I in Q 1 & Q 2 increases Causes current pulled from Q8 to increase Output current of Q8 Q9 correspondingly increase Since I c10 remains constant, it forces combined current of Q3 & Q4 to decrease

The dc analysis of the 741 input stage. =

Input Stage : The 741 op-amp circuit. I C7

The dc analysis of the 741 input stage, continued. I c5 =I Bias Current of Q

The dc analysis of the 741 input stage, continued. 50 KΩ

The 741 op-amp circuit : SECOND STAGE I C16 I C17

DC Analysis : Second Stage Neglect Base Current of Q 23 I C17 =I C13B Q 13 is lateral pnp transistor Q 13B has a scale of 0.75 times that of Q 12 I C13B =0.75 I REF I REF = 0.73 mA & β P >>1 I C13B = 550 µA & I C17 = 550 µA

Output Stage Bias : The 741 op-amp circuit.

The 741 output stage without the short-circuit protection devices. Q 13 is lateral pnp transistor I S of Q 13A is 0.25 times I S of Q 12 Neglect Base current of Q 14 & Q 20 Base Current of Q 23 is 180/50=36 μ A Negligible as assumed

Output Stage Bias Voltage V BE18 ≈ 0.6V Current Thru R 10 =0.6/40k=15 µ A V BB

Summary Collector Currents : 741 Op Amp

Small-signal analysis of the 741 input stage. Collectors Q 1 & Q 2 connected to dc voltage so are grounded Q 3 & Q 3 are biased by constant current source so are open cct

Small-signal analysis of the 741 input stage. Input appears across four input resistors

The 741 op-amp circuit.

Small signal model : The load circuit of the input stage.

The load circuit of the input stage Q 5 & Q 6 are identical and their bases are tied together So their collector currents are equal

The load circuit of the input stage

Output Resistance : 741 Op Amp R o1 is parallel equivalent of R o4 & R o6 Output Resistance R o1 Assume that common bases of Q 3 & Q 4 are at virtual ground

Output Resistance : 741 Op Amp Output Resistance R o1 Assume that the base of Q 6 is at virtual ground Because signal is very small

Output Resistance : 741 Op Amp R o1 is parallel equivalent of R o4 & R o6 R o1 =R o4 ||R o6 R o1 =6.7 MΩ Output Resistance R o1

Figure 9.22 Small-signal equivalent circuit for the input stage of the 741 op amp.

Second Stage :The 741 op-amp circuit.

Figure 9.24 The 741 second stage prepared for small-signal analysis.

Input Resistance : Second Stage

Transconductance : Second Stage Thus current through the output resistance of Q 13B is zero

Output Resistance R 02 : Second Stage R 02

Output Resistance R 02 : Second Stage R 02 Since the resistance between the base of Q17 and ground is relatively small, The base is grounded and circuit is CB

Output Resistance R 02 : Second Stage R 02

Output Resistance R 017 Since the resistance between the base of Q17 and ground is relatively small, The base is grounded and circuit is CB

Figure 9.25 Small-signal equivalent circuit model of the second stage.

Figure 9.27 Thévenin form of the small-signal model of the second stage. Open Circuit Voltage Gain =

Output Stage :The 741 op-amp circuit.

The 741 output stage.

Input from second stage Q 17 Loaded with 2 kΩ resistor Q 18 & Q 19 and R 10 provide Class AB bias to output stage. Q 14 & Q 20 are output transistors Output stage is driven by emitter follower Q 23 acts as buffer

Output Voltage Limits Maximum positive output voltage v omax is limited by input circuit Saturation of Q 13 A Minimum negative output voltage v omin is limited by input circuit Saturation of Q 17

Small Signal Model for the 741 output stage. v o2 =-G m2 R 02 v i2 G m2 = 6.5mA/V & R O2 = 81kΩ R in3 is input resistance of the output stage with load R L

Input resistance R in3 of output stage R in R in3

Input resistance R in R in20 R in R out18 R in =R in20 ||R out18 Suppose Q 20 is conducting and Q 14 is cut-off

Input resistance R in20 R in20

Input resistance R out18 R out18 R out18 is r o13A in series with output resistance of Q 18 & Q 19 r o13A >>Output resistance of Q 18 & Q 19 Output resistance of Q 18 & Q 19 = 163 Ω

Input resistance R in3 of the output stage R in20 R in R out18 R in =R in20 ||R out18 R in3 β 20 = β 23 = 50, R L = 2kΩ, r o13A = 280kΩ R in3 = 3.7 MΩ

Small Signal Model for the 741 output stage. R in3 = 3.7 MΩ R o2 = 81 kΩ R in3 >> R o2 So R in3 will have little effect On the performance of the op amp = -515 V/V

Open Circuit Overall Voltage Gain G vo Small Signal Model for the 741 output stage.

Open Circuit Overall Voltage Gain G vo Q 14, Q 20 & Q 23 are common collector circuits, So gain is unity

Circuit for finding the output resistance R out. Exact Value of R out will depend upon which transistor (Q 14 or Q 20 ) is conducting Suppose Q 20 Is conducting and Q 14 is cut-off. Input source feeding the output stage is grounded

Circuit for finding the output resistance R out. 2 1

Output Short Circuit Protection Stage :The 741 op-amp circuit.

Output Short-Circuit Protection If any terminal of the IC is short circuited to one of the power supplies, IC will burnout. Protection Circuit limits the current in the output transistors in the event of short circuit.

Output Short-Circuit Protection Against maximum current the op amp can source In normal case – Current thru the emitter of Q 14 is 20mA, voltage drop across R 6 is approx 540mV and Q 15 is off In the event of short circuit, – if current in the emitter of Q 14 exceeds 20mA, voltage drop across R 6 will increase above 540mV and Q 15 will conduct. Robs some of the current supplied by Q 13A, thus reducing the base current of Q 14. This limits the current that the op amp supplies from the output terminal in the outward direction to 20mA.

Output Short-Circuit Protection Against maximum current the op amp can source In normal case – Current thru the emitter of Q 20 is 20mA, voltage drop across R 7 is approx 540mV and Q 21 is off In the event of short circuit, – if current in the emitter of Q 20 exceeds 20mA, voltage drop across R 7 will increase above 540mV and Q 21 will conduct. Robs some of the current supplied by Q 24, thus reducing the base current of Q 20. This limits the current that the op amp supplies from the output terminal in the inward direction to 20mA.

Small Signal Gain Gain is found from the cascade of the equivalent circuits of the op amp

Frequency Response

CcCc

C c introduces a dominant low-frequency pole Using Miller’s theorem, the effective capacitance due to Cc between the base of Q 16 and ground is The total resistance between base of Q 16 and ground is CcCc

Figure 9.32 Bode plot for the 741 gain, neglecting nondominant poles. The convenience of use of internally compensated 741 is achieved at the expense of a great reduction in open loop gain--- externally compensated op amp.

Figure 9.33 A simple model for the 741 based on modeling the second stage as an integrator.

Slew Rate

Slew rate