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As for negative feedback amplifier circuit, you need to know these



Improved zoom stability

After introducing negative feedback, improvement in gain stability of an amplifier circuit is usually measured by relative change.


So, derivative:

That is:


Nonlinear Distortion Reduction and Noise Reduction

Due to presence of non-linear devices in circuit, output signal will have certain non-linear distortions. If negative feedback is introduced into amplifying circuit, then its non-linear distortion can be reduced.

It should be noted that: Negative feedback can only reduce harmonic distortion generated by amplifier itself, but negative feedback is powerless against harmonic distortion of input signal.

Amplifier circuit noise is caused by uneven thermal movement of carriers within components of amplifier circuit. Interference occurs due to influence of external factors such as power grid, lightning, and so on. The introduction of negative feedback can reduce noise and interference, but output signal will also decrease according to same law, and as a result, signal-to-noise ratio at output (called signal-to-noise ratio) has not improved.


Influence of negative feedback on input impedance

Since negative feedback can improve stability of gain, once negative feedback is introduced, amount of gain reduction in bass and treble region will be reduced, thus broadening bandwidth.

After introducing negative feedback, bandwidth can be expanded by a factor of about (1+AF) times.

(a) Review of series

(b) Parallel feedback

Fig. 1. Determining input resistance

1. Series negative feedback increases input resistance

After introducing serial negative feedback, input impedance can be increased by (1+AF) times. Namely:

Where: ri is open-loop input impedance

rif - closed loop input impedance

2. Parallel Negative Feedback Reduces Input Impedance After introducing parallel negative feedback, input impedance is reduced to 1/(1+AF) of input impedance without feedback.

That is:


Influence of negative feedback on output impedance

1. Negative voltage feedback reduces output impedance

After introducing negative voltage feedback into amplification circuit, output voltage stability is improved, that is, circuit has a constant voltage characteristic.

After introducing negative voltage feedback, output impedance rof decreases to 1/(1+AF) of original value.

2. Negative current feedback increases output impedance

After introducing negative current feedback into amplifier circuit, stability of output current is improved, that is, circuit has a constant current characteristic.

After introducing negative current feedback, output resistance rof increases to original (1+AF) times.

3. The principle of selecting negative reviews

(1) Negative DC feedback must be introduced to stabilize static operating point.

(2) In order to improve AC performance, AC negative feedback must be introduced.

(3) To stabilize output voltage, enter a negative voltage feedback, to stabilize output current, enter a negative current feedback.

(4) To increase input resistance, enter a series negative feedback, to decrease input resistance, enter a parallel negative feedback.


Characteristics of deep negative reviews

1. Conditions for evaluating a series of negative reviews

Feedback depth (1+AF)>>Negative feedback equal to 1 is called deep negative feedback. Generally, if it is a multi-stage negative feedback amplifier circuit, it can be thought of as deep negative feedback. Currently there are:





Evaluation criteria:

(1) For deep series negative feedback: ui≈uf (so-called "virtual short")

(2) Due to increase in input impedance of closed loop series negative feedback under deep negative feedback conditions: ii≈0 (the so-called “virtual gap”)

2. Evaluation Conditions for Parallel Negative Feedback

Due to strong negative feedback: xi≈xf

(1) For deep parallel negative feedback: ii≈if (or "virtual gap")

(2) The input impedance of a closed loop parallel negative feedback decreases under deep negative feedback conditions: ui ≈ 0 (so-called "virtual short circuit")


Deep Negative Feedback Gain Estimate

Example 1 Estimate voltage gain Auf of feedback amplifier circuit shown in Figure 2.



Figure 2. Negative feedback circuit with voltage sequence and negative feedback circuit with current sequence


(1) In amplification circuit shown in Figure 2(a), it can be deduced that Rf is negative feedback with successive sequence of leapfrog voltage, so it can be considered as deep negative feedback, that is, ui≈uf . . Hence, its feedback factor is:

Thus increasing voltage with feedback:

In addition, from structure of circuit, we can assume that feedback voltage is obtained after successive division of output voltage by resistor Rf and Re1, therefore:

Still available:

(2) In amplification circuit shown in fig. 2(b), one can judge Compile current series of negative reviews. So, under conditions of deep negative feedback, there is ui≈uf.


uf= i.e.× , uo=-io×Rc≈ie×Rc,

So his feedback factor is:

Thus increasing voltage with feedback:

Example 2 Estimate source voltage gain Ausf of feedback amplifier circuit shown in Figure 3.



