22-12-2012, 06:43 PM
The uA741 Operational Amplifier
Operational.ppt (Size: 554.5 KB / Downloads: 156)
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.
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
Input Differential Stage
The input stage consists of the transistors Q1 through Q7 with biasing performed by Q8, Q9, and Q10.
Transistors Q1 and Q2 are emitter followers which causes input resistance to be high and deliver the differential input signal to the common base amplifier formed by Q3 and Q4.
Transistors Q5, Q6, and Q7, and resistors R1, R2, and R3 form the load circuit of the input stage. This portion of the circuit provides a high resistance load.
Transistors Q3 and Q4 also serve as protection for Q1 and Q2. The emitter-base junction of Q1 and Q2 breaks down at around 7V but the pnp transistors have breakdown voltages around 50V. So, having them in series with Q1 and Q2 protects Q1 and Q2 from an accidental connection between the input terminals.
Output Buffering Stage
The Output Stage consists of the
complimentary pair Q14 and Q20,
and a class AB output stage
composed of Q18 and Q19. Q15
and Q21 give short circuit protection
(described later) and Q13A supplies
current to the output stage.
The purpose of the Output Stage is
to provide the amplifier with a low
output resistance. Another requirement
of the Output Stage is the ability to
dissipate large load currents without
dissipating large quantities of power.
This is done through the class AB Out-
put 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)
Concluding Remarks
The uA741 operational amplifier is a versatile circuit that is not adversely affected by outside interference.
Changes in beta, resistor values, and temperature have little effect on the op-amp.
This shows how well the uA741 was designed.
However, as technology continues to improve, CMOS amplifiers are beginning to become more popular than their BJT cousins.