Analog Signal Processing – Fall 2013 Instructor : Dr Juan Alvarez T.A : Trung Mai Van Pspice Tutorial – Operational Amplifiers Operational Amplifiers are one of the most commonly used electronics components. They are used in amplification applications in many configurations such as inverting, non – inverting, summation, differentiation or integration. The advantages of the used of OpAmps are the very high input impedance of , very high gain and low output impedance. Therefore, OpAmps can be used as connectors between different circuits. In Pspice, we have many models for OpAmps as shown on the figure below However, it is a necessary to start with an ideal model which can be launched easily by pressing the letter “P” (for place part) and then type “OPAMP” in the search typebox.
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Analog Signal Processing – Fall 2013
Instructor : Dr Juan Alvarez
T.A : Trung Mai Van
Pspice Tutorial – Operational Amplifiers
Operational Amplifiers are one of the most commonly used electronics components.
They are used in amplification applications in many configurations such as inverting,
non – inverting, summation, differentiation or integration.
The advantages of the used of OpAmps are the very high input impedance of ,
very high gain and low output impedance. Therefore, OpAmps can be used as
connectors between different circuits.
In Pspice, we have many models for OpAmps as shown on the figure below
However, it is a necessary to start with an ideal model which can be launched easily
by pressing the letter “P” (for place part) and then type “OPAMP” in the search
typebox.
In case, you want to change the characteristics of the ideal OpAmps, double click
on the model on capture cis schematic page to bring you to the device’s page.
It is important to note that the voltage output of OPAMP can’t be greater than the
positive and negative supply voltage. Hence, you must change these parameters in
case you need a voltage that higher than those aforementioned.
Now let’s get started with some simple OpAmps simulations. We will begin with the
inverting configuration. Make sure you place and wire exactly the circuit below
In order to interchange the negative and positive terminals of the OPAMP to make
the negative terminal above the positive one vertically. You should right – click the
OPAMP to launch a pop – up menu then choose mirror vertically.
Then, choose the simulation settings as with the Time Domain (Transient) analysis
type and the time to run of 30 ms due to the frequency of the Vsin source is of 100
Hz, translates to a period of 0.01 second.
To make life simpler for you to check the nodes, it is highly recommended that you
use the net alias functionality of Capture CIS to place “In” and “Out” as in the
circuit.
Run the simulation by pressing the run arrow and choose Add Trace to reach the
Add Traces window above, type in v(In) in the Trace Expression box, the type
v(Out) in Trace Expression to get the waveforms as shown
In the figure of the waveforms, the v(In) in green and the v(Out) in red, it is ease
to realize that they are out of phase due to the relation
and , so the amplitudes of the output and input voltages are the
same. This ciruit, actually, makes the phase of the output difference compared
with the input. To make things more interesting, give the circuit some amplications,
it is easy to change the ratio between and . For an example, we need the
output voltage a half the input voltage, the gain
. Here, we choose .
Non – Inverting amplifier
The gain of the non – inverting amplifer
Differential amplifier (difference amplifier)
The circuit shown computes the difference of two voltages multiplied by some
constant. In particular, the output voltage is:
The differential input impedance Zin (i.e., the impedance between the two input
pins) is approximately R1 + R2. The input currents vary with the operating point of
the circuit. Consequently, if the two sources feeding this circuit have
appreciable output impedance, then non-idealities can appear in the output, as the
equations for this circuit were derived assuming zero source impedance for both V1
and V2. An instrumentation amplifier mitigates these problems.
Under the condition that the Rf /R1 = Rg /R2, the output expression becomes:
where is the differential gain of the circuit.
Moreover, the amplifier synthesized with this choice of parameters has
good common-mode rejection in theory because components of the signals that
have V1 = V2 are not expressed on the output. Although this property is described
here with resistances, it is a more general property of the impedances in the circuit.
So, for example, if a compensation capacitor is added across any resistor (e.g., to
improve phase margin and ensure closed-loop stability of the operational amplifier),
similar changes need to be made in the rest of the circuit to maintain the ratio
balance. Otherwise, high-frequency components common to both V1 and V2 can
express themselves on the output. Additionally, because of leakage or bias currents
in a real operational amplifier, it is usually desirable for the impedance looking out
each input to the operational amplifier to be equal to the impedance looking out of
the other input of the operational amplifier. Otherwise, the same current into each
operational amplifier input will generate a parasitic differential signal and thus a