Tiristores

University of Florida

EEL 3111 — Summer 2010

Drs. E. M. Schwartz & R. Srivastava

Department of Electrical & Computer Engineering

Page 1/4

Evan Kriminger, TA

Revision 0

26-Jul-10

Lab 9: Op Amps and the 555 Timer
OBJECTIVES
 Understand the following op amp circuits:
o Voltage follower
o Rectifier
o Low pass filter
 Gain experience with the 555 timer.
MATERIALS
The lab assignment (this document)
 Your lab parts
 Printouts (required) of the below documents:
o Pre-lab analyses
o Multisim screenshots e-mailed to course e-mail
INTRODUCTION
In this lab we explore some applications of op amps and introduce the 555 timer. First we introduce the
voltage follower, which mitigates loading effects. Then we perform AC-DC conversion using the
combination ofan op amp rectifying amplifier and an active low pass filter. Finally, we work with the
555 timer, a widely used integrated circuit (IC).
Voltage Follower
Earlier in the semester we emphasized the significance of loading effects. We must make sure that
connecting a load to a circuit does not drastically change the open circuit voltage. A voltage source is
said to be “stiff” if it can supply aconstant voltage to the load, regardless of the load resistance. We can
model a voltage source by its Thévenin equivalent circuit, as in Fig. 1. The Thévenin resistance is called
the output resistance of the source, Rout. This connects to a load with input resistance given by Rload.

Figure 1 ‒ Voltage source Thévenin equivalent and load resistor.
The circuit in Fig. 1 is simply voltagedivision. The voltage delivered to the load, Vload, is

In the case that Rload >> Rout, the source resistance becomes negligible and this equation reduces to Vload
= Vsource. Therefore, for optimum voltage transmission, we want sources with low output resistances and
loads with high input resistances. This is not always the case. For example, voltage signals must drive
audio speakers with Rloadvalues as low as 4 Ω. A solution is to use the voltage follower.

University of Florida

EEL 3111 — Summer 2010

Drs. E. M. Schwartz & R. Srivastava

Department of Electrical & Computer Engineering

Page 2/4

Evan Kriminger, TA

Revision 0

26-Jul-10

Lab 9: Op Amps and the 555 Timer

Figure 2 – The voltage follower.
When placed between the source and the load, the voltagefollower, seen in Fig. 2, serves as a buffer that
isolates the source from the load. Because of its high input resistance, the voltage follower is a good
load for Vsource, and the voltage at its input (the + terminal) is approximately equal to Vsource. The voltage
follower has unity gain, so at its output, the voltage is also approximately equal to Vsource. The voltage
follower acts a low outputresistance source for Rload. The voltage source and the voltage follower
function as a stiff voltage source. No matter what Rload we use, we will deliver the desired voltage to it.

Figure 3 – Due to high input resistance, no current enters the op amp, therefore
the voltage at the + terminal is approximately equal to Vsource.
AC to DC Converter
To convert AC voltage to DC voltage, we willmake use of two cascaded (series) op amp circuits. An
AC voltage source is fed into a half-wave rectifier, which zeros the negative component and amplifies
the voltage. The output of rectifier is sent to a low pass filter, which produces a nearly DC voltage. A
filter implemented with op amps is known as an active filter, because it relies on active elements (active
elements require power sources,unlike passive elements such as resistors and capacitors).

University of Florida

EEL 3111 — Summer 2010

Drs. E. M. Schwartz & R. Srivastava

Department of Electrical & Computer Engineering

Evan Kriminger, TA

Page 3/4

Revision 0

26-Jul-10

Lab 9: Op Amps and the 555 Timer

Figure 4 – A 1V, 1kHz AC voltage source is first rectified, then filtered to produce a
negative...