ΜA741


The µA741 is an early operational amplifier integrated circuit, originally designed at Fairchild Semiconductor by Dave Fullagar and released in 1968.
It is still often used as an educational example, especially for imperfections that an operational amplifier can have, and due to its long history of production of compatible and architecturally near-identical ICs under similar names
. Being inferior in every aspect compared to modern op amps, applications are restricted to legacy and high-volume use cases where a relatively high-voltage supply is available anyways.

History

After the successful release of a monolithic opamp IC, the Bob Widlar-designed µA702 in 1963, proved that commercial demand for such components was large, and after losing its designer, Fairchild Semiconductor set out to build a higher-performance opamp to compete with LMx01-series produced by National Semiconductor, which had hired Widlar.
Dave Fullagar worked to improve the output dead zone, input stage impedance and included a compensation capacitor to improve the dynamic behaviour of the opamp.

Architecture

Image:OpAmpTransistorLevel Colored Labeled.svg|thumb|right|600px|A component-level diagram of the common 741 op amp. Dotted lines outline: current mirrors; differential amplifier; class A gain stage; voltage level shifter; output stage.
The 741 op amp shares with most op amps an internal structure consisting of three gain stages:
  1. Differential amplifier — provides high differential amplification, with rejection of common-mode signal, low noise, high input impedance, and drives a
  2. Voltage amplifier — provides high voltage gain, a single-pole frequency roll-off, and in turn drives the
  3. Output amplifier — provides high current gain, along with output current limiting, and output short-circuit protection.
Additionally, it contains current mirror bias circuitry and compensation capacitor.

Building blocks

Differential amplifier

The input stage consists of a cascaded differential amplifier followed by a current-mirror active load. This constitutes a transconductance amplifier, turning a differential voltage signal at the bases of Q1, Q2 into a current signal into the base of Q15.
It entails two cascaded transistor pairs, satisfying conflicting requirements. The first stage consists of the matched NPN emitter follower pair Q1, Q2 that provides high input impedance. The second is the matched PNP common-base pair Q3, Q4 that eliminates the undesirable Miller effect; it drives an active load Q7 plus matched pair Q5, Q6.
That active load is implemented as a modified Wilson current mirror; its role is to convert the input current signal to a single-ended signal without the attendant 50% losses. Thus, a small-signal differential current in Q3 versus Q4 appears summed at the base of Q15, the input of the voltage gain stage.

Voltage amplifier

The voltage gain stage consists of the two NPN transistors Q15 and Q19 connected in a Darlington configuration and uses the output side of the current mirror formed by Q12 and Q13 as its collector load to achieve its high voltage gain. The output sink transistor Q20 receives its base drive from the common collectors of Q15 and Q19; the level-shifter Q16 provides base drive for the output source transistor Q14. The transistor Q22 prevents this stage from delivering excessive current to Q20 and thus limits the output sink current.

Output amplifier

The output stage is a Class AB amplifier. It provides an output drive with impedance of about, in essence, current gain. Transistor Q16 provides the quiescent current for the output transistors and Q17 limits the output source current.

Characteristics

Biasing circuits

Biasing circuits provide appropriate quiescent current for each stage of the op amp.
The resistor connecting the diode-connected transistors Q11 and Q12, and the given supply voltage, determine the current in the current mirrors, Q10/Q11 and Q12/Q13. The collector current of Q11, . For the typical, the standing current in Q11 and Q12 would be about. A supply current for a typical 741 of about agrees with the notion that these two bias currents dominate the quiescent supply current.
Transistors Q11 and Q10 form a Widlar current mirror, with quiescent current in Q10 such that, where represents the emitter resistor of Q10, and is, the thermal voltage at room temperature. In this case.

Differential amplifier

The biasing circuit of this stage is set by a feedback loop that forces the collector currents of Q10 and Q9 to match. Any small difference in these currents provides drive for the common base of Q3 and Q4. The summed quiescent currents through Q1 and Q3 plus Q2 and Q4 is mirrored from Q8 into Q9, where it is summed with the collector current in Q10, the result being applied to the bases of Q3 and Q4.
The quiescent currents through Q1 and Q3 will thus be half of, of order about. Input bias current for the base of Q1 will amount to ; typically around, implying a current gain for Q1.
This feedback circuit tends to draw the common base node of Q3/Q4 to a voltage, where is the input common-mode voltage. At the same time, the magnitude of the quiescent current is relatively insensitive to the characteristics of the components Q1–Q4, such as, that would otherwise cause temperature dependence or part-to-part variations.
Transistor Q7 drives Q5 and Q6 into conduction until their collector currents match that of Q1/Q3 and Q2/Q4. The quiescent current in Q7 is, about, as is the quiescent current in Q15, with its matching operating point. Thus, the quiescent currents are pairwise matched in Q1/Q2, Q3/Q4, Q5/Q6, and Q7/Q15.

Voltage amplifier

Quiescent currents in Q16 and Q19 are set by the current mirror Q12/Q13, which is running at approximately. The collector current in Q19 tracks that standing current.

Output amplifier

In the circuit involving Q16, the resistor must be conducting about, with Q16. Then must be about, and. Because the Q16 collector is driven by a current source and the Q16 emitter drives into the Q19 collector current sink, the Q16 transistor establishes a voltage difference between the Q14 base and the Q20 base of about, regardless of the common-mode voltage of Q14/Q20 bases. The standing current in Q14/Q20 will be a factor [diode modelling|] smaller than the quiescent current in the class A portion of the op amp. This standing current in the output transistors establishes the output stage in class AB operation and reduces the crossover distortion of this stage.

