when a diode is operated inthe reverse biased condition, the width of the depletion region increases as voltage that is applied increases. Varying the width of the depletion region is equivalent to varying the plate separation of a very small capacitor such that the relationship between junction capacitance & applied reverse voltage will look something like that presented in figure below. The typical variation of capacitance supplied by a varactor is from about 50pF to 10pF as the reverse voltage goes up from 2V to 20V. The symbol used for a varactor diode has been shown above.
Showing posts with label tutorial. Show all posts
Showing posts with label tutorial. Show all posts
Varactor diodes
when a diode is operated inthe reverse biased condition, the width of the depletion region increases as voltage that is applied increases. Varying the width of the depletion region is equivalent to varying the plate separation of a very small capacitor such that the relationship between junction capacitance & applied reverse voltage will look something like that presented in figure below. The typical variation of capacitance supplied by a varactor is from about 50pF to 10pF as the reverse voltage goes up from 2V to 20V. The symbol used for a varactor diode has been shown above.Transistor operating configurations
There are 3 basic circuit configurations that are used for transistor amplifiers. These 3 circuit configurations depend upon which one connection among the 3 transistor connections is made common to both the input and the output. In the case of bipolar junction transistors, the configurations are referred to as common emitter, common collector or emitter follower, & common base, as shown in diagram below.
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The superposition theorem
This tutorial is superposition theorem. The superposition theorem states:
‘In any network which is made up of linear resistances & containing more than 1 source of e.m.f., the resultantcurrent flowing in any branch is the algebraic sum of the currents that would flow in that branch if eachsource was considered separately, all other sourcesbeing replaced at that time by their respective internal resistances.’
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‘In any network which is made up of linear resistances & containing more than 1 source of e.m.f., the resultantcurrent flowing in any branch is the algebraic sum of the currents that would flow in that branch if eachsource was considered separately, all other sourcesbeing replaced at that time by their respective internal resistances.’
Shunts and multipliers
An ammeter, which is used for measuring current, offers a low level resistance (ideally 0) & must be connected in series with the circuit.
A voltmeter, which is used for measuring p.d., offers a high level of resistance (ideally I has infinite) & must be connected in parallel with the part of the circuit whose p.d. is required. There is no difference between the basic instrument used to measure current and voltage because both make of use a milliamp meter as their basic part. This is a sensitive instrument which gives f.s.d. for currents of just a few milli amperes. When an ammeter is needed to measure currents of having large magnitudes, a proportion of the current is diverted through a low-value resistance connected in parallel with the meter. Such a diverting type of resistor is referred to as shunt.
DARLINGTON PAIR
The construction of Darlington pair consists of 2 BJTs connected as shown below in the figure. The emitter current of Q1 becomes the base current of Q2 transistor. The current gain of the pair will be equal to the product of the current gains of the individual transistors.
BYPASS CAPACITOR
The application of an emitter resistor (R4) provides improved stability but it also gives reduced gain. When needed, we can restore the gain by wiring a high-value capacitor across R4.This keeps the emitter voltage substantially constant. Without the use of capacitor, the voltage at the emitter rises and falls with the signal. And hence it will provide negative feedback.
Lets take an example, as iB rises (tending to increase vBE),iC rises, and the emitter voltage goes up. This tends to decrease vBE, which decreases iC and resists the rise in emitter voltage. Different way of analyzing this is to say that the capacitor shunts the signal at the emitter through to the ground. This is the foremost reason why C3 is called a bypass capacitor. With this capacitor inplace, the voltage gain of the amplifier is about 280.The lower cut-off point is raised to 130 Hz, so bandwidth is somewhat reduced.
Waveform harmonics
- Consider that an instantaneous voltage v be represented by the formula
v = Vm sin 2Ï€ft volts.
