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Objective
Students are to model and build a circuit resembling the LM741
Operation Amplifier using
simple parts such as transistors, resistors and capacitors. By
conducting an AC small-signal
analysis on both the LM741 Op-Amp chip and design circuit,
students are expected to receive
similar results in voltage gain.
Introduction
Operational amplifiers are among the most widely and commonly
used component used in
electronic devices today. It is considered the backbone of
electronics for circuit design. An
operational amplifier is a DC-coupled circuit used to receive
and input signal and output a signal
with a higher voltage gain. Depending on the configuration of
the circuit, the op-amp can
produce different outputs, including an inverting signal and
even a signal with lower gain. The
architecture of all operational amplifiers consist of four
different gain stages within the internal
circuitry. These include the differential amplifier, bias
generator, gain stage, and output stage.
Descriptions and explanations of these four gain stage can be
found below. The internal
circuitry of the LM741 Op-Amp consists of several PNP
transistors, NPN transistors, resistors,
and capacitors. Therefore, the op-amp can be made using the same
basic components and
theoretically will produce similar results to the IC chip.
Figure 1
Differential Amplifier
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The input stage is a transconductance amplifier, which consists
of a cascaded differential
amplifier followed by a current-mirror active load. This
transconductance amplifier turns a
differential voltage signal at the bases of Q2 and Q1 into a
current signal for the base of Q15.
Both Q2 and Q1 create a NPN emitter follower pair that provides
a high input impedance.
Below the NPN emitter follower is Q3 and Q4, a PNP common-base
pair that eliminates the
increase in input capacitance. Both the PNP transistors drive
and active load Q7 and amatched pair Q5 and Q6. The active load is
implemented as a Wilson current mirror in order to
convert the input current signal to a single-ended signal, thus
creating a small-signal differential
current in Q3 and Q5 feeding into the base of Q15.
Voltage Amplifier
The voltage gain stage is a class-A gain stage consisting of two
NPN transistors (Q15 and Q19)
utilizes the output of the current mirror from Q12 and Q13 as a
collector load to achieve its high
voltage gain. The common collectors of Q15 and Q19 drives the
base of Q20 while Q16 drives
the base of Q14. The transistor Q22 controls the current,
preventing excess current to pass into
Q20.
Output Amplifier
The output stage consists of transistors Q14 and Q20. It is a
class AB emitter follower amplifier.
It provides an output drive with a low impedance of 50 Ohms.
Transistor Q16 provides current
for the output transistors while Q17 provides output current
limiting.
PSPICE Simulation
The following image (Fig 1) is the PSPICE model of the circuit
design that represents the 741
Op-Amp IC chip. It is wired with a inverting amplifier circuit
where Rf is 10k ohms and R1 is
3.33k ohms. This configuration creates an output with a gain of
-3.
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Figure 2
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The following figure (Fig 3) represents the bias analysis of the
circuit representing the 741 Op-
Amp. The PSPICE model allows the user to view the current and
voltage at each point in the
circuit. As seen below, the current are marked with red tags
while the voltages are marked with
the voilet tags. By providing currents and voltages at each
point in the circuit, the user can
further analyze and confirm the inner workings of the op-amp
circuit.
Figure 3
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The following figure (Fig 4) is the PSPICE simulation of the
output signal versus the input signal.
Since the circuit is designed to provide an inverting gain of 3,
we expect the output signal to be
greater and opposite in magnitude as compared to the input
signal. The input signal was 3 Vp-p
at 5 kHz in frequency whereas the output signal is inverted
9Vp-p at 5 kHz frequency.
Figure 4
Frequency Analysis
In order to observe how differences in frequency affect the gain
of the amplifier, a frequency
sweep is conducted. High frequencies will affect the gain,
causing it to stray from the theoreticalgain. In addition, the
effect frequency has on the gain should be more apparent in the
circuit
created using transistors as compared to the actual 741 Op-Amp
chip. By conducting a
frequency sweep on both the circuit design and the 741 IC chip,
the following data in figure 5 is
collected.
For the circuit using the 3904 transistors,the output gain
begins to break down at 35~40 kHz in
frequency. However, the 741 IC begins to break down at around
60~70 kHz. As expected, the
breakdown frequency of the 741 IC chip is much larger compared
to the 3904 transistor circuit
design. At 100 kHz, the gain drops from 3 to roughly 1.6 volts
in the 3904 circuit design while
the 741 IC chip only drops down to 2.7. The 3904 circuit breaks
down much more rapidly and is
less stable at higher frequencies whereas the 741 IC chip is
much more stable and is moreresilient to frequency breakdown. This
is expected since the 741 IC chip is manufactured to
provide a steady and stable gain whereas the 3904 circuit design
consists of many transistors in
an attempt to replicate the functionality of the 741 IC
chip.
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Figure 5
Graphical Data
The following figures (figures 6-10) are graphs of the input
signal vs the output signal on the
same graph. These graphs include signals of the 3904 design
circuit and 741 IC chip at different
frequencies.
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Figure 6 - 3904 Circuit Design at 1kHz.
Figure 7 - 3904 Circuit Design at 100kHz.
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Figure 8 - 741 IC Chip at 1kHz.
Figure 8 - 741 IC Chip at 100kHz.
