Purpose of This Minilab

Physics 2025 : Analog Circuits / Digital Circuits

Purpose of This Minilab

• Gain some basic experience in reading and building electronic circuits.

• Test voltage dividers under load.• Build basic amplifier circuits.• Learn how digital circuits and digital logic work.


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

Vout

Ground (0V)

R1

R2


The Voltage Divider – How it Works

Vin

Vout

Ground (0V)

R1

R2

The total resistance of the circuit is: Rtotal = R1+R2 (1)

The current from Vin to ground is:

I

)2(0

21 RRVI in

)3(0 2IRVVout

21

2

RRRVV inout

Combining (2) and (3):

Ohm’s law for R2:


The Voltage Divider – How to Choose R1 and R2

Vin

Vout

Ground (0V)

R1

R2

I

21

2

RRRVV inout

Example task: Vin = 5V ………..create Vout = 2V

52

21

2 RRR

Many Possible Solutions:R1 = 3 W R2 = 2 WR1 = 30 W R2 = 20 WR1 = 300 W R2 = 200 WR1 = 3000 W R2 = 2000 W etc.


The Voltage Divider – Which Solution to Choose?

Many Possible Solutions:R1 = 3 W R2 = 2 WR1 = 30 W R2 = 20 WR1 = 300 W R2 = 200 W………………………….R1 = 300 K W R2 = 200 K W etc.

Current I is very large(maybe too large for thepower supply to handle)

Current I is very small(Problem when attachingcircuits with smaller resistances to Vout).


Attaching a Simple Circuit to Voltage Divider

Vin

Vout

R1

R2R3

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.

attached circuit


Voltage Divider on the Bread Board

To 5V(Vin)

To Ground(0V)

VoutR1

R2


Measuring Vout of Voltage Divider

For correct polaritymake sure GNDindicator goes into“COM” input onDMM.

Black clipshould beon ground.


Inverting Amplifier Circuit – How it WorksR4 Negative

feedback loop

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 hasvery high input resistance)

Current through R3 = Current through R4

+

-R3

Vout

Vin

V+

V-I I


Inverting Amplifier Circuit – How it Works

+

-R3

R4

Vout

Vin

V+

V-I I

Applying Ohm’s Law on R3 :333

0RV

RV

RVVI ininin

Applying Ohm’s Law on R4:444

0RV

RV

RVVI outoutout

43 RV

RV outin

3

4

RR

VV

in

out


Inverting Amplifier Circuit – How it Works

+

-R3

R4

Vout

Vin

V+

V-I I

""3

4 amplifiertheofGainRR

VV

in

out

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 gainsimply by choosing the proper ratio of R4 and R3 !!!


Inverting Amplifier Circuit – Amplifying a Signal(just to show you more applications…)

+

-R3

R4

Vout

V+

V-I I

Vin Vout

Sinusoidal input signal Sinusoidal output signal:• Is inverted• Has different amplitude


The Inverting Amplifier Circuit YOU Will Build

+

-R3

R4

Vout

VAR1

R2

5V

Voltage divider from Problem 11

Gain of amplifier circuit:

3

4

RR

VVGin

out

Note: +12V and-12V connectionsfor amplifier not shown in diagram.


Amplifier is an Integrated Circuit (IC): LF351

8 pins (connections)4 on each side

Notice thesemicircularcutout (helpsto identify pinnumber)

pin chart for LF351 (view from top)(pins 1, 5, 8 are not used)

-12V

pin 1

All pin diagrams are shown in the lab manual.

1

2

3

4

8

7

6

5

-

+

+12V

Out

-

+


Connecting LF351 to Create Amplifier Circuit

1

2

3

4

8

7

6

5

R4+12V

-12V

R3R1

R2

5V

Vout


Using the Breadboard for IC connection

5 holes in a“column” areelectricallyconnected.

But:Red and Greenare NOT connectedacross the center break.

The center break


Inserting IC into Bread Board

Insert IC into bread board across thecenter 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


Connecting +12V and –12V Power to the IC

1 2 3 4

8 7 6 5

- +

+12V

Out

- +

-12V


Complete Amplifier Circuit

Voltage divider

R3

R4

Clips attached asshown measureVin of amplifiercircuit.


Measuring Vout of Amplifier Circuit

The output voltage of the amplifier circuit is measured where R4 attaches to pin 6 of theLF351 IC.


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.


Amplifying an AC Signal

R4

Vout p-p

R3

A

B

Function Generator Output

Oscilloscope Channel 1

VA p-p

-

+

+12V

-12V



Amplifying an AC Signal

R4

Vout p-p

R3

A

B

Function Generator Output


VA p-p

-

+

+12V

-12V



Binary Numbers

In digital electronics information is coded as binary numbers whichcontain only Ones and Zeroes.

Example: 1001 (binary) = 1x23+0x22+0x21+1x20 = 9 (decimal)

Any decimal number can be converted to a binary number and storedelectronically (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 5V0V 0V

1 0 0 1


Digital Circuits – The Basic Idea

Digital CircuitInput #1

Input #2Output

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).


Digital Circuits – The Basic Idea


Input #2Output

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.


Digital Circuits – TTL


Input #2Output

“TTL” (Transistor-Transistor Logic) circuits are digital circuitsthat use the following “High” and “Low” voltages:

High = 5 Volts = “1”Low = 0 Volts = “0”


Digital Circuits – Example: The Inverter

InverterInput 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 output0V on input 5V on output

“1” on input ”0” on output“0” on input “1” on output

…in other words …

…in other words …


Digital Circuits – The Inverter

Truth Table for Inverter

Input Output

0 11 0

The official symbol

This ring symbolizes“inverting”.


The “AND” Gate – Another Digital Circuit

A

BQ

Truth Table for AND Gate

Input A Input B Output Q = A•B

0 0 01 0 00 1 01 1 1


The “NAND” Gate – Another Digital Circuit

A

BQ

Truth Table for NAND Gate

Input A Input B Output Q = A•B

0 0 11 0 10 0 11 1 0

Just like “AND”gate but additionallyinverted”.

Indicates“invert”


What Good are Digital Circuits?

Digital circuits are basically automated decision makers.

Circuit that rings a bellwhen 5V is applied.

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 produces5V signal if door is open and 0V when closed.

Circuit that produces 5V when alarm is “ON”, 0V when it is “OFF”.

By combining digital circuits you can build very complicateddecision making machines.


4081 : The AND Gate IC (contains 4 gates)5 Volt

Input A

Input B Output Q

View from the top

A and B could, for example,be connected to SW1 and 2on the bread board.

Output Q could, for example,be connected to the logic indicator(green LED) on the bread board.


Remember: These power point presentationsare available on our website and on Canvas, so you can download them on the computer at your lab table.

Purpose of This Minilab

Documents

voltage vout

electronic circuits

voltage dividersuppose

resistors r1

basic amplifier circuits

test voltage dividers

fixed voltage power

digital logic work