Wednesday, 10 April 2013

Simplex, Half Duplex, Full Duplex Communication Channel

A communications channel can be defined as a pathway over which information can be conveyed. A channel can be defined by a physical wire that connects communicating devices, or by a radio, laser, or other radiated energy source that has no obvious physical presence. The required data for transmission should be sent through a communications channel has a source from which the information originates, and a destination to which the information is delivered, the source is called as the transmitter and the destination is called as the receiver. In between the transmitter and the receiver, the transmission medium of the data, this is usually called as the channel of a communication system.  Although the required information for transmission originates from a single source, there may be more than one destination (more than one receiver), depending upon how many receive stations are linked to the channel and how much energy the transmitted signal possesses. If the channel length (transmission distance) is more and the transmission power is less, the receiver situated at a long distance cannot receive the data properly.

In a digital communications channel, the information can be represented by individual data bits that are encapsulated into multi bit message units. An example of a message unit that may be conveyed through a digital communications channel is a byte, which consists of eight bits. A collection of bytes can be grouped to form a frame or other higher-level message unit. These types of multiple levels of encapsulation facilitate the handling of messages in a complex data communications network.

If we consider any communications channel, it has a direction associated:

Simplex Channel: We know that the message source is the transmitter, and the destination is the receiver. A channel whose direction of transmission is unchanging is called as a simplex channel. In other words, a type of data transmission, which is taken place only in one direction (from one antenna to the other only), for example, a radio station is a simplex channel because it always transmits the signal to its listeners and never allows them to transmit back. A television set up can also be considered as the simplex type. The advantage of simplex mode of transmission is, since the data can be transmitted only in one direction, the entire band width can be used.

Half Duplex Channel: A half-duplex channel can be considered as a single physical channel in which the direction may be reversed. Messages can flow in two directions in a half-duplex type, but never at the same time. In other words it can be said that at a single time, the transmission of data are done in only one direction. For example, in a telephone call, one party speaks while the other listens. After a pause (when one party stops his speech), the other party speaks and the first party listens. Speaking simultaneously will result in a garbled sound that cannot be understood. The main difficulty of half-duplex mode of transmission is since two channels are used, the band width of the channel should be decreased.

Full Duplex Channel: A full-duplex channel can be used for transmitting simultaneous message exchange in both directions. It consists of two simplex channels, a forward channel and a backward (reverse) channel, linking at the same points. The transmission rate of the reverse channel will be very slow if it is used only for flow control of the forward channel. The main problem of the full duplex mode of transmission is, since we are using two channels, the band width should be decreased.

Tuesday, 9 April 2013

NPN and PNP Transistor Structure

The BJT (Bipolar junction Transistor) can be constructed by using three doped semiconductor regions separated by two p-n junctions. These three regions are called as emitter, base, and collector. The emitter will emits the electrons, while the collector will collects the electrons emitted from the emitter. The emitter region and the collector regions are separated by the help of a less doped region called as the base. Based on the combination of p and n regions, there are two types of BJTs, either pnp (two p regions separated by one n region) and npn (two n regions separated by one p region).

The npn and pnp BJTs are represented below:


The easiest method to identify a transistor is just check the arrow mark represented in the transistor circuit. The arrow mark points to the n-region of the transistor. The regions near to the arrow mark show the p-region. In figure (a), the emitter section shows the arrow mark. So the emitter section is a n-region. The region near to emitter from figure is base. So from the configuration of transistor, it is clear that no two n regions occur near. Thus the Base region is P type. Also by the configuration of transistor it is clear that the side near to p-region should be n-type. Hence the transistor is a npn transistor. In the similar way from figure (b), it is clear that the arrow mark points to the base region. So the base region is n-type. The regions opposite to base should be p-regions. Hence the structure is a pnp transistor.

The C, E, and B symbols represent the collector, emitter, and base regions, respectively. The base region is lightly doped and it is very thin region. The emitter side is heavily doped and the collector region is moderately doped.


Monday, 8 April 2013

Basic NPN and PNP Transistor Operation

For correct operation if a transistor, the two pn junctions must be correctly biased with  the externally applied dc voltages.

