cict lect 03

8/10/2019 Cict Lect 03

1/22

8/10/2019 Cict Lect 03

2/22

Voltage Comparators, Flip-Flops

Contents

1 Voltage comparators

2 Flip-FlopsBistable flip-flopsMonostable flip-flopsAstable flip-flops
http://find/

8/10/2019 Cict Lect 03

3/22


Voltage comparators

Contents


http://find/

8/10/2019 Cict Lect 03

4/22


Voltage comparators

Comparators

DefinitionVoltage comparators are circuits which compare the input voltage with a certain

reference value and, according to the result, give a switched or opened signal on

the output.

The input voltage of a comparator can change continuously, the output valuechanges in step (a choice of two values) at the input voltage transition throughthe reference value (decision level). Thus the comparator represents an interfacebetween the analogue and digital part of the system.

The comparators can be divided into two groups:comparators with an asymmetric input circuit (with one input) such comparatorcompares the input voltage with voltage created inside the comparator,

comparators with a symmetric input circuit (differential) compare two input voltages.

The basis of a comparator is an amplifier with a high amplification, so thata part of its characteristics, where it works as an active element, is narrow enoughfor its width to be neglected.
http://find/

8/10/2019 Cict Lect 03

5/22


Voltage comparators

Comparator one step amplifierUnlike the pulse signal amplifiers, whose dominant working area is the active area,

where the input signal is amplified, dominant working area for amplifiers used incomparators is saturation orblocking area of the active element, where theoutput voltage of the comparator does not change at small changes of the inputvoltage.

Figure: Comparator as transistor amplifier (switch) and transfer characteristic.

The simplest comparator can be a plain transistor amplifier (switch).Decision level is given by the threshold value of the base-emitter junction,U=0.65V.

Amplification about ten to hundred corresponds with a considerable width ofthe active area. Temperature dependence of voltageonB-Ejunction.

V lt C t Fli Fl
http://find/

8/10/2019 Cict Lect 03

6/22


Voltage comparators

Comparator two step amplifier

Figure: Comparator two step amplifier.

Bigger amplification hundred to thousand.Still temperature dependence of voltage on first transistor B-E junction

solution: differential inputs.Due to parasitic parameters the circuit can oscillate solution: positive feedbackwith a resistor Rv.Hysteresis in transfer characteristic.The comparators with discrete transistors are used only in the simplest cases.Even though it would be possible to use this method for creating a more

sophisticated connection, for this purpose are nowadays used almost entirelyintegrated amplifiers.

Voltage Comparators Flip Flops
http://find/

8/10/2019 Cict Lect 03

7/22


Voltage comparators

Comparators with Integrated Amplifiers

Temperature dependence of decision levels is significantly reduced with the use of

differential amplifier input, where the base emitter voltage changes of bothinput transistors are mutually compensated.

Figure: Simplified internal structure of the comparator LM 311.

Input differential stage, differential amplifier (converting signal into asymmetricform), output stage (transistor with open collector and emitter).

Operating range (LM311) of input voltage from UCC+ 0.5 V to UCC+ 1.5V

if the UCC= 0 V, the zero level isout of range.

Voltage Comparators Flip Flops
http://find/

8/10/2019 Cict Lect 03

8/22


Voltage comparators

Comparators with one decision level

Figure: Typical connection of comparator LM311 non-inverting (and transfer characteristic).

Figure: Typical connection of comparator LM311 inverting.

Voltage Comparators Flip-Flops
http://find/

8/10/2019 Cict Lect 03

9/22

Voltage Comparators, Flip Flops

Voltage comparators

Comparators with one decision level (shifted)

Figure: Inverting connection of comparator LM311 with shifted decision level.

http://find/

8/10/2019 Cict Lect 03

10/22

Voltage Comparators, Flip Flops

Voltage comparators

Comparators with two decision levels

Figure: Inverting connection of comparator LM311 with hysteresis.

Inverting character, feedback (Rv), reference voltage divider, decision levels:

U1HL = R2R2+...

U1LH = R2||Rv

R2||...

http://find/

8/10/2019 Cict Lect 03

11/22

g p , p p

Voltage comparators

Comparators with two decision levels

Figure: Part of LM311 data sheet.

http://find/

8/10/2019 Cict Lect 03

12/22

Voltage comparators

Other types of comparatorsThe pair of comparators whose output voltages are as logical quantities beingprocessed in a logical AND circuit creates a window comparator.

High level voltage at the output of a AND circuit input voltage is in the range fromUr1 to Ur2.

Precision comparator with hysteresis:The output signals from the pair of comparators are being processed by the RS flip-flop(presented later).Comparator does not invert and the decision levels are given by reference voltages Ur1and Ur2.

Figure: Precision comparator with hysteresis.

