Transcript The uA741 Operational Amplifier

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 HighGain 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 resistance.
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 Output 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)
DC Analysis
Reference Bias Current
• This current is generated by Q11, Q12 and resistor R5. From
these, we can write:
IREF  
Vcc  Vbe  Vbe   Vee
R5
IREF  0.733mA
• From this value of IREF, the current in the collector of Q10
can beGcalculated.
iv en
IC10  R4
 IREF 
VT  l n

I
C10


IC10  18.421
A
• This value (IC10) is twice the value of I (which is used later in
the DC analysis.
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
IC13B  0.75 IREF
IC16
IE16
IC13B  550A
IB17  IE17  R8  Vbe
R9
1 6.2A
DC Analysis (cont)
Output Stage
Using the fact that Q13A delivers ¼ of IREF, the
following outputs result:
IC23
IE23
0.25 IREF
180A
If Vbe is assumed to be 0.7V, the current in R10 is 18uA
which causes the following:
IC18
IE18
IC23  IR10
162A
Since the base current of Q18 is IC18 / beta = 165u /
200:
IC19
IE19
IR10  IB18
18.8A
DC Analysis (cont)
Table of Results
Below is a table that lists all of the
transistors and their collector currents.
Q1
Q2
Q3
Q4
Q5
Q6
Q7
9.5
9.5
9.5
9.5
9.5
9.5
10.5
DC Collector Currents of the 741 op-amp (uA)
Q8
19
Q13B
550
Q19
Q9
19
Q14
154
Q20
Q10
19
Q15
0
Q21
Q11
730
Q16
16.2
Q22
Q23
730
Q17
550
Q23
Q13A
180
Q18
165
Q24
15.8
154
0
0
180
0
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 opamp 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.