Class A with transformer

pp 50-59

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3.1 Introduction
O ultimate goal of support and other functions of electronic devices is to stimulate action to correct a final order which is the output load. This output load varies, depending on the type of device, and may be a headset, a speakerphone, a cathode-ray tube, a measuring device or an electromechanical control.
O workload exit to function properly, you should be excited by an electrical signal suitable form and with sufficient width and power. This power may be small, such as in the case of headphones, or more commonly as large in the case of loudspeakers or electromechanical control devices.
If we have great power required to excite the output load, low-signal amplifiers (low power output and low distortion) we met so far are inadequate. Thus, the final stage of the excitation load output amplifiers require specialists who strengthen the power level of the input signal at the same time retaining its shape. The amplifiers are called power amplifiers.
The output power level of a power amplifier should be high on the output power level of the previous step, while the absolute value of exported power may be low. This power depends on the active element (eg, transistors) that we use, the design of the circuit and supply voltage. So, we have as good a power amplifier using a transistor of 0.5 W with a supply 9 V, like a transistor 20 W of power with 15 V. Primarily, the power output of an amplifier power is limited by three key factors that characterize the power transistor, ie the maximum (allowed) power losses, Pmaj, the maximum collector voltage, VCma, and the maximum collector current, ICmax . It is worth noting that the first factor can be improved, ie the permissible power losses increase, if we use the

transistors suitable heat sink or heat sink and configured correctly the dimensions of the fins of the heat losses sink it.

Due to the relatively high level iochyos output, a power amplifier must have high power output, n = P0 / Pdc, where the output power P0 and Pdc power supply (dc). This aspiration is an important design element of a

this amplifier. Besides, one of the most serious problems occurring in a power amplifier is the problem of strain amplitude, ie, when a pure sinusoidal signal applied to the input, the output is not pure sinusoidal signal, but is imported from the enhanced signal, plus a number of additional components with frequencies multiple of imported (harmonic components). The distortion is due to what, given the extensive area of operation,

parameters of power amplifiers vary widely throughout the rotation cycle of the signal. Therefore, the method of solving or design based on low signal equivalent circuit is no longer applicable and used in Fourier analysis or graphical methods.
For the elimination of these distortions resort to special techniques such as fitting push-pull, which will see below, or supply the amplifier with suitable resonant circuits (see resonant power amplifiers Special Topics in Electronics, Third year).

3.2 Classification of power amplifiers

In order modulation (increase) in performance power amplifiers n the function and are classified into three basic categories or classes, depending on how the polarization of the transistor. The classes are class A, class B and class C. Sometimes, however,

an amplifier designed to operate at an intermediate grade between A and B, called class AB.
The class A amplifier, a current circulates throughout the rotation cycle (ie during 360 °) of the input signal, as shown in Sch.3.1. In class A operating point Q is calm in the middle of the linear portion of the characteristic

entry. In this way the ac signal base-emitter creates a full circle of the power base and therefore the current collector.

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The class B amplifier, the operating point Q is calm at 0, such that

hence the base current and collector to run the half (ie 1800) the rotation cycle of the input voltage, as shown in Sch.3.1. If, however, polarization amplifier at point Q ', to avoid the strong non-linear portion of the typical entry, we operate in class AB. In class, the base-collector current, is available in more than 180 ° and less than 360 ° cycle of the input signal.
Figure 3.1. The various classes of operation amplifiers (A, B, AB, C), depending on the polarity of the transistor
Finally, the class C amplifier operating point is in a position, as shown in

Sch.3.1 so that the base current and hence the collector to run less than 180 ° cycle of the input signal.
The amplifiers of class A amplifiers find applications in low voltage signal of the previous chapter and amplifiers

low power, using only one amplifier in output stage.
In class B amplifiers rarely used a single transistor amplifiers especially at audio frequencies, due to increased distortion created. Usually uses two transistors in connection push-pull.

With this connection, as we shall see, first we eliminate distortion and also increases maximum force.
Finally, we note that the class C amplifiers are mainly used in output stage of transmitters and receivers in the area of radio frequency RF. Their burden is tuned circuit to output the sinusoidal signal without distortion (see Special Topics in Electronics III year).

3.3 Adjust the transformer

Often, the burden that stimulate the output power amplifiers has been resistance Rl Low Price. This happens for example with speakers or antennas that have an impedance magnitude of O 3 to 30 W. If you are connecting both a low load directly to the output (collector) of a transistor power amplifier which has a relatively high output impedance, RCE, we would have a significant deviation from the requirement to adjust resistors so the output of the amplifier, which would be transferred to the load, it would not be big enough.
According to the theorem of the maximum transmission power to achieve perfect adjustment, ie a maximum transfer of power from the amplifier output power in the output load RL, should be about equal RL = RO, ie the load resistance is equal to the output impedance of the transistor. Because this equality is not the case, nowhere in the case of low power, resorting to inserting a special adapter (adapter) between the output transistor and the output load. The transformer has been regulated or determined why metaschimatimou

