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2.1 Introduction
The reset (feedback) a part of the output amplifier at the entrance gives a special operating behavior and individual characteristics that may be useful in several applications. In this chapter we examine the basic principles of amplifiers with negative feedback * and the advantages and disadvantages, in terms of amps without feedback.
feedback called the phenomenon in which a portion of the output signal is allowed to return from the exit and applied to the amplifier input.
There are two types feedbacks, negative and positive.
Negative feedback we have, when all or part of the output signal (voltage or current) returns with appropriate wiring, the input of the amplifier in such a way that the signal feedbacks (return signal) be removed from the original input signal.
Thus, the total signal is applied every time the amplifier input with a negative feedback is the original input signal minus the signal feedbacks (as instantaneous values).
Because the original input signal decreases with negative feedback depending reduced and the output signal. For this reason, the amplifiers are characterized by negative feedback small gain (amplification), compared to amplifiers without negative anasyxefxi.
Positive feedback we have, when all or part of the output signal (voltage or current) returns with appropriate wiring, the input of the amplifier in such a way that the signal feedbacks (return signal), added to the original input signal.
Generally, the amplifiers positive feedback is undesirable, since the amplifier becomes unstable and it tends to work as an oscillator.
The positive feedback used in oscillator circuits, which are discussed in another chapter.
* Some books, instead of feedback, use the term feedback or feedback.
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2.2 General principles of feedbacks
The phenomenon of feedbacks finds many practical applications. A first important application is in automatic control systems.
The negative feedback can be used in an amplifier for the following reasons:
1. To bring stability to the voltage gain or power.
2. To make a larger linear portion of curves. This leads to an improvement of the deformity.
3. To widen the passband frequency.
4. To reduce or increase the input impedance.
5. To reduce or increase the output impedance.
6. To reduce the internal noise.
7. To limit the size change of the characteristics of the amplifier operation due to thermal effects.
When we talk about stabilizing the gain, we mean to make the voltage gain or current less dependent on the parameters of transistors.
Besides seeking greater linearity in the amplifier mode, since we want the output signals with less distortion.
Even right across the amplifiers are created due to thermal effects, electrical signals conduct random disturbances, called noise.
O noise, especially in amplifiers with very little input, creates embarrassing situations in extracting information from the output signal. The export of this information becomes very difficult when the magnitude of the output signal exceeds the magnitude of the noise. Then, the output signal covered by noise. If this support is meaningless, since boosting the signal, equally reinforced and noise. So to reduce the noise, we use negative feedback.
Depending on the effect of feedbacks the enjoyment, we have the following two basic types feedbacks.
1. feedback power.
2. feedback voltage.
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The most important common feature of these two types of negative feedbacks
is that we reduce the gain.
We also have two other types and feedbacks.
O is called a feedback parallel branch and the other feedback series.
The following depicts the Sch.2.1 block diagram of a typical amplifier feedback. The feedback can refer to a voltage or current.
Figure 2.1. Amplifier block diagram feedback
2.3 Amplifier with voltage feedback
Assuming that all signals in the chart is Sch.2.1 voltage signal, the diagram that depicts a voltage amplifier feedback. However, more detail can we represent a feedback voltage amplifier as shown in Sch.2.2.
Figure 2.2. Amplifier block diagram with a negative voltage feedback
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The negative voltage feedback achieved when part of the signal returned by the output voltage to the input of the amplifier is such, that such a phase has to be removed from the input voltage.
2.3.1 Voltage Gain with feedback
In Sch.2.2 the output voltage V0 is the voltage obtained at the ends of the load resistor RL, <at the entrance of the circuit (panel) feedbacks.
We define coefficient <feedbacks why BN voltage circuit feedbacks reason:
where Vf is the voltage signal feedbacks or otherwise the voltage signal from the output and displayed at the entrance of the amplifier.
Also designated as the voltage gain of the amplifier without AV feedback reason:
Vi where the input voltage of the amplifier without (to cut) the feedback.
As shown in Sch.2.2, compose three input voltages to the amplifier. Along with the Vi and Vf, we have the total voltage of the input signal Vs (with feedback). These three trends associated with the following equation:
If we solve the Eq. (2.3.1) and (2.3.2) on Vf and Vi, respectively, and substitute in Eq. (2.3.3), easily proved that the voltage gain of the amplifier feedback (closed loop gain) Avf given by:
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In the case of negative anasyzexis, the, vAv = Vf / V <0, ie the voltage Vf feedbacks back at the entrance to the 180 phase difference to the input voltage Vi (Vf so this will be the abstract Vi). Since the vnAn >> 1 the above relation is written:
Thus, the voltage gain with proven practical feedback independent of the parameters of the amplifier without feedback.
2.3.2 Input Impedance
The input impedance amplifier with a feedback, Rif, is given by the ratio of the voltage of the input signal (with feedback) Vs to the input current I, namely:
Similarly, the input impedance of an amplifier without feedback Ri defined by the ratio of the input voltage (without feedback) Vi to the input current Ii ie: Ri = y (2.3.7)
It turns out that the input impedance with feedback, given by the equation:
Rif = Ri (1 + ba) (2.3.8)
This equation connects the input impedance of the corresponding feedback without feedback. If feedbacks negative voltage, the input impedance with feedback is greater than the resistance without feedback. This is because, if you meet the above criterion feedbacks negative, ie, | BN AO | >> 1, the brackets of Eq. (2.3.8) is, in absolute value, much greater than one.
From this equation, we see that for the negative voltage feedback with | BN Aj >> 1, the output impedance with feedback is much smaller than the corresponding feedback without resistance.
2.3.4 Equivalent Circuit
To design the amplifier equivalent circuit feedback voltage, and we create the circuitry of Eq. (2.3.8). Based on this equation shows the equivalent circuit shown in Sch.2.3.
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2.3.3 Output Impedance
The output impedance of an amplifier with feedback denoted R0f, R0 and without feedback with R0. It turns out that the resistors are connected with the following
equation:
Figure 2.3. Equivalent circuit voltage amplifier feedback
Example 2-1
The circuit represents one of Sch.2.4 feedback voltage amplifier. The data parameters of the circuit without feedback is: = 100, = 2 KOS and KOS R0 = 5.
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Analogue Electronics
VCC
Figure 2.4. Practical voltage amplifier with feedback
Find the parameters of the circuit feedback.
Solution
Based on Eq. (2.3.1) and the divisor of line-anasyzef
opment, the rate feedbacks calculated by:
We observe that, Avf = 18.7 very close to the approximate value would be expected from Eq. (2.3.5), which gives us:
Avf = 00435 23
Good approximation of the above relationship we would have if vnAn was greater than 10.
It should also be noted that the input impedance with feedback Rif calculated resistance is "seen" after the circuit resistances R and R2 of Sch.2.4. To calculate the total input resistance Rif (cA) with feedback should also consider the parallel combination of R1 and R2. The input impedance is that "sees" the circuit before the resistors R and R2.
Therefore, the total input impedance is the parallel combination of all these resistances, ie fol = Rif / / Ri / / R2 (2.3.10)
