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SUBJECT: RADIO

BIPOLAR TRANSISTORS

Introduction

The unipolar transistor is an element whose internal resistance can vary depending on the input signal applied, this variation caused makes it capable of regulating the current through the circuit that is connected. It is formed by the union of three pills semiconductor (N or P) linked together, being the central than at the ends and in this way, we can find NPN or PNP transistors. The central bar is the base and is the smallest of all and the end are the emitter and collector (higher than the issuer). The transmitter heavily doped carriers, and its mission is to inject them into the base. The base is lightly doped (less doped), and here is where they spend the carriers coming from the emitter collector road, in this way creates a current. The collector is more doped than the base, but unless the issuer, and this is who collects the carriers coming from the issuer and has not collected the base.

transistor is true in any respect to voltages and currents, the following:

vcb + Vbe =

Vce Ic + Ib = Ie

addition, as a very important parameter, we have:

B (beta or hfe) = Ic / Ib

and is the current gain collector-base when the load resistance is zero.

Polarization is to get the appropriate voltages at each point of the circuit, the desired current and the point of rest (or work) Q. All this involves connecting the transistors to some resistance that, through the voltage drop produced in them, they could institute the intended values \u200b\u200band stability. This will be done from DC voltages.

static load line will be a line located in the first quarter, which cut the curves

Ic = f (Vce) - collector current function of the collector-emitter voltage -.

For two points defining the line, we will raise the equation for the collector mesh in the circuit that we analyze, we will:

Ic = 0

and get Vce (point of intersection with the horizontal axis and maximum voltage that can be applied).

Then do:

Vce = 0

and get Ic (point of intersection with the vertical axis and maximum current that we can supply).

working point Q

is always located in the load line and within a curve, specifying a certain collector current Ic and a specific collector-emitter voltage Vce.

For the working point Q will raise three equations: the base mesh, the mesh collector and finally the equation of transistor

Ic = Beta x Ib.

applications then we will see this in various application circuits.

Work Zones
Depending on the position of the working point, we can distinguish three zones: Zone

cutting hotspot and saturated zone.

*
cutting area This area always have Ib = 0, Ic = 0, Vce = Vcc. The transistor behaves almost like an open circuit.

* Hotspot
This is where the transistor usually works, being the area where the transistor amplifier, fulfilling Ib Ic = B, Vce = 0.6 V (0.2 V for the case of germanium transistors.)

* Zone of saturation
The transistor behaves approximately like a short circuit.

Vce = 0.2 V, Ibsat> Ib, Icsat = B Ibsat, Ic = Icsat.

Stabilization The stabilization is to prevent thermal runaway and reduce the displacement of the point. To get used methods by which an increase in collector current lead, by feedback, to a variation of another magnitude to cause a compensatory decrease of the collector current, so that the resulting increase in Ic is much smaller than Ic increased without the stabilizing system.

bias circuit examples

Polarization fixed emitter resistor

Getting the point

Q:

equation of currents:

Ie = Ic + Ib

Grid collector: Vcc-Vce = ICRC + (Ic + Ib) Re

mesh base: Vcc-Vbe = IbRb + (Ic + Ib) Re Equation

transistor: Ic = BIb (assuming B = 120)

base mesh ---> Ib = 38.87 microamps. Trt of the equation ---> Ic = 4.58 milliamps. Collector mesh ---> Vce = 5.42 volts.

Polarization by collecting feedback about obtaining

Q:

current equation: Ie = Ic + Ib

I = Ic + Ib Grid collector: Vcc-Vce = IR + (Ic + Ib) Re (R = 810 ohms)

mesh base: Vcc-Vbe = IR + IbRb + (Ic + Ib) Equation trt Re: Ic = BIb (assuming B = 110)

base mesh ---> Ib = 42.53 microamps. Trt of the equation ---> Ic = 4.58 milliamps

collector mesh ---> Vce = 4.95 volts.

Stabilization emitter resistance (Re) and voltage divider bias based (self-bias)

The electrical This circuit is very effective and is developed as follows: If we assume an increase in Ic, the voltage drop across Re increases and counteracts the increase of the current Ic because there is a decrease in the base bias voltage Vbe. R1 and R2 are the resistances for varying the working point Q and consequently the work area.

Getting the point Q:

equation of currents: Ie = Ic + Ib

Grid collector: Vcc-Vce = ICRC + (Ic + Ib) Re

Equation voltage base: VBB Vcc = R2 / (R1 + R2)

Rb = R1R2 / (R1 + R2)

Mesh base: VBB-Vbe = IbRb + (Ic + Ib) Re

transistor equation: Ic = BIb (assuming B = 110)
base mesh ---> Ib = 55.11 microamps. Trt of the equation ---> Ic = 5.31 milliamperes collector mesh ---> Vce = 5 volts.

Basic Settings

They are: * Issuer

common: The entrance is on the base and the collector output. * Base

common emitter input and output per collector. * Collector

common base input and output by the issuer. Each configuration

has its own characteristics such as voltage amplification and / or current, impedance, input / output high, medium or low, etc..

