ECEN 2420, Electronics for Wireless, Spring 2016
Prof. Zoya Popovic
IN-LAB HOMEWORK 11 EVEN– AUDIO AMPLIFIER AND AGC
Assigned
Due
Covers pages
Additional sources needed
th
th
April 17
April 17
250-253
No
th
Turn in: Monday, March 4 in Homework Session 6:15-9:30pm
Today you will be building and testing Board 9, which contains the AGC and Audio Amplifier.
It is VERY important that close attention is paid to the direction of diodes, FETs and 8 pin IC’s
as the circuit is super sensitive. Make sure to remove all leads from the bottom of the board to
avoid shorting.
Acquire a PCB from one of the TA’s as well as the bag of parts for Board 9 along with one
speaker. Before you do anything with the boards, verify that your bag contains all the parts
necessary to build both the AGC and audio amplifier. You will lose points for soldering the
entire board immediately, so please follow along.
Part 1: Audio Amplifier
To begin, we will rebuild the audio amplifier on the printed board. This will consist of soldering
in all components of the diagram, boxed in red below. You will also need to solder in R8, C26,
C30, and the Speaker Jack. Finally solder a wire to each terminal of the speaker in your bag.
Verify all connections look good with a TA after you have finished.
TA Signature: ______________________________
1
Now we will again hook up a signal generator to the board as follows, please be careful in what
side of C20 you hook the ends of the 1.5 kΩ resistors to. Set the frequency of your signal
generator to 620 Hz, with 15 Vpp amplitude and hook it up as shown below. Also please do not
solder the leads of the resistors into any FET holes; these are difficult to clean out and will be
generally hard for everyone involved.
Ground one terminal of your speaker and insert the other into the speaker jack so you see it touch
either of the metal contacts in the plug. How clean does your signal sound? Explain any defects
in the sound that you might hear.
R6 controls the overall gain of the amplifier, adjust this so that the cleanest sound is produced.
R8 acts as your volume control, adjust this to a level that isn’t too loud so as not to blow your
speakers. What happens when you turn off the signal generator and hook up the speaker as
before? Where does this come from?
2
Draw a picture of the oscilloscope reading at the positive terminal of C26 for an input voltage of
5 mVpp and 30 mVpp. Explain any differences.
Part 2: AGC
Now, solder in the remaining components of board 9. Make sure to again remove all extra length
on the leads to your board. Again, check off with a TA, that indeed the boards look good and
testing can continue.
TA Signature:
________________________________
In order to set up tests you will need to solder in wires as shown in the picture below and hook
these up through a bread board or banana clips. AF1 and AF2 are where the signal generator
should be connected.
3
Set the signal generator to produce 620 Hz at 15 mVpp and hook up your speaker to the audio
jack. Note any differences between the circuit with the AGC and just the audio amplifier.
Now vary your input voltage level as well as your volume (R8) and record all observations
below. How does your input voltage level affect your overall sound quality?
Now we will plot your input voltage versus your output voltage. Probe the positive terminal of
C26 and read the output Vpp at increments of 5 mVpp. Plot in the range of 5 – 55 mVpp. Describe
dynamic range.
Vin
(mVpp)
5
10
15
20
25
30
35
40
45
50
55
Vout
(Vpp)
4