Operation of the pnp transistor is same as that of a npn transistor, but the change is that the  roles of electrons and holes, bias polarities, and current directions are all reversed.

The figure shows the correct biasing of a BJT circuit.

The base-emitter (BE) junction should be forward biased and the base-collector (BC) junction is reverse biased for the proper working of the BJT..

Due to the forward bias condition, base to emitter narrows the BE depletion region. The reverse bias voltage applied from base to collector will leads to the widening of  the BC depletion region. The heavily doped n¬-type emitter region is packed with conduction-band (free) electrons and it will emit electrons when the emitter-base junction is forward biased. The free electrons from the emitter diffuse through the forward biased BE junction into the base region (p-type). The electrons become minority carriers (like in a forward biased diode) in the base region. Since the base region is lightly doped and very thin, it has a limited number of holes only. Only a small percentage of all the electrons flowing through the BE junction can combine with the available holes in the base region because of that light doping. A small base electron current is formed due to these relatively few recombined electrons flow out of the base lead as valence electrons. Most of the electrons flowing from the emitter to the base region will not recombine. These electrons will diffuse into the BC depletion region.

These electrons are pulled through the reverse-biased BC junction by the electric field set up. It is mainly due to the force of attraction between the negative and positive ions.

Electrons can move through the collector region, out through the collector lead, and into the positive terminal of the collector voltage source due to the externally applied voltage.

The collector electron current is thus formed. It is clear that the collector current (Iccr) is much larger (higher) than the base current.

Due to this reason, transistors exhibit current gain.

It is clear that the emitter current is higher than the base and collector currents. So the emitter current will constitute the base and the collector currents.

Hence,

IEcr = ICcr + IBcr

The Capital letters shows the dc values of current.

Saturday, 6 April 2013

Full Wave Rectifiers Theory and Circuit Operation

Full-wave rectifiers are used for various electronic applications. Full-wave rectifiers are the most commonly used devices for the dc power supplies.

A full-wave rectifier is as same as the half-wave rectifier circuit, but the difference is a full wave rectifier allows unidirectional current through the load during the entire sinusoidal cycle (as opposed to only half the cycle in the half-wave). In other words, for both the positive and negative half cycles of the input sinusoidal wave, the full wave rectifier conducts current through the load resistance. This will leads to a constant dc voltage through out the input wave (sinusoidal wave).


Average value of output of the full wave rectifier becomes twice that of the half wave rectifier output:

VA = 2Vpi/p

Full wave rectifiers are of two types:

i) Center-tapped Full-Wave Rectifier:

To the secondary of a center-tapped transformer, two diodes connected.
Between the center tap and each secondary half of Vin  is shows up.
Only one of the diodes is forward biased, at any point in time.
A continuous conduction  of current through load will flows due to this.


The peak inverse voltage (PIV) across then diode  D2 is:

PIV = (Vpi(sec)/2 – 0.7) – (-Vpi(sec)/2)
      = (Vpi(sec)/2 + Vpi(sec)/2 – 0.7)
      = Vpi(sec) – 0.7

Since we know that,

Vpi(out) = Vpi(sec)/2 – 0.7, we get:
Vpi(sec) = 2Vp(out) + 1.4

SO the PIV across each diode will be:

PIV = 2Vpi(out) + 0.7 V

ii) Bridge Full-Wave Rectifier:


The diodes D1 and D2 are forward biased during the positive half cycle of the input. 
The diodes D3 and D4 are the conducing diodes during the  negative half cycle of the input.
The output voltage can be written as:
The PIV is a lot smaller, we can use a full bridge rectifier than a center-tap:

PIV = Vpi(out) + 0.7 Volts

Friday, 5 April 2013

Half Wave Rectifiers Theory and Circuit Operation

Half Wave Rectifiers Theory and Circuit Operation:

In order to convert an AC voltage into DC voltage, rectifiers are used  In all power supplies that operate from an ac voltage source, rectifiers are used 

1) Basic Power Supply:

The power supply converts the standard 110 Vac (AC voltage) into a constant dc voltage. All electronic devices such as (TVs, VCRs, DVDs, etc) have at least one rectifier circuit. A rectifier circuit is used to convert an ac input voltage to a pulsating dc voltage. The filter circuit can eliminates the fluctuations in the rectified output voltage. The regulator circuit is used for maintaining a constant dc voltage for various inputs and load resistances. The circuit (or device) receiving power from the source s called as the load .
Figure: Block diagram of a rectifier ciruit and a dc power supply with a load
a) Half-Wave Rectifier :

The half-wave rectification process is illustrated below.

