EE1620 Op Amps online - practice questions PDF

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Op Amps Aims: • To familiarise students with some basic Op Amp circuits. • to reinforce the understanding of OrCAD PSpice simulation software. Learning Outcomes – students should be able to: • use OrCad/PSPICE program to simulate and analysis some basic Op Amp circuits.

Introduction A precision differential amplifier is known as an operational amplifier and is one of the most powerful analog devices available to the designer. When the operational amplifier is combined with negative and positive feedback, variety of devices such as amplifiers, buffers, Integrators, differentiators, oscillators and filters can be created. You MUST record all your work in a laboratory notebook (lab. book) electronically, for example as a Word document. The workshop consists of several tasks that must be completed. You need to work steadily to finish the whole workshop and some of you may not reach the end. Do not rush your work as it is more important to understand what you are doing than it is to complete all the tasks. You MUST submit your report via Wiseflow by the deadline (23:59 noon on the day after your lab).

THEORY The term operational amplifier (Op-Amp) is used to describe a high gain voltage amplifier capable of amplifying both ac and dc (zero frequency) signals. It is used as a building block in electronic systems. The Op-Amp consists of a number of transistor stages in a single chip, and provides the characteristics of a voltage-controlled voltage source. An ideal Op-Amp would have a zero output impedance and an infinite input impedance. In reality the Op-Amp departs from this ideal but it is normal to consider it as ideal in circuit calculations. The inputs are ‘floating’ in voltage level and one (either) is usually connected to a reference voltage (often for convenience 0 volts). The device is usually operated from balanced positive and negative dc supply rails. As shown in Fig. 1, it has 2 input terminals, one is inverting (V-) so that it makes the output change in the opposite direction, the other non-inverting (V+) that makes the output change in the same direction. The pin numbers shown are standard for single 8-pin DIL Op-Amps.

Figure 1. The fundamental equation for the device is: where Av is the voltage gain. At low frequencies and when working in its linear region, Av is at least 100,000. The input to the Op-Amp is the difference between A+ and A-, but they must be considered as separate voltages, both referenced to 0 volts. A floating voltage source connected between the 2 input pins will not enable the transistors in the IC to be biased with respect to the supply rails and the circuit will not work. From the equation, A+ = A- = 0, Ao should also be zero, i.e. there is no input/output offset. This is the ideal situation that can only be approached in practice. Large inputs cause the output to saturate a volt or so inside the dc rail voltages. A further increase in the input does not change the output, so the effective gain is zero.

Before running the program Before starting the OrCad Capture CIS program you need to create a folder called \pspice examples on the H: drive (your home directory). You may already have such folder from a previous experiment (Introduction to OrCad).

Starting the OrCad Capture CIS program From the “Start” menu. Select the option File/New/Project. This option will invoke the New Project dialog box. You should select a meaningful project name (of your choice) and select the h:\pspice_examples directory for the location. Select the Analog or MixedA/D option. Press OK and choose Create a blank project option and then press OK. Now you should have a circuit layout grid and the Capture toolbar (on the right hand side). Now you are ready to draw your circuits. 1.

Circuit 1 – Testing the specifications of the 741 operational amplifier

a) Begin by selecting the menu option Place/Part, or by clicking on the second vertical toolbar button. This will invoke the Place Part dialog. Select Add Library , choose opamp , then type ua741 under Part and then OK. Place the OpAmp in the grid. b) Place two sinwave sources :Place/Part → Add Library → Source → vsin . Give zero values for VOFF, VAMPL and FREQ. c) Place two DC sources: Place/Part → Add Library → Source → vdc. Give 15V for one and -15 V for the other. d) Place a resistor: Place/Part → Add Library → Analog → R. d) The last step in drawing the circuit is to add the circuit ground, which must be at the node numbered zero (0). The easiest way to do this is to click on the vertical toolbar button labelled GND, and select the part named 0. (If you don’t see this part, you will need to click the Add Library button and add the Source library from the PSpice subdirectory.) This is a very important step and easy to forget, but your circuit cannot be simulated by PSpice unless it contains a ground at node 0. Place the ground component on the layout grid, and wire it to your circuit. The completed circuit should look like the one below. Vcc 15

0 U1 7

V1 3

VOFF = 0 VAMPL = 0 FREQ = 0

+

V+ OS2 OUT

2

0 V2 VOFF = 0 VAMPL = 0 FREQ = 0

uA741

-

4

OS1 V-

5 Rload 6 1

10k V

Vee -15

0 0

0

d) Now you are ready to specify the type of analysis to perform on this circuit. You can ask PSpice to perform several different types of analysis on the circuit you have drawn. To begin, you need to select a name for the simulation. This is done by choosing the menu option PSpice/New Simulation Profile, which will generate a dialog box. Write any name under the Name and set up the following simulation profiles:

•

A DC Sweep of V1 from -200 uV to 200 uV, in steps of 0.1 uV.

•

A transient analysis (0 to 0,2 ms), with a Maximum step size of 0.2 us.

•

An AC Sweep from 1 Hz to 1 GHz, with 100 Points/Decade. select the Time Domain (Transient) under Analysis type. Put 2ms for Run to Time and then click OK to return to your schematic.

e) Place a voltage marker at the input of Rload, and perform a DC Sweep to display Vout. Simulate your circuit by selecting PSpice/Run option (or press F11) to start the circuit analysis. f) Comment on the output voltage waveform.

2.

Circuit 2 – The Inverting and Non-Inverting Op Amps Use your skills in part 1 above to simulate the following two circuits. (Set the simulation profile to transient analysis (0 to 2 ms))Comment on the output voltage waveforms. 100 k

100 k

+15V

1 k VOFF = 0 FREQ = 1k VAMPL = 1mV

+15V

1 k

_ V out

V out

+

~

+ -15V

3.

_

VOFF = 0 FREQ = 1k VAMPL = 1mV

50 k

~

-15V

50 k

Circuit 3 – The Summing Op Amps Simulate the following Circuit. (Set the simulation profile to transient analysis (0 to 2 ms)). Comment on the output voltage waveforms. 100 k +15V

2 k VOFF = 0 FREQ = 1k VAMPL = 1mV

_ V out

2 k

~

+ VOFF = 0 FREQ = 1k VAMPL = 2mV

~

UA741 -15V

50 k

Further task: Using the Summing Op Amp circuit design an amplifier and calculate the appropriate gain in order to add two similar AC signals (2Vpp, 1kHz) and give a signal of 10Vpp at the output. Use simulation to evaluate your design. Conclusions Draw conclusions on the work you have performed for this workshop. Think about what you have achieved and remember the aims of the workshop....