You get a bonus - 1 coin for daily activity. Now you have 1 coin

Condition of stable operation of the amplifier

Lecture



The condition of self-excitation is equality

  Condition of stable operation of the amplifier ,

the first of which is the phase balance condition, and the second the amplitude balance condition.

The amplifier will not be self-excited if the conductance of the circuit at its input, taking into account the feedback, will be positive:

  Condition of stable operation of the amplifier .

But the absence of self-excitation does not mean the invariance of the amplifier.

We introduce the stability coefficient

  Condition of stable operation of the amplifier .

If a   Condition of stable operation of the amplifier , the amplifier can be self-excited. With   Condition of stable operation of the amplifier no feedback, which corresponds to the maximum stability of the amplifier. Usually take   Condition of stable operation of the amplifier . In this case, the change of the gain and bandwidth under the action of the OS does not exceed 10 ... 20%. the closer   Condition of stable operation of the amplifier to one, the more stable the amplifier.

Similar reasoning is true for a negative OS: the properties of the amplifier should not undergo significant changes, so choose   Condition of stable operation of the amplifier .

Let us find the stability condition of the amplifier with a given stability margin. When considering the previous question, we received

          Condition of stable operation of the amplifier .

Where   Condition of stable operation of the amplifier - the function that determines the dependence   Condition of stable operation of the amplifier from disorder   Condition of stable operation of the amplifier and argument   Condition of stable operation of the amplifier .

The nature of this dependence, and consequently, the active component of the input dynamic conductivity from   Condition of stable operation of the amplifier shown in the figure (solid line). As a function of the argument   Condition of stable operation of the amplifier , it will change (dashed lines), approaching   Condition of stable operation of the amplifier or   Condition of stable operation of the amplifier . At pic   Condition of stable operation of the amplifier negative, with OOS - positive.

  Condition of stable operation of the amplifier


Figure 10.7.

Substitute the expression for   Condition of stable operation of the amplifier in the expression for   Condition of stable operation of the amplifier

  Condition of stable operation of the amplifier .

At the last lesson, we obtained an expression for the resonant gain

  Condition of stable operation of the amplifier ;

  Condition of stable operation of the amplifier .

In this expression entered the signs of the absolute value   Condition of stable operation of the amplifier and   Condition of stable operation of the amplifier to combine both cases of OS in one formula, since with pic   Condition of stable operation of the amplifier and   Condition of stable operation of the amplifier , and at OOS   Condition of stable operation of the amplifier ,   Condition of stable operation of the amplifier .

Multiplying both sides of the resulting equality by   Condition of stable operation of the amplifier get

  Condition of stable operation of the amplifier .

Solving this equation for   Condition of stable operation of the amplifier get

  Condition of stable operation of the amplifier .

Sustainability is ensured if

  Condition of stable operation of the amplifier .

With identical circuits at the input and output of the amplifier   Condition of stable operation of the amplifier ;   Condition of stable operation of the amplifier and the expression for   Condition of stable operation of the amplifier take the form

  Condition of stable operation of the amplifier .

This expression shows that to obtain a large, stable gain, an amplifying device must be chosen with the maximum value of the ratio   Condition of stable operation of the amplifier . This relationship simultaneously characterizes the amplification capabilities of an electronic device and spurious feedback. Gain is possible if   Condition of stable operation of the amplifier . Expression Study   Condition of stable operation of the amplifier by   Condition of stable operation of the amplifier shows that it has extreme points:

  Condition of stable operation of the amplifier ;

  Condition of stable operation of the amplifier .

When studying the effect of the internal OS on the performance of the amplifier, it is necessary to proceed from the largest absolute value of the active component of the input dynamic conductivity.

Then

  Condition of stable operation of the amplifier .

In a multi-stage amplifier, the output circuit of this stage is the input of the next stage. In the presence of an OS, it is shunted by the input dynamic conductivity of the next cascade. Because of this, its equivalent conductivity changes, which entails a change in the input dynamic conductivity and parameters of the input circuit of this cascade to an even greater degree. Thus, a multi-stage amplifier is less stable than a single-stage amplifier. However, if in each cascade the condition

  Condition of stable operation of the amplifier ,

then the decrease in stability is insignificant.


Comments


To leave a comment
If you have any suggestion, idea, thanks or comment, feel free to write. We really value feedback and are glad to hear your opinion.
To reply

Devices for the reception and processing of radio signals, Transmission, reception and processing of signals

Terms: Devices for the reception and processing of radio signals, Transmission, reception and processing of signals