1. Purpose of This Minilab
Gain some basic experience in reading and building electronic circuits.
Learn how digital circuits and digital logic work.
Learn some basic digital to analog interfacing.

2. Analog Circuits – The Voltage Divider
Suppose you have a fixed voltage power supply (Vin).
To generate a voltage Vout (between 0 and Vin): Build a “voltage divider”
using two resistors (R1 and R2).
Vin R1
Vout R2
Ground (0V)

3. The Voltage Divider – How it Works
The total resistance of the circuit is: Rtotal = R1+R (1)
 The current from Vin to ground is:
Vin
Vout
Ground (0V) R1 R2
Ohm’s law for R2: I
Combining (2) and (3):

4. The Voltage Divider – How to Choose R1 and R2
Example task:
Vin = 5V ………..create Vout = 2V
Vin
Vout
Ground (0V) R1 R2
Many Possible Solutions:
R1 = 3 W R2 = 2 W
R1 = 30 W R2 = 20 W
R1 = 300 W R2 = 200 W
R1 = 3000 W R2 = 2000 W
etc. I

5. The Voltage Divider – Which Solution to Choose?
Many Possible Solutions:
R1 = 3 W R2 = 2 W
R1 = 30 W R2 = 20 W
R1 = 300 W R2 = 200 W
………………………….
R1 = 300 K W R2 = 200 K W
etc.
Current I is very large
(maybe too large for the
power supply to handle)
Current I is very small
(Problem when attaching
circuits with smaller resistances
to Vout).

6. Attaching a Simple Circuit to Voltage Divider
Choose R1 and R2 such that:
R1<<R3
R2<<R3
Otherwise Vout drops much lower and is no longer what you designed it to be.
Vin R1
Vout R3 R2
For Activity 1 you should
choose R1 and R2 to be less
than 10kW but not too low.
Recommended range: a few
few hundred W.
attached circuit

7. Voltage Divider on the Bread Board
To 5V
(Vin)
To Ground
(0V) R2 R1
Vout

8. Measuring Vout of Voltage Divider
Black clip
should be
on ground.
For correct polarity
make sure GND
indicator goes into
“COM” input on
DMM.

9. Inverting Amplifier Circuit – How it Works
Negative
feedback loop + - R3
Vout
Vin V+ V- I
Virtual equality:  Voltage at “-” input = Voltage at “+” input
(V- = 0Volt because V+ = 0Volt)
Current flows around op-amp (and basically none into it, because op-amp has
very high input resistance)
 Current through R3 = Current through R4

10. Inverting Amplifier Circuit – How it Works
Vin -
Vout R3 + V+
Applying Ohm’s Law on R3 :
Applying Ohm’s Law on R4:

11. Inverting Amplifier Circuit – How it Works
Vin -
Vout R3 + V+
Example: R4 = 10 kW R3 = 5 kW  Gain = - 2
This means: If Vin = 2V then Vout = – 4V
Notice how EASY it is to design an amplifier with a specific gain
simply by choosing the proper ratio of R4 and R3 !!!

12. Inverting Amplifier Circuit – Amplifying a Signal (just to show you more applications…)
Vin
Vout I I V- -
Vout R3 + V+
Sinusoidal output signal:
Is inverted
Has different amplitude
Sinusoidal input signal

13. The Inverting Amplifier Circuit YOU Will Build
Note: +12V and
-12V connections
for amplifier not
shown in diagram.
Vin -
Vout R3 +
Gain of amplifier circuit:
Voltage divider
from Activity 1
Note: Vin is now the input voltage of the amplifier
circuit (don’t confuse this with prior Vin from voltage
divider).

14. Amplifier is an Integrated Circuit (IC): LF351
Notice the
semicircular
cutout (helps
to identify pin
number) 1 2 3 4 8 7 6 5 - +
+12V
Out
pin 1
8 pins (connections)
4 on each side
All pin diagrams are
shown in the lab manual.
-12V
pin chart for LF351 (view from top)
(pins 1, 5, 8 are not used)

15. Connecting LF351 to Create Amplifier Circuit
+12V R1 R2 5V 1 8 R3 2 7
Vout 3 6 4 5
-12V

16. Using the Breadboard for IC connection
5 holes in a
“column” are
electrically
connected.
But:
Red and Green
are NOT connected
across the center break.
The center break

17. Inserting IC into Bread Board
Insert IC into bread board across the
center divide: 4 pins on each side.
Push IC all the way down.
indentation
pin 1
Example: Use any of these 4
holes to connect to pin 4
pin 4

18. Connecting +12V and –12V Power to the IC
Out 8 7 6 5 - + 1 2 3 4 - +
-12V

19. Complete Amplifier Circuit
Voltage divider R4
Clips attached as
shown measure
Vin of amplifier
circuit. R3

20. Measuring Vout of Amplifier Circuit
The output voltage
of the amplifier
circuit is measured
where R4 attaches
to pin 6 of the
LF351 IC.

21. Taking out an IC Grab the IC with the yellow IC removal tool.
Pull evenly and straight
upwards.
The IC removal tool helps to
avoid bent or broken pins.