Once polarized the transistor to work in a given area, we will introduce an alternating signal at its input to amplify it. The amplification is to increase the amplitude of an electrical signal, then the amplifier output will have an identical signal to the input but with greater amplitude.

Depending on where you place the operating point Q we have the following types of amplifiers:

* amplifier in class A: The working point is located in the hotspot. *

Class B Amplifier: The working point is situated on the edge of the hotspot. Only amplify the positive half cycle of the input signal, so you will need two transistors to amplify both half-cycles (positive and negative). *

Class AB Amplifier: The working point is located at the bottom of the conduction zone. *

Amplifier Class C: The working point is at the cutting area. Also here you need two transistors.

If we amplify the scale can also make classification:

- Amplicen voltage. -

power amplifier. -

power amplifier. -

continuous amplifier. -

low frequency amplifier. -

high frequency amplifier. -

frequency video amplifier

POWER AMPLIFIER COMPLEMENTARY SYMMETRY

PINCIPIOS

BASIC POWER amplifiers, also called Power, whose mission is to deliver to the load signal great power with minimum distortion and maximum efficiency. The output impedance should be small since the load is usually a speaker (4 or 8 ohms), so these amplifiers tend to be common collector because its current gain is very high and this makes the output intensity is large enough to move the cone. There are various assemblies such as common-emitter amplifier output coupling transformer-amplifier output and push-pull output amplifier with complementary symmetry

The circuit consists of two transistors with identical characteristics but different type, one PNP and one NPN (from Hence the name of "complementary"). Class B are polarized so that each transistor lead in opposing half cycles of the input signal.

As can be seen on the oscilloscope, the output signal has a distortion called crossover distortion. This distortion is a more than it can be in any electronic circuit, the most common (and all undesirable) the frequency, phase or amplitude.

crossover distortion occurs because to be biased in class B (very close to the cutting area) transistors do not start driving until a voltage of about 0.6 volts between base and emitter. To avoid this distortion is biased to class AB transistors by increasing the resistance value R1, or having two diodes in series such as seen in the following circuit, thus producing a voltage drop equal to the threshold base-emitter junctions of transistors, so crossover distortion disappears (see signal on the oscilloscope).

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SUBJECT: RADIO AM FM

AM MODULATION PRINCIPLES

modulation is called the effect of "add" a low frequency signal (information) to a high frequency or carrier.

There are three basic methods of modulating the carrier. Are amplitude modulation, modulation Frequency and phase modulation. In our case we will study the amplitude modulation.

Figure 1 shows a wave of radio frequency (RF) pure AF signal and an amplitude modulated wave signal AF

The amplitude of the RF signal varies with modulating signal amplitude. Moreover, the speed with which varies the amplitude of the RF signal depends on the frequency modulation signal.

A RF signal is called a carrier and the envelope or modulating AF.



modulation rate

.- Relationship tension between BF and RF voltage is designated as the degree of modulation, represented by the letter m. The degree of modulation in% is given by the formula:

% modulation = Vmax-Vmin x 100

Vmax + Vmin



The degree of modulation is chosen so that the signal does not reach BF to modulate the carrier by more than 100%, on the maximum amount the 90% modulation.


ISSUER SYSTEM .-

is responsible for transmitting the signal into space. Consists of the following blocks:
- sound signal converter power, which is the modulating signal.
- Oscillator carrier, which will be responsible for generating a frequency wave pattern that has the station.

modulator, we mix the two signals, the LF and RF, giving us the amplitude modulated signal.


RF Amplifier us conveniently amplify the signal to pass through space.


RECEIVER SYSTEM .-

receptor occurs in the reverse process of modulation (Fig. 3). This process is called demodulation or detection.

The desired station signal is received by the antenna and selected by the RF switch. Then it is detected. The resulting audio signal is amplified and applied to the speaker.

superheterodyne receiver .-

The previous recipient was overwhelmed by the superheterodyne receiver. This is more practical because it provides more selectivity, sensitivity and stability. The building blocks of a receiver of this type are:


a) input circuit, which is done by coupling the antenna to the first transistor and is tuned to the frequency of the station you want to receive fs.

b) local oscillator, which generates the frequency signal Buddha, that to achieve the effect heterodyne, must be mixed (broken) with the received signal. The frequency of this oscillator is variable at will and must maintain a constant difference, IF of 460 KHz, with the signal tuned input circuits (fs).

d) mixer, in which the two heterodyne signals fs and Buddhas. Normally the same way makes local oscillator and mixer then called converter.

e) Intermediate Frequency, which consists of one or more amplifier stages tuned to the IF frequency of 460 KHz. This amplifier between the different frequencies that occur in the heterodyning and are present at the output of the converter, selects and amplifies only the FI.

d) detector, which demodulates the IF, thus obtaining
BF signal that is modulated. This detector must receive the IF signal strong enough to work in the linear area of \u200b\u200bthe characteristic of the diode, so there is no distortion.
usually derived from a component detector for continuous operation of AGC (Automatic Gain Control).