4ms 5ms 6ms
Circuit simulation software allows designers,
hobbyists, and experimenters to quickly verify proper circuit
operation through computer simulation before going to the time,
trouble, and expense of actual circuit construction. Most software
for the PC (PSpice, hSpice, etc.) is based on the program Spice
(Simulation Program with Integrated Circuit Emphasis) that was
developed in the late 1960s at the University of California
Berkeley. Modern PC software combines the original Spice
simulation code with a user-friendly, Windows-based graphical
interface. This allows the circuit designer to graphically lay out
the circuit by connecting the schematic symbols (schematic
capture) instead of through a text file listing (net list) as the
original Spice code required. A free student version of PSpice
is available for download from www.electronics-lab.com/
downloads/schematic/013/. The student version is identical to
the professional version except that the number of nodes in the
circuit is limited.
as shown in Figure 11. The 1 kHz fundamental has an
amplitude of 1V and is clearly visible on the left side of the plot.
The next most prominent signals are harmonic distortion at
odd multiples of the fundamental signal ( 3 kHz, 5 kHz, 7 kHz,
etc.), but the largest of these harmonics is only about one millivolt — three orders of magnitude smaller than the fundamental
— so the amplifier’s harmonic distortion is indeed very low.
With the proper selection of resistors, either inverting
Fr equ e ncy
■ FIGURE 10. Simulated output
of audio amplifier shown in
or non-inverting amplifiers
can be designed to trade an
op-amp’s gain and bandwidth.
If you like to get your
hands dirty, your next step is to
build the circuits shown here in
order to verify the simulation
results. A simple signal generator
and an oscilloscope are adequate
7ms 8ms 9ms 10ms for this task. A word of caution:
due to the large open loop
gain in the uA741, testing the
circuit in Figure 1 requires input waveforms at the microvolt
level in order to prevent clipping at the output of the op-amp. Such a low level input can be difficult to achieve in
an uncontrolled benchtop environment. The other circuits
are suitable even for beginners. A Parts List is included for
the amplifier shown in Figure 9 for your convenience. The
supply voltages are limited to 5V to avoid exceeding the
transistor’s maximum allowable power dissipation. The
purpose of the two 100 μF electrolytic capacitors is to
decouple the power supply lines to reduce the potential
for oscillation. You can replace the output resistor shown
in the figure (R5) with a small eight ohm speaker if you
want to use the circuit as an audio amplifier.
We hope you enjoy designing, simulating, and
constructing inverting and non-inverting amplifiers built
with op-amps. You may contact us with any comments or
questions at firstname.lastname@example.org. NV
Comer, D, Comer, D. Fundamentals of Electronic Circuit Design.
John Wiley and Sons, New York, NY, 2003.
Millman, J, Grabel, A. Microelectronics. McGraw-Hill Book
Company, New York, NY, 1987.
1 Midband gain is the ratio of the output voltage to the
input voltage at frequencies in
between the low and high rolloff
frequencies. Because an op-amp
experiences no low frequency
rolloff, the midband gain is the
same as the gain at DC.
2 The cutoff frequency is the
frequency at which the voltage
gain drops to 70.7% of the
midband value. This is equivalent
to a 50% drop in power gain or a
reduction of 3 dB. In an op-amp
— which amplifies down to DC —
the cutoff frequency and the
bandwidth are identical.
■ FIGURE 11. Frequency spectrum
70KHz of the output waveform of the
audio amplifier shown in Figure 9.