22. Binary Numbers
In digital electronics information is coded as binary numbers which
contain only Ones and Zeroes.
Example: (binary) = 1x23+0x22+0x21+1x20 = 9 (decimal)
Any decimal number can be converted to a binary number and stored
electronically (e.g., in a computer).
1’s and 0’s are often stored as High (5Volt) and Low (0 Volt) voltages.
For example, the number shown above (1001) could be represented
by 4 “data lines” that have either high or low voltages. 5V 0V 0V 5V

23. Digital Circuits – The Basic Idea
Input #1
Output
Input #2
Digital circuits have one or more “inputs” and one or more “outputs”.
Inputs are wires or pins to which a given voltage is applied.
Outputs are wires or pins that provide a certain voltage. The value of the
output voltage depends on the value of the voltages applied to the inputs.
Never apply a voltage to an output! The output already generates its
own voltage. You can “read” that voltage (e.g., with a DMM).

24. Digital Circuits – The Basic Idea
Input #1
Output
Input #2
Why are they called “digital”? Because we apply only two specific voltages to the inputs and we can only receive one of these two voltages on the output, nothing else.
These two voltages are called “High” and “Low” voltage. They are also called “1” and “0”  They can represent a binary number (“digit”).
Digital circuits are some of the basic building blocks in computers.

25. Digital Circuits – TTL Input #1 Digital Circuit Output Input #2
“TTL” (Transistor-Transistor Logic) circuits are digital circuits
that use the following “High” and “Low” voltages:
High = 5 Volts = “1”
Low = 0 Volts = “0”

26. Digital Circuits – Example: The Inverter
Input
Output
Inverter has only one input and one output.
How the inverter behaves:
If you apply a “high” voltage to the input  You get “low” voltage at the output.
If you apply a “low” voltage to the input  You get “high” voltage at the output.
5V on input  0V on output
0V on input  5V on output
…in other words …
“1” on input  ”0” on output
“0” on input  “1” on output
…in other words …

27. Digital Circuits – The Inverter
The official symbol
This ring symbolizes
“inverting”.
Truth Table for Inverter
Input
Output

28. The “AND” Gate – Another Digital CircuitQ B
Truth Table for AND Gate
Input A Input B Output Q = A•B

29. The “NAND” Gate – Another Digital Circuit
Indicates
“invert” A Q B
Truth Table for NAND Gate
Input A Input B Output Q = A•B
Just like “AND”
gate but additionally
inverted”.

30. What Good are Digital Circuits?
Digital circuits are basically automated decision makers.
Very simple example:
A burglar alarm that rings a bell when a door is open but only
when the alarm is actually activated. You can use an “AND” gate.
Circuit that produces
5V signal if door is open
and 0V when closed.
Circuit that rings a bell
when 5V is applied.
Circuit that produces 5V
when alarm is “ON”, 0V
when it is “OFF”.
By combining digital circuits you
can build very complicated
decision making machines.

31. 4081 : The AND Gate IC (contains 4 gates)
5 Volt
View from the top
Input A
Input B
Output Q
A and B could, for example,
be connected to SW1 and 2
on the bread board.
Output Q could, for example,
be connected to the logic indicator
(green LED) on the bread board.

32. Digital to Analog Converter (DAC)
The music on an I-pod or on a CD is stored digitally as lots of binary numbers. To actually create sound when playing the music these binary numbers (digital) must be converted into “analog” voltages (the garden-variety that is not limited to specific “High” and “Low” voltages).
Once those analog voltages are generated from the digital data they can then be applied to a loudspeaker.
Speaker CD
(digital
data)
DAC

33. Building a Basic DAC With an Op-Amp+ - R4
Vout V+ V- I
5.6kW
10kW
22kW
47kW
SW1
SW2
SW3
SW4
Switches (5V or 0V)
0 Volt
We know this voltage
(virtual equality) I2 I3 I4
More current
means higher
Vout.

34. + - R4 Vout I 5.6kW 10kW 22kW 47kW SW1 SW2 SW3 SW4 I2 I3 I4 I1
0 Volt I2 I3 I4 I1
If SW1 is “0”  I1=0
If SW1 is “1” (5V)  I1=5V/5.6kW
Similar for I2, I3, I4

35. With SW1….SW4 you can switch on and off I1…..I4. + - R4
Vout V+ V- I
5.6kW
10kW
22kW
47kW
SW1
SW2
SW3
SW4
0 Volt I2 I3 I4 I1
With SW1….SW4 you can switch on and off I1…..I4.
I will become more or less.
Vout will become more or less.

36. + - R4 Vout I 5.6kW 10kW 22kW 47kW SW1 SW2 SW3 SW4 I2 I3 I4 I1 V-
0 Volt I2 I3 I4 I1
Because the 4 resistances are different, the 4 currents (when switched on) will be different.
When SW1 is switched on more current is added than when SW2 or SW3 or SW4 is switched on.
SW1 has a greater influence on Vout than SW2 (contributes more to I).
SW2 has a greater influence on Vout than SW3.
Etc..

37. + - R4 Vout I 5.6kW 10kW 22kW 47kW SW1 SW2 SW3 SW4 I2 I3 I4 I1
0 Volt I2 I3 I4 I1
The 4 resistances are weighted differently by factors of about 2, 4, and 8 just like digits in binary numbers.
If the 4 inputs represent a 4 digit binary number, the output voltage is proportional to the value represented by that binary number.
Build the circuit and try it out!

38. Remember: These power point presentations
are available on our website, so you can download
them on the computer at your lab table.