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3.5 Class B Amplifier
The low efficiency of class A amplifiers are a serious problem in
systems have great power. Thus, the transmitter emits large force of several kW and therefore, if the output is low operating costs will be high. Also the initial cost of construction of these transmitters will be great when it is needed most expensive and high power components and construction elements.
A large part of the power losses of power amplifiers due to losses in the collector. Therefore, we can have a significant performance improvement if you reduce this power loss. Because power is equal to the product of current times the voltage, the power losses in the collector is proportional to the voltage and current collector. Therefore, if we can reduce the effect of forcing the collector current to travel only during the voltage at the collector is below the summit we will have a significant downgrading of the losses.
This can be done polonontas the amplifier to "cut", ie making the amplifier non-conductive for zero signal at the entrance. For sinusoidal input signal transistor conducts only half of the input signal. During the other half of the transistor is off and does not conduct, ie operating in class B. Sch.3.6
shows the waveform for operation in class B compared with the class A.
Figure 3.6. Operating points Q amplifiers in class A and B and output waveforms for one cycle of the input signal
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While the class A, the collector current circulates during the entire cycle of the input signal, the class B is the collector current and outstanding during the half cycle of the input signal. This leads to a strong distortion of the output signal. For most cases the distortion is not acceptable. Two such, but amplifiers (class B) can work together to give the waveform output normal-form without distortion. The Sch.3.7 shows such an amplifier is called a push-pull amplifier in class B (with adapter). The
basic requirement for this amplifier is tsn two transistors to
have the same power, so as to reinforce the two halves of each cycle of the waveform.
3.5.1 Push-Pull Amplifier
It will now become a small outline of the amplifier push-pull, with the help of waveforms at various points in this, as depicted in Sch.3.8, assuming perfect information and perfect conditions. At the entrance of the whole amplifier assume an input signal expressed by a sinusoidal input current amplitude I0, ie
VCC
Figure 3.7. Amplifier push-pull class B
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the transformer primary entrance will have a current, as shown by the
A Sch.3.8 with instantaneous value
(3.5.1)
O input transformer turns ratio is 1:1 and median intake (right in the middle of the secondary). Therefore, we will send the bases of two transistors, T1, T2, two streams of the same width, Im, but being in phase contrast (ie, phase difference
180 °). In the first half cycle of rotation of the input signal, the transistor T1
polarized by the label in a state cut, so does not conduct, so as the current base, iB1, and the current collector, iC1, zero (Sch.3.8 b, d). In the same
time, the other transistor T2 is conducting, polarized in the active region, so the base and collector currents of, iB2 and iC2,, to monitor the input signal, as shown in c and e Sch.3.8
As shown in Sch.3.7 two currents IC1 and IC2 (instantaneous values) leads to
taking the middle of the output transformer primary diarreontas in the opposite direction over, so will induce voltages on the secondary 'opposite polarity induction and therefore load current in the opposite direction. So if n is the turns ratio of
output transformer, the recommended load current to be
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Figure 3.8. Waveforms at various points in the push-pull amplifier and matching these
(3.5.2)
This relationship is verified by removing the graphical wave-tomorfon of Sch.3.8
d and e, which leads to the load current waveform of f Sch.3.8
The above applies under ideal conditions. In practice, however, at points where the waveform crosses the
V2C
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years horizontal axis (0, 0 ', 0'', ...) displayed a strong local deformation, called the beam deformation and because the characteristic of a transistor is not linear near
to zero. To reduce this distortion, the transistors of the push-pull amplifier polarized not just in class B but slightly dexiotera, ie in class AB. However, this deviation is small (about 0.65 V), so the amps are considered roughly polarized in class B.
Inputs of a class B
The power (ac) output is still given by Eq. (3.4.6) which applies regardless of the operating system. But in the case of class B, according to Sch.3.8 b, the maximum value collector voltage amplitude (ac variation) is VCO = VCC. So, in Eq. (3.4.6), the maximum output power will be given by the expression Pxc = VC2CO (3.5.3)
Besides, it turns out that in class B, the dc power supplied
from the power source to each module is given by
Pdc = 2VcCco (3.5.4)
Hence, the maximum performance in a class B amplifier is
(3.5.5)
This relationship provides the performance class B or normal aid
schyti power or power amplifier push-pull.
Finally, it is shown that the maximum value of the power given
the load RL is given by
PL, max = 2RT (3.5.6)
where R'L = N2RL resistance RL reduced (as reflected impedance)
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the primary output of the transformer (see Sch.3.5 b). Based on these relationships can calculate the resistance of the load required for a given force pUirax.
It is useful to close this section by giving
the advantages of centralized power in aid class B
which are:
• First, as shown by comparison of Eq. (3.4.5) and (3.5.5), the
operation ensures class B (max) power output up
78.5%, so much greater than the maximum output power
allows the class A with resistive load, which does not exceed 25%.