Figure 3. Negative feedback circuit in parallel with voltage and negative feedback circuit in parallel with current


(1) In amplification circuit shown in Figure 3(a), Rb is a parallel voltage negative feedback. Under condition of deep negative feedback from formula (4-16) you can find out that ii≈if (or—virtual break), and there is also ui≈0 (false closure).

From input circuit in fig. 3 (a) we can get:

So, increasing voltage of feedback source:

(2) In amplification circuit shown in Figure 3(b), Rf is a hopping parallel negative voltage feedback. Under condition of deep negative feedback ii≈if (virtual gap) and ui≈0 (virtual short circuit), therefore there are:

From output in fig. 3 (b) shows that:

Thus, increasing the feedback source voltage:

From above analysis process, it can be seen that under deep negative feedback conditions, gain is determined only by some resistors and has almost nothing to do with gain circuit. If it is not deep negative feedback, error of result calculated by above method is relatively large, and other methods should be used for analysis at this time.


Estimating negative feedback of an amplifying circuit

1. Evaluation of feedback loop

The reinforced part of circuit is main circuit of transistor or operational amplifier. And feedback is a circuit that feeds some or all of signal at output of amplification circuit back to input, so feedback loop must be a loop that leads back from output of amplification circuit to input. end. This circuit usually consists of resistors and capacitors. When looking for this loop, pay special attention not to go directly through power terminal and ground terminal, which is most common problem for beginners. For example, if only electrode-to-electrode feedback is considered in Figure 5, amplification path is from T1 base to T1 collector and then through T2 base to T2 collector, and feedback loop is from T2 collector to T1 emission via Rf pole. The feedback signal uf=ve1 influences pure input voltage signal ube1.

Fig. 4. Negative voltage feedback

2. Court of AC and DC

From characteristics of "forward blocking" capacitors, you can judge characteristics of AC and DC feedback currents. If there is a grounded capacitor in feedback loop, then it is DC feedback, and its function is to stabilize static operating point; if capacitor is connected in series in loop, this is AC feedback, which improves dynamic performance of converter. amplifier circuit; if there is only resistance or only wires in feedback loop, then feedback is coexistence of AC and DC.

Feedback in fig. 1 is coexistence of AC and DC.

3. Evaluation of positive and negative reviews

The method of instantaneous polarity is used to evaluate positive and negative feedback. Instantaneous polarity is a hypothetical condition that assumes an instantaneous increase in signal at input of amplification circuit. This signal is returned to input through amplification circuit and feedback loop. If feedback signal increases input signal, it is positive feedback, otherwise it is negative feedback. At this point, it is necessary to figure out configuration of amplifier circuit, whether it is a common-emitter, common-collector, or common-base amplification. The signal input and output points of each configuration amplifier circuit are different, and their instantaneous polarity is also different. As shown in Figure 5. If phase difference is 180°, instantaneous polarity is reversed, and if phase difference is 0°, instantaneous polarity is same. Op-amp circuits also suffer from feedback problems. The op-amp output has same instantaneous polarity as non-inverting input and opposite instantaneous polarity as inverting input.

Table 1 Phase difference of amplifier circuits with different configurations

In accordance with instantaneous polarity detection method described above, start at input end of amplifying circuit marked with instantaneous polarity and work your way back to input end along amplifying circuit and feedback loop. At this time, evaluate whether feedback is positive or negative based on principle that negative feedback always attenuates pure input signal, and positive feedback always strengthens pure input signal.

In a transistor amplifier circuit, if instantaneous polarity of feedback signal back to input pole is same as original polarity, it is positive feedback, otherwise it is negative feedback. Note that feedback of common-emitter amplifier circuit sometimes returns to common pole - emitter, while instantaneous polarity of feedback back to emitter is same as that of base, i.e. negative feedback, but vice versa positive feedback. The instantaneous polarity detection sequence in Figure 4 is as follows: T1 base (+) → T1 collector (-) → T2 base (-) → T2 collector (+) → Rf to T1 emitter (+), this is instantaneous polarity feedback to emitter same as to base, so circuit is negative feedback. In feedback loop of an op-amp, if instantaneous polarity fed back is same as original instantaneous polarity at same end, it is positive feedback, otherwise it is negative feedback; if instantaneous polarity being fed back is same as original instantaneous polarity polarity at other end, then negative feedback, otherwise positive feedback.

4. Feedback Type Evaluation

Feedback type refers to type of negative AC feedback in circuit, so feedback type can only be judged when there is negative AC feedback in circuit. Feedback returns all or part of output signal (voltage or current) back to input and somehow affects input signal (voltage or current). Therefore, feedback is divided into voltage feedback and current feedback according to output waveform. According to form of influence on input signal, it is divided into serial feedback and parallel feedback.

