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

parasitic output component. Consequently, choosing R1 = R2 and Rf = Rg is

common in practice.

In the special case when Rf /R1 = Rg /R2, as before, and Rf = R1, the differential

gain A = 1, and the circuit is a differential follower with:

In this circuit, we use the resistors with the same value to have an output

voltage as .

We have the same waveforms when trace the output voltage and the difference

between the two voltages.

Voltage follower (unity buffer amplifier)

Used as a buffer amplifier to eliminate loading effects (e.g., connecting a device

with a high source impedance to a device with a low input impedance).

(realistically, the differential input impedance of the op-amp itself,

1 MΩ to 1 TΩ)

Due to the strong (i.e., unity gain) feedback and certain non-ideal characteristics of

real operational amplifiers, this feedback system is prone to have poor stability

margins. Consequently, the system may be unstable when connected to sufficiently

capacitive loads. In these cases, a lag compensation network (e.g., connecting the

load to the voltage follower through a resistor) can be used to restore stability. The

manufacturer data sheet for the operational amplifier may provide guidance for the

selection of components in external compensation networks. Alternatively, another

operational amplifier can be chosen that has more appropriate internal

compensation.

Summing amplifier

A summing amplifier sums several (weighted) voltages:

When , and independent

When

Output is inverted

Input impedance of the nth input is ( is a virtual ground)

To get the result as in the figure above, we use the bias point analysis.

Instrumentation Amplifier

Inverting integrator

Integrates the (inverted) signal over time

(where and are functions of time, is the output voltage of the

integrator at time t = 0.)

This can also be viewed as a low-pass electronic filter. It is a filter with a

single pole at DC (i.e., where ) and gain.

There are several potential problems with this circuit.

It is usually assumed that the input has zero DC component (i.e., has

a zero average value). Otherwise, unless the capacitor is periodically

discharged, the output will drift outside of the operational amplifier's

operating range.

Even when has no offset, the leakage or bias currents into the

operational amplifier inputs can add an unexpected offset voltage

to that causes the output to drift. Balancing input

currents and replacing the non-inverting ( ) short-circuit to ground with

a resistor with resistance can reduce the severity of this problem.

Because this circuit provides no DC feedback (i.e., the capacitor appears

like an open circuit to signals with ), the offset of the output may

not agree with expectations (i.e., may be out of the designer's

control with the present circuit).

Many of these problems can be made less severe by adding

a large resistor in parallel with the feedback capacitor. At significantly

high frequencies, this resistor will have negligible effect. However, at low

frequencies where there are drift and offset problems, the resistor provides

the necessary feedback to hold the output steady at the correct value. In

effect, this resistor reduces the DC gain of the "integrator" – it goes from

infinite to some finite value .