Briefly introduce the analysis method of negative feedback amplification circuit

Briefly describe the analysis method of negative feedback amplification circuit

1. Approximate calculation under deep negative feedback conditions

use Calculating the gain of the negative feedback amplifying circuit is more accurate but more troublesome. Because you must first find the open loop gain And feedback coefficient It is necessary to first divide the feedback amplifying circuit into a basic amplifying circuit and a feedback network, but this is not simply a matter of disconnecting the feedback network, but it is necessary to remove the feedback and also consider the load effect of the feedback network on the basic amplifying circuit. 1. Therefore, it is usually based on engineering practice to use a certain approximation condition, that is, to estimate the closed-loop gain under deep negative feedback conditions. In general, most feedback amplifier circuits, especially those composed of integrated op amps, can satisfy the conditions of deep negative feedback.

according to with Definition

,

in If, if ,then

, which is

So have

This formula shows that when Feedback signal With input signal Little difference, net input signal Very small, so there is

For series negative feedback (virtual short), For parallel negative feedback (virtual) . The concept of “virtual short” and “virtual break” can be used to quickly and easily estimate the gain of the negative feedback amplifier circuit. Closed loop voltage gain .

Second, the small signal model analysis method of the negative feedback amplifier circuit

It is discussed above that the gain of the feedback amplifying circuit is approximated under the condition of deep negative feedback, and the input resistance and output resistance of the circuit are qualitatively analyzed. This has important implications in engineering approximations and can establish and be familiar with certain important concepts.

Here we will introduce the method and steps of analyzing the small-signal model of the negative feedback amplifier circuit, calculating the closed-loop gain, input resistance and output resistance. Specific steps are as follows:

1. A small signal equivalent circuit of the feedback amplifying circuit is drawn, which includes a small signal equivalent circuit of the basic amplifying circuit and an equivalent circuit of the feedback network.

(1) The drawing method of the small signal equivalent circuit of the basic amplifying circuit:

For the basic amplifying circuit composed of discrete components (triode and field effect transistor), it is processed according to the methods of Chapters 3 and 4. For the basic amplifying circuit composed of the integrated operational amplifier, it can be processed according to the method in LT_01 of this section. However, it should be noted that the integrated op amp usually gives the parameters of open-loop differential mode voltage gain AVO, input resistance ri and output resistance ro, while the amplifier circuit has four types (voltage amplification, transimpedance amplification, mutual conduction amplification and current amplification). Therefore, it is necessary to consider the mutual conversion relationship between the four basic amplifying circuit models, which has been briefly introduced in the introduction.

(2) The drawing method of the equivalent circuit of the feedback network:

1 The main function of the feedback network is to transmit feedback signals To the input of the amplifier circuit (with For comparison), so the output port of the feedback network has a controlled source with internal resistance The type of the controlled source is determined by the type of AC feedback, such as voltage series negative feedback, then for If it is current parallel negative feedback, then Is and many more. The internal resistance of the controlled source is called the output resistance of the feedback network and is expressed in rOF. The rule for finding rOF is that if voltage feedback is used, the output node of the amplifier circuit is short-circuited (ie, ); if current feedback, the output circuit of the amplifier circuit is opened (order) ). rOF reflects the load effect of the feedback network on the input of the amplifier circuit.

2 Although the forward transmission of the feedback network (for the input signal The transmission can be ignored, but its load effect on the output of the amplifier circuit should be preserved. The input resistance riF of the feedback network reflects this load effect. The rule for finding riF is that if the series feedback is used, the input circuit of the amplifying circuit is opened ( ); if parallel feedback, the input node of the amplifier circuit is shorted to ground ( ).

2. Find the gain of the feedback amplifier circuit

(1) Find the open loop gain Feedback coefficient

Finding the open-loop gain by the small-signal equivalent circuit of the feedback amplifier circuit Whenever the control source in the equivalent circuit of the feedback network is made Just fine. This treatment conforms to the principle of separating the basic amplifying circuit (ie, the open-loop state) from the feedback amplifying circuit: both the effect of the feedback is removed, and the load effect of the feedback network on the input and output of the basic amplifying circuit is retained.

(2) by Get

3. Find the input resistance Rif and output resistance of the feedback amplifier circuit

The method of finding Rif and Rof by the small-signal equivalent circuit of the feedback amplifying circuit is the same as that described in Chapter 3.

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