This document provides instructions for building and testing a differentiator circuit using an op amp. Key points: - The circuit uses an LM356 op amp instead of the diagrammed uA741. Resistors and capacitors can be combined to achieve desired values. - A series resistor and feedback capacitor are added to the ideal differentiator circuit to form high-pass and low-pass filters, stabilizing the circuit and reducing noise. - As frequency increases, the capacitor acts less like an open circuit and more like a short circuit. This changes the circuit's behavior from a differentiator to an inverting amplifier to an integrator. - Phase shift between input and output will vary from 90°

1. Lab 8
Experiment 17
A Differentiator Circuit

2. Note Comment in Lab Manual
• All of the diagrams use a uA741 op amp.
– You are to construct your circuits using an
LM 356 op amp.
• There is a statement that values for R1, R2, and
C2 should be limited by what is in your kit.
– You may combine multiple resistors (capacitors) to
obtain a desired resistance (or capacitance).

3. Ideal Circuit

4. Capacitors
)
(
)
(
1
)
(
)
(
1
o
C
t
t
C
C
C
C
t
v
dt
t
i
C
t
v
dt
dv
C
t
i
o





5. iC
i = 0
iR iR + iC + i =0
where i = 0mA
iC =C1 dV3/dt
iR = [0V – Vo]/R1
Vo = -R1C1 dV3/dt

6. Practical Circuit

7. Why Two Different Circuits
• If the input contains electronic noise with high
frequency components, the magnitude of the
high frequency components will be amplified
significantly over the signal of interest and the
system likely will become unstable.
– It is thus necessary to modify the circuit to reduce
or eliminate such effects.

8. Modifications to Ideal Circuit
• Two modifications to the circuit – both of which
results in the formation of frequency filters.
– First, a series resistor is inserted before the negative
input terminal of op amp. The effect of this resistor is
to act as an attenuator for the high frequency
components.
– Second, a capacitor is placed in the feedback network.
This capacitor provides more feed-back for the high
frequency components than for the low frequency
components and also acts to stabilize the circuit.

9. Practical Circuit

10. Capacitors
)
(
)
(
1
)
(
)
(
1
o
C
t
t
C
C
C
C
t
v
dt
t
i
C
t
v
dt
dv
C
t
i
o





11. From these equations:
• When the voltage across the capacitor doesn’t
change (i.e., d.c. voltage), the capacitor’s current
is equal to zero.
– The capacitor acts like an open circuit.
• When the voltage across the capacitor is
changing rapidly (e.g., high frequency sine wave),
the capacitor’s current is large and also changes
with time.
– The capacitor acts like a short circuit. The current
through the circuit is limited by the other components
in the circuit (i.e., the resistors).

12. Practical Circuit
R2 with C1 forms a high pass filter.
If V3 is a d.c. voltage source, C1 acts like an open circuit
and all of the input voltage (V3) is dropped across the
capacitor (VC1) and the current through R2 and C1 will
be determined primarily by the first derivative of the
V3.
If V3 is a high frequency a.c. voltage source, C1 acts like
a short circuit and the current through R2 and C1 will
be determined primarily by V3 divided by R2.

13. Practical Circuit
R1 with C2 forms a low pass filter.
If the difference in the voltage between the negative input
terminal on the op amp and Vo is relatively constant, C1
acts like an open circuit and all of the current through R2
and C1 will flow through R1.
If the difference in the voltage between the negative input
terminal on the op amp and Vo varies a lot with time, C1
acts like a short circuit and all of the current through R2
and C1 will flow through C2 and the output voltage will be
approximately equal to the voltage on the negative input
terminal, which will be 0 V.

14. Operation as a Function of Frequency

15. Design Constraints
f
C
R
C
R
C
R
C
R
f
C
R
f
C
R
f
unity
H
C






2
1
1
1
2
1
2
1
and
2
1
2
1
1
2
1
1
1
1
2
1
1
2








16. Design Constraints
F
C
Hz
f
Hz
f
Hz
f
unity
H
C

1
.
0
1500
5000
3000
1 




17. dB
• dB is an abbreviation for decibels
V
V
log
20
dB
P
P
log
10
dB
in
out
in
out


















0.707
2
2
V
V
when
occurs
3dB
-
2
1
P
P
when
occurs
3dB
-
in
out
in
out




















18. Follow the Directions in the Lab
Manual
• Except:
– Use the function generator on the Velleman
oscilloscope
• Remember that you have to set the Amplitude to 10V to
have 5V sin(t) outputted.
– Do not use the 10X probes with the Velleman
oscilloscope when performing the oscilloscope
measurements.
• Just use the standard BNC-to-alligator or BNC-to-IC clip
cables.
– All plots should be made using MatLAB.

19. PSpice Simulation:
AC Sweep
Differentiator Gain Integrator

20. Phase Shift
--Dt --












2
f
1
T
is
wave
sine
the
of
period
the
where
degrees
360
angle.
phase
the
is
where
)
sin(
)
90
sin(
)
cos(
)
sin(


D
-






T
t
t
t
t
dt
t
d o

21. Measurement of Phase Angle
• There are two sets of instructions in the Lab 8
folder under resources
– Phase Delay.pdf, which explains how to make a phase
angle calculation using the information displayed
when the Oscilloscope function of the Velleman
oscilloscope is used.
• You should become familiar with this technique.
– Magnitude and Phase.pdf, which explains how to use
the automated measurement tools on the Velleman
scope to obtain the magnitude and phase of a signal
at a single frequency and over a range of frequencies.

22. Phase Shift as a Function of Frequency
• The phase shift between the input voltage and
the output voltage of the op amp will change
from 90o to 180o to 270o as the operation of
the circuit changes from a differentiator to
inverting amplifier to integrator.

23. Caution:
PSpice Transient Analysis Issue
Information in first half cycle is incorrect because
the initial charge on the capacitor is zero.