The uA741 Operational

Amplifier

Outline

• Brief History

• Stages

• DC Bias Point Analysis

• Small Signal Analysis

• Concluding Remarks

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.

Brief History (cont)

• 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

Schematic

Stages

• Input Differential Stage

• Intermediate Signal-Ended High-Gain

Stage

• Output Buffering Stage

• Current Source / Short Circuit Protection

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.

Intermediate Single-Ended

High-Gain Stage

The second stage is composed of Q16, Q17,
Q13B, and the resistors R8 and R9.

Transistor Q16 acts as an emitter follower
giving the second stage a high input resis-
tance.

Transistor Q17 is a common-emitter amplifier
with a 100-Ώ resistor in the emitter. The load
of this amplifier is composed of the output
resistance of Q13B. This use of a transistor
as a load resistance is called active load.

The output of this amplifier (the collector of
Q17) has a feedback loop through Cc. This
capacitor causes the op-amp to have a pole
at about 4Hz.

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
4

th

addition, pg 813)

DC Analysis

Reference Bias Current

• This current is generated by Q11, Q12 and resistor

R5. From these, we can write:

• From this value of IREF, the current in the collector of

Q10 can be calculated.

• This value (IC10) is twice the value of I (which is used

later in the DC analysis.

I

REF

V

cc

V

be



V

be



V

ee









R

5



I

REF

0.733mA



Given

I

C10

R

4



V

T

ln

I

REF

I

C10















I

C10

18.421

A



DC Analysis (cont)

Input Stage

Using the value IC10 found before, the analysis
unfolds as shown in the schematic.

This analysis is done using the
standard BJT, current mirror, and
differential amplifier textbook
equations.

DC Analysis (cont)

Second Stage

Assuming beta to be >> 0, the
following DC biasing equations result

I

C13B

550A



I

C13B

0.75 I

REF





I

C16

I

E16

I

B17

I

E17

R

8





V

be



R

9

16.2A

DC Analysis (cont)

Output Stage

Using the fact that Q13A delivers ¼ of IREF, the
following outputs result:

If Vbe is assumed to be 0.7V, the current in R10 is
18uA which causes the following:

Since the base current of Q18 is IC18 / beta =
165u / 200:

I

C23

I

E23

0.25 I

REF



180A

I

C18

I

E18

I

C23

I

R10



162A

I

C19

I

E19

I

R10

I

B18



18.8A

DC Analysis (cont)

Table of Results

Below is a table that lists all of the
transistors and their collector
currents.

DC Collector Currents of the 741 op-amp (uA)

Q1

9.5

Q8

19

Q13B

550

Q19

15.8

Q2

9.5

Q9

19

Q14

154

Q20

154

Q3

9.5

Q10

19

Q15

0

Q21

0

Q4

9.5

Q11

730

Q16

16.2

Q22

0

Q5

9.5

Q23

730

Q17

550

Q23

180

Q6

9.5

Q13A

180

Q18

165

Q24

0

Q7

10.5

Small Signal Analysis

To better visualize the various small
signal properties of the uA741 op-
amp, a simple inverting circuit is
constructed around the op-amp.

This circuit is the circuit that will
be used in the following
analysis. It has a gain of 100
(Rf / R).

Small Signal Analysis (cont)

1. Frequency Response

The op-amp circuit is
supplied by a 1mV AC
signal and a Frequency
analysis is performed.

The inverting amplifier
circuit outputs a gain
of 100 until a
frequency of 8kHz is
reached. After this
point, it attenuates at
20dB per decade until
it reaches unity gain at
1MHz.

Small Signal Analysis (cont)

2. Transient analysis

The op-amp circuit is
now supplied with a
1mV 1kHz sinusoidal
source and a transient
analysis is performed.

The op-amp outputs a
100mV signal that is
the exact inverse of
the input signal. This
verifies that the op-
amp is indeed
magnifying the signal
appropriately as well
as inverting the signal.

Small Signal Analysis (cont)

3. Monte Carlo Analysis

a. The resistors of the circuit are to be given

a

2% tolerance and the frequency/transient
analysis are to be performed again.
b. Next, the beta values of the transistors
are to be given a tolerance of 50%.
c. Finally, the temperature of the circuit is

varies from –150C to 100C

Monte Carlo Analysis

The resistor values are allowed to
vary by 2%

– Transient Analysis

The resistor values, as can
be seen on the left, do cause
changes in the output signal,
however, the general output
shape is retained. Note: The
61mV offset is still present

Monte Carlo Analysis

The resistor values are allowed to
vary by 2%

– Frequency Response

As is to be expected, the
resistor variances have
little (almost none) effect
on the frequency
response of the op-amp.
This is expected because
the resistors have no
effect on the
capacitances and poles of
the amplifier.

Monte Carlo Analysis

The transistor beta values are allowed to vary by
plus/minus 50

– Transient Analysis

As is evident by the plot on the left,
the beta value of the transistors
have very little effect on the output
signal. The design of the uA741 op-
amp is such that the circuit is beta
independent.

The plot on the right is a histogram
showing the number of times that
a particular output value (from the
above simulation) occurred. As
can be seen, a vast majority of the
output signals are within 5mV of
the expected value.

Monte Carlo Analysis

The transistor beta values are allowed to vary by
plus/minus 50

– Frequency Analysis

As is evident by the
simulation/histogram on the left,
the uA741 operational amplifier’s
frequency response is not
effected by changes in beta.
Once again, this is due to the op-
amp’s relative beta
independence. This beta
independence is quite beneficial
because in the mass production
of transistors, their beta values
can vary by a large amount.
Having the op-amp operate
regardless of beta variations
assures that the amplifier will
operate properly in a wide range
of conditions.

Monte Carlo Analysis

The temperature of the system is set
to

-150C, 0C, and 100C

As can be seen by the
simulation on the left,
variances in temperature do
not effect the shape of the
output nor do they effect the
amplitude of the output (the
gain stays the same).
Temperature does, however,
effect the DC offset.

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.