N2 n = N ^

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Figure 3.2. Adapter power amplifier
Where N, and N2 is the number of turns (revolutions) of the primary and secondary, respectively. O adapter lag between the output transistor and the output load (speaker) in a manner that shows

It turns out that under these conditions, the secondary circuit of the transformer can be removed and replaced by an equivalent resistance of the primary (glancing resistance) equal to Ri
R = 2 (3.3.2)

Figure 3.3. Syndesmologisi adjustment of the transformer

3.4 in class A amplifier
3.4.1 Class A amplifier with resistive loads

In this section we will determine a maximum price, output, power output and power losses that characterize
Figure 3.4. Power amplifier in class A with resistive load

power amplifier with a resistive load and direct coupling (ie without transformer) when operating in class A.
The Sch.3.4 shows a typical class A amplifier with resistive load and the load line corresponding to the flock designed output characteristics of transistors. From this figure, given the power supply voltage Vcc, we find that the maximum ac voltage peak to peak (pp) which can be given to a resistive load is Vcc. Furthermore, to have this output voltage or voltage relaxation should

VCQ = VCC / 2. The minimization of RL maximizes power (V2CQ / RL) attached to the load. But the minimum possible value of RL is determined by the current collector, as shown in Sch.3.4. The minimum allowed value of RL, which does not give excessive collector current, but a large permissible value of Imax, are:
It turns out that the power attributed to the ac load is given by:
while the power supplied from the dc supply voltage Vcc is:
Pdc = VccIcQ (3.4.3)
The output of amplifier n defined by:

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Considering that, for optimum operation in class A should be
We IcQ = Icmax / 2, from Eq. (3.4.2) - (3.4.4), the performance of the amplifier
power in class A, resistive load is:

nmax = 4 <nmax = 25% (3.4.5)
So, the direct coupling of resistive loads in power amplifiers
Class A results give a small yield (25%).
Finally, the power losses in the transistor PT, without the input signal
, prove that is twice the maximum power delivered to load. Ie by Eq. (3.4.2),
Pt = 2p ^ c = (3.4.6)

Example 3-1
Identify the elements of a power transistor which, when supplied with a voltage of 12 V and load is 10 ohms, are suitable for class A power amplifier with maximum efficiency.
The maximum power delivered to load is given by Eq. (3.4.6)
Vcc 122 144 P 12 = 144 = 1,8 W Pac 8Rl 8 x 10 80 1,8 W
As we have seen, no signal transistor consumes twice the power PT Pnc,

therefore, PT = 3.6 W. Besides, the basis of Eq. (3.4.1), the maximum
collector current is:

Therefore, the transistor should be open collector current of at least 1.2 A and power is 3.6 W.

3.4.2 Class D Amplifier Transformer
As we have seen - for reasons having to adjust resistance
is sometimes the coupling of a power amplifier output to the load through a transformer. Because of this, the operating conditions are modified as we shall see below.
O class A amplifier output power is relatively low and high bias current. When designing such an amplifier main objective is to ensure maximum output power. To get maximum output point of calm (operating)

Q must lie on the hyperbolic curve of maximum power transistor. Under these conditions formed two straight load, the dc and ac load line. The dc load line, which corresponds to the ohmic resistance of the transformer primary is almost
vertical, because the coil ohmic resistance is very small (several Ohm). So VcQ = Vcc. H ac load line, corresponding to the reflected resistance (Rac = RL / n2), shall be such price not to exceed the maximum power loss of the transistor.
Figure 3.5. Class A amplifier with transformer and load it straight

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In the following we accept that the characteristic of the transistor

is linear and that the ac voltage and ac current, and hence the output can be varied from zero to the maximum permissible values. Assuming that the output transformer is lossless, the output is:
where Vco Ico and widths, ac fluctuation of voltage and current collector, respectively.
As shown in the diagram Sch.3.5 for excellent polarization class A (Q average burden of direct ac), the maximum possible values of ac amplitudes, not deformed we are Vco = VcQ S Vcc and Ico = IcQ. Therefore, based on Eq. (3.4.7), the

maximum output power (ac) of the amplifier is:
The corresponding forward power chorigeitaii from the dc source is:
Pdc = VccIcQ VcQ ICQ (3.4.9)
The yield is:

When the input signal is zero, the power supplied from the power supply will still be back VcQIcQ. This power should now be consumed in its entirety, as losses from the power transistors. So, expresses the strength of this effect. Therefore, to achieve strengthening class A power output level, eg 1 W, we must use transistors to withstand losses power 2 W.
The ohmic resistance ac load is given by:
 Example 3-2
To select the power transistors and the power amplifier output power of Class A, if the ac output power is 1 W and the output load RL = 4 for O supply voltage Vcc = 12 V.
The transistors have a maximum power losses 2x1 W = 2 W



To use transistors that will have maximum collector voltage
On the basis of Eq. (3.4.1) and (3.4.2) the real burden (eg loudspeaker) is ohmic resistance RL, the ratio of the transformer coil is:
Hence, the transistor should have a maximum current of 6 A and power consumption of 2 W. losses
O the transformer turns ratio will be, Eq. (3.4.13)