Figure: Operation of a half-wave rectifier circuit
–The diode is forward biased, when the sinusoidal input (Vin) goes positive thus the diode conducts current on the positive half cycle of the input voltage. This will makes the output voltage keeps as the shape of the input voltage.

– When the input voltage (Vin ) becomes negative (second half of cycle), the diode is reverse biased. (This is due to the condition that in order to make a diode as forward biased condition; a higher voltage should be applied at the positive side (P side) of the diode compared to the negative part of the diode. In other words if a higher voltage is applied at the P side of the diode, it conducts).

When the diode is in the reverse biased state, there is no current.

AT this time, the voltage across the load resistor RL is 0V.

SO the net result is a pulsating dc voltage with the same frequency as the input voltage.

The average value of the pulsating wave is

VAVG = Vp/p
Since the cut off voltage for a silicon diode is 0.7 Volts, we also have to take the 0.7 V from the barrier potential into account.

Thus we should get:
Vp(out) = Vp(in) – 0.7 V

a) Peak Inverse Voltage (PIV):  PIV equals the peak value of the input voltage. In other words PIV can be defined as the maximum reverse voltage that can be with stand by the diode. After the PIV voltage, the diode will get damaged or (breakdown condition occurs after PIV).

A diode must be able to withstand this amount of the applied repetitive reverse voltage.

PIV = Vp(in)

Thursday, 4 April 2013

Power Supply Filters and Regulators

Power Supply Filters and Regulators: 

We know that a pulsating dc wave is the output of the rectifier circuit. Our ultimate aim is to obtain a constant dc output. In order to obtain the constant dc output, we need to filter out the oscillations from the pulsating dc wave. with the help of  a diode capacitor combination, this can be achieved.

The charging and discharging of a capacitor-input filter is such that it fills in the “gaps” between each peak value. The variations of voltage can be reduced by this action. This variation in voltage can be defined as  ripple voltage.

We know that the performance (advantage) of a full-wave rectifier over a half-wave is much good. When the time between peaks is shorter, the capacitor can more effectively reduce the ripple.


The capacitor appears as a short circuit While charging.
This will leads to a large current flow through the diodes.
A surge resistor (Rsurge) is added, in order to avoid damaging the devices.
The surge resistance value (Rsurge)should be small in comparison to the load resistor (RL).

We can use IC voltage regulator, to effectively reduce the ripple occurring after filtering.

We know that a regulator consists of 3 terminals: input, output and reference (or adjust) terminal. It is better to add capacitors after (and before) the regulator circuit. Further filtering of the signal can be done by the help of a large capacitor between the input voltage and the input terminal.

A smaller capacitor is added after the regulator in order to improve transient response.


Examples of positive output regulators are the 78XX series .
Examples of negative output regulators are the 79XX series.

Type Number
(Series)
Output Voltage
(in Volts)
7805
5 V
7806
6 V
7808
8 V
7809
9 V
7812
12 V
7815
15 V
7818
18 V
7824
24 V

Voltage regulation can be measured by two means:
Line regulation : For a given change in input voltage, how much change occurs in the output voltage.
Line regulation = (DVout/DVin)*100%
Load regulation : The rate of output voltage change over a certain range of current values: minimum (no load, NL) to maximum current (full load, FL).
Load regulation = (VNL – VFL)/VFL 100%

Wednesday, 3 April 2013

Diode Limiting and Clamping Circuits

Diode limiting and Clamping Circuits
.
1) Limiters:

The Diodes can be used to clip off (To cut the waveform) portions of signal voltages (above or below certain specified levels). If we want to get a wave form which is in the form of clip off, we can generate that types of wave forms by the help of diodes. In electronic circuits, diodes are considered as the limiting circuit.