This is a waveform which varies sinusoidally with time t, has a frequency f, and a maximum value represented by Vm. It is normally assumed that alternating voltages have wave shapes which are sinusoidal where only 1 frequency is present. If the waveform is not happened to be sinusoidal it is called a complex wave, & whatever its shape is, itmay be split up mathematically into components called the fundamental and a number of harmonics.This process is referred to as harmonic analysis.The fundamental, which is the first harmonic, is sinusoidal and has the supply frequency, f ; the other harmonics are also sine waves having frequencies which are integer multiples of f . Thus, if supply frequency is fifty hertz, then the third harmonic frequency is 150Hz, the fifth 250Hz, & so on.
Main effects of electric current
There are three main effects of an electric current:
(a) magnetic effect
(b) chemical effect
(c) heating effect
Listed below are few practical applications and uses of these effects of an electric current:
- Magnetic effect:
Magneto motive force and magnetic field strength
The Magneto-motive force (mmf) is the cause due to the presence of the a magnetic flux in a magnetic circuit,
mmf,Fm=NI amperes
where N is used to represent the number of conductors (or turns) and I represents the current in amperes. We sometimes express the unit of mmf as ‘ampere-turns’. As we know, since ‘turns’ have no dimensions, the SI unit of mmf is the ampere. Magnetic field strength (or magnetizing force),
H= NI/l ampere per metre
where l stands for the mean length of the flux path in metres.
Thus mmf =NI= Hl amperes.
Hysteresis loss
A disturbance in the alignment of the domains, referred as groups of atoms, of ferro-magnetic material give rise to energy to be expended in taking it through a cycle of magnetization. This energy acts as heat in the specimen and is known as the hysteresis loss. The amount of energy loss associated with hysteresis is proportional to the area of the hysteresis loop.The area of a hysteresis loop greatly depends on the type of material. The area, and thus the energy loss, is very bigger for hard materials than for soft type materials.
Diagram below shows typical hysteresis loops for:
Field effect transistors
We can find Field effect transistors in 2 basic forms; junction gate & insulated gate. The gate source junction of a junction gate field effect transistor (JFET) is well a reverse-biased p-n junction.The gate connection of an insulated gate field effect transistor(IGFET), on the other end is insulated from the channel and charge is capacitively coupled to the channel. To keep matter simple, only JFET devices are considered in this tutorial. Diagram below presents the basic structure of an n-channel JFET.
Bipolar junction Transistors (BJT)
The Bipolar transistors normally made up of n-p-n or p-n-p junctions of either silicon (Si) or germanium (Ge) material. These junctions are, in fact, produced in a single slice of silicon by diffusing impurities through a photo graphically reduced mask. Silicon transistors are work better when compared with germanium transistors in the wide-spread majority of applications (mainly at high levels of temperature) and thus germanium devices are very rarely encountered in modern electronic equipment.The construction of typical n-p-n and p-n-p transistors is shown in diagrams 12.1 and 12.2. For conducting the heat away from the junction (important in medium and other high-power applications) the collector is allied with the metal case of the transistor.
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COMMON-COLLECTOR AMPLIFIER
The common-collector amplifier shown below contains an emitter resistor but the collector is connected directly to the positive rail. The base is biased by 2 resistors. Supposing that
· the quiescent emitter current is about 1 mA,
· the voltage present across the emitter-resistor R3 is 7.5 Volt,
· bringing the output to exactly half-way between the supply rails.
· To provide for a vBE of 0.7 V, it needs to hold base at 8.2 V.
· The values of R1 and R2are calculated to provide this.
COLPITTS OSCILLATOR
The type of oscillator we are discussing in this tutorial largely depends on a resonant network that consists of 2 capacitors (series capacitance is equal C in total) and an inductor (L) connected in parallel with them. This L-C network resonates at a frequency, f = 1/2 π(LC)-1/2. The op amp is wired in the circuit as an inverting amplifier with a gain of about 30. Its non-inverting (1) input is kept at half the supply voltage (1V/2) by the two 22 kΩ resistors that are acting as a potential divider. The LC network is placed in the +ve feedback loop of the op amp. At the resonant frequency level the output coming from the op amp makes the network to resonate. The tapped point between the capacitors exists at 1V/2, but the part of the oscillating signal across C2 is fed to the inverting amplifier. It is then amplified and maintains the network oscillating strongly.