Fig. 5. Current parallel negative review

(1) Evaluation of serial and parallel connections

Series-parallel feedback type refers to how feedback signal affects input signal, that is, how it is connected at input. Series feedback means that net input voltage and feedback voltage are connected in series in input loop, for example, net voltage input signal ube1 and feedback signal uf=ue1 in figure 1; parallel feedback means that net input current and feedback current in input circuit are connected in parallel, as shown in pure input current connection diagram ib1 and if in figure 4. To sum up, if feedback signal returns to emitter of input circuit, it is a series feedback connection, and if it returns to base, it is a parallel feedback. In negative feedback circuit of an op amp, if feedback returns to other end of input, it is series feedback, as shown in Figure 6. In figure, uD and uF are connected in series, if back to other end of input, it is series feedback, as shown in fig. 7, and iD Connect in parallel with iF.

Fig. 6. Negative voltage feedback

Fig. 7. Current Parallel Negative Feedback

(2) Voltage and Current Estimation

Voltage and current feedback refers to form in which feedback signal is taken from output signal (voltage or current). Voltage Feedback 6 shows as an example that feedback voltage uF is obtained by sampling output voltage uO through a voltage divider consisting of resistors R1 and R2. The feedback voltage is part of output voltage, so it is voltage feedback. When evaluating voltage feedback, you can use a simple method, i.e. according to definition of voltage feedback - feedback signal is proportional to output voltage, imagine a short circuit of both ends of load RL of amplifying circuit, and after a short circuit, if uF=0 ( or IF=0), i.e. voltage feedback.

Current Feedback Let's take Figure 7 as an example. In this figure, feedback current iF is shunting output current iO by resistors R1 and R2, so it is current feedback. Another convenient way is to open load RL (RL=∞) so that iO=0 to iF=0, that is, feedback signal caused by output has disappeared, so it is defined as current feedback.


Parallel Negative Feedback Voltage

A parallel negative voltage feedback circuit is shown in Figure 8. Since feedback signal and input signal are added at same point, it is a parallel feedback. According to instantaneous polarity method, this is negative feedback, and this is negative voltage feedback. Due to parallel feedback, currents add and subtract at input.

Fig. 8. Parallel Negative Voltage Feedback

Dimensions with resistance

Dimensions with resistance

Has dimension of conductivity

is called transimpedance gain, is called feedback coefficient for mutual conductivity, Multiplication is dimensionless. For deep negative feedback, trans resistance gain is

Voltage Gain:


Negative voltage series feedback

(a) discrete amplifier circuit (b) integrated op-amp circuit

Fig. 9. Negative voltage feedback

(1) Estimation Method

For circuit shown in fig. 9(a), according to instantaneous polarity method, feedback voltage applied to emitter of E1 through Rf is "+", which corresponds to same polarity as input voltage, and is applied to both sides of input circuit point, so that this serial negative feedback. The feedback signal is proportional to output voltage and is voltage feedback. This feedback from back stage to front stage is AC feedback, and at same time there is negative feedback from very first stage on Re1, which will be analyzed below.

For figure (b), since input signal and feedback signal are added to two input terminals of op-amp, this is series feedback and it is evaluated as negative feedback according to instantaneous polarity. and this is negative voltage feedback. The output is negative feedback of series AC and DC voltage.

Series current negative feedback circuit is shown in fig. 7-7. Figure 10(a) is formed by removing Ce from basic amplifier circuit,

Figure 10(b) consists of integrated op-amps.

In fig. 10(a), feedback voltage is taken from Re, and according to instantaneous polarity and way feedback voltage is connected, it can be considered as series negative feedback. Because output voltage is short-circuited, the feedback voltage is still there, so it is series current negative feedback.

(а) (b)

Figure 10. Negative feedback of current series

For circuit in fig. 10(b) find the gain gain

Thus 1/R, shunting effect of Rf is ignored here. Voltage Gain


Current parallel negative review

The current parallel negative feedback circuit is shown in fig. 11(a) and (b). For circuit in (a), feedback node is same as entry point, so it is a parallel negative current feedback. For circuit in figure (b), this is also parallel negative current feedback.

(а) (b)

Figure 11. Negative parallel current feedback

Current feedback factor take pic. 11(b) as an example

Current magnification

Obviously, current amplification is mainly related only to parameters of external circuit and has nothing to do with internal parameters of operational amplifier. Increasing voltage