The diode will become forward biased as soon as VA (The voltage across the positive side of the diode) becomes larger than VBIAS+0.7. When diode is in forward biased condition, the voltage VA cannot become larger than VBIAS + 0.7 V (Since the cut off voltage for silicon diode is +0.7 V, in order to turn the diode ON we need to apply a positive voltage of +0.7 Volts at the positive part of the diode for the diode to get forward biased).

Thus, the voltage across the load resistance RL, will also be equal to VBIAS + 0.7.

When diode is in reverse biased condition, it appears as an open circuit (since the positive part of the diode is always getting a negative voltage in the reverse biased condition), so the output voltage is the voltage of RL alone will be obtained.

With the help of a voltage divider, the desired (required) voltage levels can be attained.


Here we replace the voltage source with a resistive voltage divider circuit.

VBIAS = R3/(R2 + R3) VSUPPLY ;  

VSUPPLY is the supply voltage.

Example wave form for diode limiter:


b) Diode Clampers :

Any device that adds a dc level to an ac voltage is called as diode clampers.

Diode clampers are also called as dc restorers.

The diode is forward biased, when the input voltage goes initially negative,

The capacitor charges to near peak voltage of input voltage (Vp(in) – 0.7).

After the negative peak voltage, the diode is reverse biased (because cathode is held near Vp(in) – 0.7 by charge on the capacitor).

Capacitor can discharge through the load resistance (RL) only.

The capacitor discharges very little in each period, since the load resistance has high resistance.

The time constant should be large enough (at least 10 times the period of the input voltage).

The capacitors can acts like a battery in series with the input voltage, since the capacitor retains charge.

Tuesday, 2 April 2013

Voltage Multipliers Using Diodes and Capacitors

Voltage Multipliers Using Diodes and Capacitors:

To increase peak rectified voltages, voltage multipliers use clamping action without increasing input transformer’s rating.
The commonly used multiplication factors of 2, 3, and 4.
 Voltage multipliers are generally used in high-voltage, low-current applications.
i) Voltage doubler.
–There are mainly two types of voltage doublers:

1) Half-wave doubler:


During the positive half-cycle of the applied secondary voltage at the input side, diode D1 is forward-biased and D2 is reverse-biased.
The Capacitor C1 is charged to the peak (Maximum) of the secondary voltage (Vp) less diode drop
 During the negative half-cycle of the input voltage, diode D2 is forward-biased and D1 is reverse-biased.
At this time, C1 cannot discharge.
So, C1’s voltage adds to the applied input secondary voltage for charging C2 to approximately 2Vp.
Under zero-load conditions, the capacitor C2 remains charged.
If a load is added, then the capacitor C2 will discharge through the load on the next positive half-cycle. Only recharged in the following negative half-cycle.
Obtained wave form is a half-wave, capacitor-filtered voltage.
The Peak Inverse Voltage (PIV) across each diode is 2VP.

2) Full-wave doubler:


– When the applied input secondary voltage is positive, the diode D¬1 is forward biased and C1 charges to approximately Vp.
– During the negative half-cycle, the diode D2 is forward biased (FB) and C2 charges to approximately V¬p.
- Output voltage can be taken across the two capacitors which is in series connection.

3) Voltage tripler:

The circuit diagram for voltage tripler is exactly same as the half-wave doubler, but another diode-capacitor pair is added to the circuit.