A precision comparator with hysteresis is manufactured in integrated versioncalled timer labelled as 555. In the connection according to it can be used asa comparator with a relatively well defined input voltage thresholds.(Timer 555 is designed especially for applications with lower frequencies, as

a square signal generator it can operate up to frequencies in order of hundreds ofkHz. A circuit of a similar type faster by one or two ordersisNE521.)

http://find/

8/10/2019 Cict Lect 03

13/22

Flip-Flops

Contents



http://find/

8/10/2019 Cict Lect 03

14/22

Flip-Flops

Types of flip-flops

Flip-flops are circuits that can be found in two stable balanced states circuitvalues (voltages, currents) do not change.

Stable either permanently (change only due to an external pulse) or temporarily(after a certain time change into other state spontaneously).

The circuit state when the voltages are changing is called non-stable.

Flip-flops can be divided into the following groups, according to the character of

the stable balanced states:bistable both states are permanently stable,monostable (often called one-shots) one of the states is permanently stable, the otherone is stable only temporarily,astable both states are temporarily stable.

The basis for the flip-flops is an amplifier with a positive feedback (pair of simpletransistor amplifying stages, one amplifying stage and a transformer for creating

the positive feedback, etc.).In digital technique pair of inverters or logical elements with similar behaviourare often used.

In practice, flip-flops made of discrete transistors are rarely used today.Being constructed on the basis of different types of integrated circuits, such as timer555 (integrated circuit most commonly used for this purpose).

http://find/

8/10/2019 Cict Lect 03

15/22

Flip-Flops

Bistable flip-flops

Bistable flip-flops

Figure: Construction of bistable flip-flop from two transistor amplifying stages.

http://find/

8/10/2019 Cict Lect 03

16/22

Flip-Flops

Bistable flip-flops

Bistable flip-flop with driving transistors

Figure: Bistable flip-flop with driving transistors (setting, resetting inputs).

There is a traditional circuit connection with transistor amplifying stages T1

, T2

,to which the driving transistors Ta1, Ta2 are connected. The circuit has twooutputs main one uq, complementary one uq andtwo driving inputs settingand resetting.

Ifus > 0.65 Ta1 is switched (saturating region), T2 blocked (cut-offoperating region), T1 switched, Ta2 blocked, uq UCC.

http://find/

8/10/2019 Cict Lect 03

17/22

Flip-Flops

Bistable flip-flops

Bistable flip-flop logical symbol

Figure: Bistable flip-flop with driving transistors (setting, resetting inputs).

From the point of view of the logical meaning, both transistors of each half of thecircuit (an amplifier and a corresponding driving transistor) represent invertersand their interconnected collectors represent a circuit for realization of a logicalproduct AND could be replaced by NOR circuits.


Fli Fl
http://find/

8/10/2019 Cict Lect 03

18/22

Flip-Flops

Bistable flip-flops

Bistable flip-flop logical symbol

Figure: Bistable flip-flop from NOR circuits.

Table: Functional table for bistable FF.

s r q q State description

0 0 q q Remember state.0 1 0 1 Resetting state.1 0 1 0 Setting state.1 1 0 0 Outputs are not complementary.


Flip Flops
http://find/

8/10/2019 Cict Lect 03

19/22

Flip-Flops

Monostable flip-flops


Figure: Connection of monostable flip-flop (so called one-shot) with driving transistor.

In bistable flip-flops, the positive feedback loop transfers always the DC signalcomponent. If we use a network transferring only the AC component in this loop,we obtain either a monostable or an astable flip-flop. If we connecta differentiating RC network between the collector of the transistor T1 and thebase of the transistor T2 we receive a classical connection of a transistormonostable flip-flop.

Activation of this circuit can be organized as for a bistable circuit with transistor

Ta1.


Flip Flops
http://find/

8/10/2019 Cict Lect 03

20/22

Flip-Flops


Operating of monostable flip-flops

Figure: Voltage waveforms in monostable

flip-flop.

It is obvious that in the quiescent state thecurrent from the supply voltage is runningthrough the resistor RB2 into the base ofthe transistor T2, therefore this transistor

in the quiescent state is switched and thetransistor T1 is blocked.

Time of switched/saturated transistor T2:ti = RB2C ln 2

Recovery time.


Flip-Flops
http://goforward/http://find/http://goback/

8/10/2019 Cict Lect 03

21/22

Flip Flops

Astable flip-flops

Astable flip-flops

Figure: Connection of astable flip-flop.

An astable flip-flop is created from a bistable flip-flop by substituting both DCcouplings between transistor collectors and bases by couplings that do nottransfer the DC component, i.e. by CR networks.

Pulse time on collectors of both transistors Tj: tij = RBjCj ln 2, where j=1, 2.

If circuit is symmetrical, output signal frequency is: f = 12RBCln 2

, for

RB =RB1 = RB2 and C =C1 =C2.


Flip-Flops
http://find/

8/10/2019 Cict Lect 03

22/22

Flip Flops

Astable flip-flops

Operating of monostable flip-flops

Figure: Voltage waveforms in monostable flip-flop.

Astable flip-flops are used for square signal generating, again mainly in an

integrated version.
http://find/

cict lect 03

Documents