pp 35-39
2.4 feedback power amplifiers
Assuming that the circuit Sch.2.1 signals are streams (rather than trends) then this circuit represents amplifier with breath-link current, as shown in Sch.2.5. To have a negative breath-link current, the stream should feedbacks If returning from the exit to the entrance be removed from the input current Ii (ie to have the phase difference in this 1800). This is illustrated in Sch.2.5.
Figure 2.5. Structural diagram of power amplifier feedback
Demonstrated that:
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2.4.1 Power Gain with feedback
As observed, the output current I0 is fueling both the load resistor Rl and the circuit feedbacks.
We define the factor Power feedbacks reason, i of the circuit (the panel) feedbacks reason:
From this relationship and, Sch.2.5 follows, that when the output current I0 flows through the circuit feedbacks, then the part which reaches the input of the amplifier is:
This current, If, called feedbacks stream. We define power gain of the amplifier Ai no feedback with the output shorted to ground:
Similarly we define the gain and power amplifier with Aif feedback shorted and the output:
This equation connects the power to gain feedback, with shorted output, to gain power without feedback a shorted output.
Because in practice usually apply |, i Ai >> 1, can not leave the unit in the denominator of Eq. (2.4.5) and we approach:
The Eq. (2.4.6) is very basic, since it expresses the fact that the gain in power with feedback shorted output can be independent of the parameters
amplifier and depends only on the circuit elements feedbacks.
2.4.2 Input Impedance
The input impedance of the amplifier feedback determined from the voltage Vs of the input signal corresponding to the input current of Is, ie:
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It turns out that the input resistance with negative feedback given by:
Because usually 1 +, A >> 1, the input impedance of the amplifier with negative feedback is much smaller than the input impedance of the amplifier without feedback. That feedback negative current can significantly degrade the input impedance of the amplifier.
2.4.3 Output Impedance
The output impedance of the amplifier feedback determined from the output voltage (open) to the output current (in the short-circuiting). It turns out that valid on
the equation:
From this equation, we conclude that the amplifier has a negative resistance feedback output increased by the factor (1 + ba), compared to the amplifier without feedback.
Example 2-2
The Sch.2.7 represents a practical power amplifier feedback. The parameters of the amplifier without feedback is: A = 800, ^ = 1 KO and R0 = 10 KO The circuit
feedbacks has the following elements ^ Z = 220 and R = 4.7 KO
Figure 2.7. Practical power amplifier feedback
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2.4.4 Equivalent Circuit
The Sch.2.6 shows the equivalent circuit of the power amplifier feedback. As we see, the output consists of a generator / power source with output resistance feedback, while the input consists of the input impedance with feedback.
Figure 2.6. Equivalent circuit power amplifier feedback
Solution
Based on Eq. (2.4.1) and the current divider circuit
feedbacks, feedbacks power factor is obtained by
relationship:
Therefore,
So, according to Eq. (2.4.5) (2.4.8) and (2.4.9), the parameters of the amplifier
feedback will be:
We observe that the approach was made in Eq. (2.4.6) to gain power is good enough for the present case because here the NE is greater than ten.
The price Aif = 21.8 and the approximate Aif - 1 /, i - 1 / 0.045 - 22.2 differ slightly.
In this example we see that the power gain
is independent of the parameters of the amplifier without feedback, so the transistor, and depends only on the resistors R8 and R9, ie only the circuit feedbacks.
Generally, when calculating the parameters of an amplifier feedback, we should be careful. So to calculate the input parameters should be the output current is zero. That is, the circuit in the second show (the second transistor) of Sch.2.7 should be considered open.
Similarly, to calculate the output parameters, should the mains inlet. is zero. That is, the circuit to first base (first transistor) should be considered open. In this way, limiting the effect of circuit feedbacks. At the same time, however, the calculation takes into account the elements that make up this circuit.