Small-signal differential mode

A small differential input voltage signal gives rise, through multiple stages of current amplification, to a much larger voltage signal on output.

Input impedance

The input stage with Q1 and Q3 is similar to an emitter-coupled pair, with Q2 and Q4 adding some degenerating impedance. The input impedance is relatively high because of the small current through Q1–Q4. A typical 741 op amp has a differential input impedance of about. The common mode input impedance is even higher, as the input stage works at an essentially constant current.

Differential amplifier

A differential voltage at the op amp inputs gives rise to a small differential current in the bases of Q1 and Q2. This differential base current causes a change in the differential collector current in each leg by. Introducing the transconductance of Q1,, the current at the base of Q15 is.
This portion of the op amp cleverly changes a differential signal at the op amp inputs to a single-ended signal at the base of Q15, and in a way that avoids wastefully discarding the signal in either leg. To see how, notice that a small negative change in voltage at the inverting input drives it out of conduction, and this incremental decrease in current passes directly from Q4 collector to its emitter, resulting in a decrease in base drive for Q15. On the other hand, a small positive change in voltage at the non-inverting input drives this transistor into conduction, reflected in an increase in current at the collector of Q3. This current drives Q7 further into conduction, which turns on current mirror Q5/Q6. Thus, the increase in Q3 emitter current is mirrored in an increase in Q6 collector current; the increased collector currents shunts more from the collector node and results in a decrease in base drive current for Q15. Besides avoiding wasting of gain here, this technique decreases common-mode gain and feedthrough of power supply noise.

Voltage amplifier

A current signal at Q15's base gives rise to a current in Q19 of order . This current signal develops a voltage at the bases of output transistors Q14 and Q20 proportional to the of the respective transistor.

Output amplifier

Output transistors Q14 and Q20 are each configured as an emitter follower, so no voltage gain occurs there; instead, this stage provides current gain, equal to the of Q14 and Q20.
The current gain lowers the output impedance and although the output impedance is not zero, as it would be in an ideal op amp, with negative feedback, it approaches zero at low frequencies.

Other linear characteristics

Overall open-loop gain

The net open-loop small-signal voltage gain of the op amp is determined by the product of the current gain of some 4 transistors. In practice, the voltage gain for a typical 741-style op amp is of order 200,000, and the current gain, the ratio of input impedance to output impedance provides yet more gain.

Small-signal common mode gain

The ideal op amp has infinite common-mode rejection ratio, or zero common-mode gain.
In the present circuit, if the input voltages change in the same direction, the negative feedback makes Q3/Q4 base voltage follow the input voltage variations. Now the output part of Q10–Q11 current mirror keeps up the common current through Q9/Q8 constant in spite of varying voltage. Q3/Q4 collector currents, and accordingly the output current at the base of Q15, remain unchanged.
In the typical 741 op amp, the common-mode rejection ratio is, implying an open-loop common-mode voltage gain of about 6.

Frequency compensation

The innovation of the Fairchild μA741 was the introduction of frequency compensation via an on-chip capacitor, simplifying application of the op amp by eliminating the need for external components for this function. The capacitor stabilizes the amplifier via Miller compensation and functions in a manner similar to an op-amp integrator circuit. Also known as dominant pole compensation because it introduces a pole that masks the effects of other poles into the open loop frequency response; in a 741 op amp this pole can be as low as .
This internal compensation is provided to achieve unconditional stability of the amplifier in negative feedback configurations where the feedback network is non-reactive and the loop gain is unity or higher. In contrast, amplifiers requiring external compensation, such as the μA748, may require external compensation or closed-loop gains significantly higher than unity.

Input offset voltage

The offset null pins may be used to place external resistors in parallel with the emitter resistors of Q5 and Q6, to adjust the balance of the Q5/Q6 current mirror. The potentiometer is adjusted such that the output is null when the inputs are shorted together.

Non-linear characteristics

Input breakdown voltage

The transistors Q3, Q4 help to increase the reverse rating; The base-emitter junctions of the NPN transistors Q1 and Q2 break down at around, but the PNP transistors Q3 and Q4 have breakdown voltages around.

Output-stage voltage swing and current limiting

Variations in the quiescent current with temperature, or due to manufacturing variations, are common, so crossover distortion may be subject to significant variation.
The output range of the amplifier is about one volt less than the supply voltage, owing in part to of the output transistors Q14 and Q20.
The resistor at the Q14 emitter, along with Q17, limits Q14 current to about ; otherwise, Q17 conducts no current. Current limiting for Q20 is performed in the voltage gain stage: Q22 senses the voltage across Q19's emitter resistor ; as it turns on, it diminishes the drive current to Q15 base. Later versions of this amplifier schematic may show a somewhat different method of output current limiting.

Applicability

While the 741 was historically used in audio and other sensitive equipment, such use is now rare because of the improved noise performance of more modern op amps. Apart from generating noticeable hiss, 741s and other older op amps may have poor common-mode rejection ratios and so will often introduce cable-borne mains hum and other common-mode interference, such as switch "clicks", into sensitive equipment.
The '741' has come to often mean a generic op-amp IC. The description of the 741 output stage is qualitatively similar for many other designs, except:
  • Some devices are not internally compensated. Hence, they provide a pin for wiring an external capacitor from output to some point within the operational amplifier, if used in low closed-loop gain applications.
  • Some modern devices have rail-to-rail output capability, meaning that the output can range from within a few millivolts of the positive supply voltage to within a few millivolts of the negative supply voltage.