Diode Data Sheet & its Electrical Characteristics


The data sheets for diodes Provides maximum ratings, electrical characteristics, mechanical data, graphs of parameters, etc. for electrical device which are currently in use.
We can see several parameters in a data sheet for diodes:

Maximum ratings of Diodes:

i) VRRM: (Peak repetitive reverse voltage).
–The Peak repetitive reverse voltage can be defined as the maximum reverse peak voltage that can be applied repetitively across the diode.

ii) VR: (DC blocking voltage)
DC blocking voltage can be defined as the maximum reverse dc voltage that can be applied across the diode.

iii) VRSM: (Nonrepetitive peak reverse voltage)
Non repetitive peak reverse voltage is the maximum reverse peak value of non repetitive voltage that can be applied across the diode.

iv) IO: (Average rectified forward current)
 IO is the maximum average value of a 60Hz rectified forward current.

v) IFSM: (Nonrepetitive peak surge current)
IFSM is the maximum peak value of non repetitive (of one cycle) forward surge current..

vi) TA:
– Ambient temperature can be denoted by TA.

vii) TJ:
– Operating junction temperature range can be defined by TJ.

viii) Tstg:
Tstg is the storage junction temperature range.

Electrical Characteristics of a diode :

vF: vF can be defined as the instantaneous voltage across the diode in forward-biased condition when forward current is 1 A at 25oC. That can be generally shown by the help of a graph.

VF(avg): VF(avg) is the maximum forward voltage drop averaged over a full cycle.

IR: IR is the maximum current occurred when the diode is in reverse-biased condition.

IR(avg): IR(avg) can be defined as the maximum reverse current averaged over one cycle (when reverse-biased with an ac voltage).

The Transistor as an Amplifier

The process of linearly increasing the amplitude of an electrical signal is called as Amplification. The signal can be defined as anything which carries some relevant information. By using the gain , a transistor can act as an amplifier. Amplifier is any device which strengthens the amplitude of a signal. The important applications of amplifier are in loud speakers, Bio medical applications etc. When a transistor gets biased in the active (linear) region, the Base-Emitter (BE) junction has a small (Low) resistance due to the forward biasing condition and due to reverse biasing condition; the Base-Collector (BC) junction has a high (Large) resistance.

The Speculations used for the design of transistor as an amplifier are:

i) Consider the DC and AC quantities:

1. Both ac and dc quantities are used in amplifier circuits.
2. For both ac and dc currents capital letters are used.
3. For dc quantities subscript will be capital letter (Uppercase).
4. For ac quantities Subscript will be lowercase (Small letters) .

ii) Transistor Amplification :

– Since the collector current is equal to the base current multiplied by the current gain (, a transistor can amplify the current.
The Base current (IB) is very small (comparatively very low) compared to IC and IE.
– So it is clear that, IC is almost equal to IE.

Now we can consider the following circuit for the transistor application


– From the circuit, it is clear that at the input side, an ac voltage Vin¬, is superimposed on the applied dc bias voltage VBB.

– Through the collector resistance, RC, the Dc bias voltage VCC is connected to the collector.

– The ac input voltage can produces an ac base current (at the Base), which will results in a much higher ac collector current than the base current.

– The ac collector current yields to an ac voltage across RC (The collector resistance), will produces an amplified, inverted, reproduction of the ac input voltage in the active region of the transistor.

To the ac wave, the forward biased base-emitter junction present at low resistance.
This internal ac emitter resistance can be denoted (represented) by r’e.
Ie (or) Ic = Vb/ r’e
Vc = Ic*RC , which is the ac collector voltage,
Since Ie ? Ic, the ac collector voltage can be Vc ? IeRC.
Vb = Vin – IbRB, where Vb can be considered as the ac input voltage of the transistor
The transistor ac output voltage can be considered as Vc

– The ratio of Vc to V¬b (Vc /V¬b) can be defined as the ac voltage gain Av, of the above  transistor circuit.
ie, Av = Vc/Vb       (1)
Now we can Substitute IeRC for Vc and Ie r’e for Vb  which will make (1) as
Av = Vc/Vb ? (IeRC)/(Ie r’e) = RC/ r’e     (2)

– Thus from (2) it is clear that the amplification depends on the ratio of RC and r’e.

– RC will be always much larger in value than r’e, In other words, the output voltage is larger than the input voltage.

Now we can consider one example for the transistor circuit:
Determine the voltage gain and the ac output voltage for the following circuit if r’e = 120 .


Solution:
We know that the voltage gain is
Av ? RC/r’e = 1 k /120  = 8.333
Hence the output voltage is
Vout = AvVb = (8.333)*(100 mV) = 0.8333 Vrms