EE222 Lab 9 Instructions

### EE 222 Electronics I Fall 1995 LAB 9

Lab dates: 12/4/95 and 12/6/95

Lab report due: This write-up is due by Tuesday December 12, 1995 at 2 pm. You should prepare your memo in HTML format and drop it into the EE222 folder on my Macintosh. Be sure to include all necessary graphics files.

Related Reading: S &S, Ch 1

## INTRODUCTION TO AMPLIFIERS

Input Resistance, Output Resistance: What are they and who cares?

Note: If you have a portable tape player or CD player, please bring it with you to lab.

In this lab, you will investigate the importance of the concepts of input resistance and output resistance for sources, amplifiers, and loads. We will use these concepts extensively in EE 323.

### LAB WRITE-UP

THIS WRITE-UP IS OPTIONAL! If you choose to do it, I will drop your lowest one-week lab grade in computing your final lab grade. (Note that two-week labs count twice as much as one-week labs.) This does NOT mean that the write-up for Lab 8 is optional!

Your write-up for this lab should be a ONE-PAGE memo on INPUT and OUTPUT RESISTANCES OF SOURCES, AMPLIFIERS, AND LOADS. This memo should describe what input and output resistances are, how they are measured, and what their significance is. Consider this year's EE sophomores as your audience. Be specific so that your audience could do the measurements themselves with no other references. You may want to include specific data from this lab to illustrate your procedure.

### PROCEDURE

#### a) Function Generator

What is the output resistance of your HP function generator (FG) specified to be? How could you measure this? Should you measure it with the FG ON or OFF? Should there be a load?

First, use the Hydra as an ohmmeter to MEASURE the resistance across the output of the FG.

Now, MEASURE the open circuit voltage of your FG (Voc). Then CONNECT a resistor (of known magnitude, R) across the output and MEASURE the voltage at the output (Vo). Can you use this data to determine the output resistance of the FG? (Remember the FG can be modeled by its Thevenin equivalent circuit with a voltage of Voc and a resistance of Ro.) Are there specific values of R that simplify your calculations?

Which of these methods do you think is better for determining the output resistance of a device such as a FG?

Connect the line-out (or earphone jack) or your portable stereo (CD player or "Walkman") directly to a speaker. How does it sound? Why?

MEASURE the resistance of your speaker (RL).

MEASURE the output resistance of your portable stereo (Rs). How do the resistances of the line-out and earphone jacks compare? Note that the line-out does not usually have volume control while the earphone jack does. Use the storage feature on the oscilloscope for this measurement. Try to keep the input signal as constant as possible. (Maybe use the same portion of a song.)

#### c) HP 6825A Programmable Power Supply/Amplifier

MEASURE input and output resistance of the HP 6825A as an amplifier. Is the input resistance higher or lower than the output resistance?

Do you think this will work as an amplifier for your portable stereo? Try it! AC couple (use a capacitor in between) the output of your portable stereo to the input of the HP 6825A. CONNECT the output of the HP to your speaker. How does it sound? Can you vary the gain?

 Bucknell University College of Engineering Electrical Engineering