• The power consumption of a transistor used in classroom
B is significantly less than is in class A.
• The class B covers to help large force, while the class A is only applicable for low and average inputs of.
Furthermore, the use of a class B push-pull connection adds the following advantages:
a) Because the two-mode quiescent current in the collectors of transistors
circulate in opposite direction of flow, is inconsistent with each other and fire at the transformer output is not marketed in the core saturation of the magnetic current, thus improving performance.
b) The ac output current because there is no second harmonic components, because
components have opposite directions of flow and thus the corresponding currents mutually neutralized.
c) The quiescent current collector and base and the two transistors are small.
Example 3-3
O push-pull amplifier Sch.3.7 uses the 15 V supply voltage and output transformer with primary and secondary coil 100 turns 20. What is the maximum value of power delivered to load, if he is Speaker 8 Ohm and how wide ac
current must
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present in each transistor collector to take the above as a strength (maximum) output power.
Solution
O transformation ratio of the transformer output is n = 20:100 = 1:5, so a secondary coil corresponding 1 N = 5 turns primary. So, based on Eq. (3.5.6) the maximum output power will be attributed to the load RL = 8 O is the speaker:
The issue width ICO obtained if we solve Eq. (3.5.3), so we have:
= 2x0.56 = i. ^ = 74.7mA co Vcc 15 15
3.6 Deformations
As we have seen, the transition from class A to B, the power amplifiers elevate their performance. But if we do not take appropriate measures, about sacrifice
form of the signal. So the issue of deformation is very important.
The distortion in amplifiers due to the nonlinearity of the input and output characteristics. The Sch.3.9 shows a typical input transistors in connection CE. As we see the characteristic is highly curved at the bottom of.
So when the applied input signal vbe = V sin vi, the resulting waveform is compressed to more negative (down) the top, Figure 3 a-9,
than when the bias point is at a higher point (Q2), b Sch.3.9 Also, the lowest bias because the signal is smaller.
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Figure 3.9. Typical input CE
When a sinusoidal signal distorted, proved mathematically, (broken down by Fourier), that created additional sinusoidal signals. The frequencies of these sinusoidal signals "deformity" are multiples of the original signal. Ie, if f is the initial frequency, creating additional frequencies 2f, 3f, 4f, etc. The initial frequency is called basic or fundamental frequency, and multiples of frequencies, called harmonics. In particular, the second harmonic 2f called the third harmonic 3f, etc.
The various harmonic components contained in a signal can be detected and measured using narrowband filters coordinated. The filter in a given coordinate and harmonic output signal has the same frequency. If the project is the filter unit, the width of the output is the width of the given harmonic.
There are commercial devices (spectral analyzer) using this technique to measure the deformation. In most cases the lower frequency harmonics are wider, making the measurement and analysis of deformation to focus only on the 2 nd and 3 rd harmonic.
The Sch.3.10 shows a sinusoidal signal vs applied in series, two circuits have non-linear characteristic curves or transport. The output current in each case can be designed graphics for a given transfer curve. In Sch.3.10
a point function Q and the signal amplitude such that the
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signal occupies only the lower portion of the curve. The result is that the signal is compressed at the bottom. But if you include the two (upper and lower) the characteristic curved sections, the signal is squeezed on both peaks.
At the same shape and depicts the output signals. In the first case there is a strong second harmonic component resulting output signal has the form of a mark consisting shown. The corresponding distortion is called harmonic distortion sound. 2 In the event the output is displayed in the large third harmonic amplitude so the signal consisting of the pattern shown. This pattern of deformation is called odd harmonics distortion.
Figure 3.10. Distortion 2nd and 3rd harmonic
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3.7 Paramofosi in push-pull amplifiers
When polarized in class A, push-pull amplifier is the idiom that
tends to counteract the unnecessary harmonic distortion.
This is because both transistors operate with a difference
phase 180 °, so that when one transistor in the signal is the maximum positive value in the other is the maximum negative. The Sch.3.11 shows only the second harmonic. After iC2 removed from iC1, as we have seen that happen in the output push-pull, the harmonics are shown in Figure mutually neutralized, while the fundamentals (which are fundamental frequency) are added.
H neutralization is done only for the even harmonics, but not for the odd harmonics. The Sch.3.12 shows, why not offset the odd harmonics, which are additive to each other as the fundamental components.
Figure 3.11. Effect of 180 ° phase difference in
excellent harmonic output of push-pull
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The elimination of the even harmonics is an important advantage of the push-pull amplifier class A compared to a transistor amplifier. The improvement is achieved completely but only if the two transistors in push-pull with the same characteristic. If the characteristic is not identical, the neutralization is not complete and left some distortion in the output.
the push-pull
When the push-pull amplifiers operate in Class B should not forget the existence of the deflection beam already developed in Section 3.5.
