Download pdf - ElectronicIII - L3

Nahrain University

College of Engineering

Computer Engineering Department

Electronic III Eng. Ahmed Ali

Controlled Sources:

Op-Amp can be used in Controlled sources circuits where an input signal is used to control

an output current or voltage source. Varying the input signal can affect on the output

source. There are four controlled sources types as described:

a- Voltage controlled Voltage source

This circuit can be built same as voltage amplifiers (inverting and non-inverting) as

shown bellow

b- Voltage controlled Current source

This circuit can be built same as inverting voltage amplifier where the Feedback

resistor is the load.

c- Current controlled voltage source

This circuit can be built same as inverting voltage amplifier where the Feedback

resistor is the load and no input resistance.

d- Current controlled current source

Current controlled current source circuit is shown below

And the output current is defined by

Voltage controlled current source Current controlled voltage source

Chapter 13

Linear Digital ICs

In this chapter we introduce the linear Digital ICs that utilize the linearity in the analog

signals to form a kind of Digital output or vice versa like Comparators, DAC, ADC,

timer, VCO and PLL.

Comparator circuit:

A comparator IC is based on an op-amp (too high gain) and some additional circuits to

protect the IC.

Op-amp by itself without any feedback is a very high gain amplifier (over 100,000) but

its output voltage is limited the Bias voltage (VCC,VEE) thus a very small difference

between the two inputs or the op amp can be amplified to reach the VCC or VEE limits.

The circuit bellow describe the usage of op-amp as a comparator to turn ON/OFF led.

Although Op-amp IC 741 can be used as a comparator an improvement are built inside

the IC for faster switching between the two levels, noise immunity and others.

311 Comparator this IC has some features

1- It can operate as well from 15V as from single +5V supply (used in TTL digital

circuits).

2- The output is taken from a bipolar transistor (Open collector) to support variety of

loads (This will separate the load current from basing throw the op-amp and enable

the designer to attach two or more open collector outputs together).

3- A strobe pin is available to control the output (enable/disable the op-amp).

(Note: without open collector outputs it is forbidden to attach two or more outputs

together directly).

IC 311 Comparator usage

Circuit description

Input signal on the inverting port is compared to the ground (or any reference signal on

the non-inverting port) [For ground comparison, it is called a Zero crossing detector].

1 K resistor is attached between the output load and the VCC, this will provide a

VCC voltage to load throw this resistor when comparator is digital zero [input signal

is above ground voltage] which cause the transistor to be in cut-off region (open

circuit) while directly attaching the load to ground when the comparator is digital

one which cause the transistor to be in saturation-region (Short circuit).

Strobe input is connected to collector transistor biased by a TTL (0/5V digital)

signal when strobe goes high (transistor short cct to ground) causing the output to

remain high regardless to the input signal, otherwise the comparator works.

Another application of IC 311 is to drive a relay as shown below

Relay is electric magnet coil that when activated pulls mechanically a switch to

alter the contacts situation; most relays have five connections two for coil, N.O.

(Normally open), N.C. (Normally close) and common.

When activated the N.O. [NO] will be connected [Closed] to the common and

N.C.[NC] is disconnected [opened] from the common.

When input is below ground voltage the output will be low providing a short cct

between VEE throw the coil that will activate it. When input is above ground voltage

the comparator will be disconnected allowing the diode to short any back EMF to

the ground then turn off to deactivate the coil.

339 Quad Comparator

This IC contain four independent comparators with one supply voltage

Since the outputs of these comparators are open-circuit collector [open collector],

these outputs can be OR-ed directly as shown below

Circuit description

Input voltage Comp 1 output Comp 2 output System Output

5 Ground Open Ground (Low)

This is called windowing that only a range of input voltage can activate the output.

Timer Circuit Versatile 555

This IC is made of a combination of linear comparators and digital flip-flops

The three R resistance in series (input impedance for op-amp are almost infinity [open])

divides the VCC voltage to VCC on the first comparator inverting input and VCC on

the non-inverting input of the second comparator. Comparator 2 outputs high when

Trigger is less than VCC that will reset the Flip flop FF to start charging phase, while

comparator 1 output high when Threshold is above VCC to start the discharge phase.

Depending on the connection of Trigger and Threshold inputs, the IC can be used as

Astable mode (non-stop clock) or monostable mode (single pulse).

Astable mode

Where capacitor voltage is connected to both trigger and threshold pins

Circuit description

The Voltage on the Capacitor C is the input for both comparators Initially Capacitor C

charge is zero volt thus

Comp. 1 is grounded S = 0

Comp. 2 is high R = 1

FF reset, NPN transistor is open and capacitor charges throw RA + RB

This situation remains till charge is VCC


Comp. 2 is grounded R = 0

FF no change, NPN transistor is still open and capacitor charges throw RA + RB untill capacitor voltage is above VCC



FF no change, NPN transistor will be short and capacitor discharges throw RB only, after capacitor voltage became lower than VCC but above VCC



FF no change, NPN transistor will be short and capacitor discharges throw RB only.

after capacitor voltage became lower than VCC



FF reset, NPN transistor is open and capacitor charges throw RA + RB And loop again and again

Monostable operation

555 timer can be used as a one shot triggered timer using low voltage triggering signal

shorter than active time (Thigh) , timing starts from the moment when trigger goes low

and stay on no matter when the trigger goes high, if time is finished and trigger is still

low timing will restart.

This is done by connecting Threshold pin to capacitor and shorted to the discharge pin

Initially a Trigger must set low (this is done automatically during circuit loading)

When Trigger is low and capacitor voltage below VCC

Comp. 1 is high S = 0


FF reset, NPN transistor is open and capacitor recharge till it reaches threshold VCC by this time Trigger should be high

When Trigger is high no matter what voltage on the capacitor



FF reset, NPN transistor is open and capacitor charges throw RA till it reaches