Overview. 0 、 introduction 1 、 what is feedback? 2 、 why we need feedback in our circuit? 3 、 how many types of feedback? 4 、 how many types of negative feedback? 5 、 how to judge all kinds of negative feedback? 6 、 advantages of negative feedback?
1、what is feedback?
2、why we need feedback in our circuit?
3、how many types of feedback?
4、how many types of negative feedback?
5、how to judge all kinds of negative feedback?
6、 advantages of negative feedback?
7、how to analyse the negative feedback circuit?
An audio-frequency amplifier can be designed to have a certain current, voltage or power gain together with particular values of input and output impedance.
The amplifier will add noise and distortion to the signals. The components employed in the amplifier, both passive (resistors, capacitors, etc.) and active (transistors and f.e.t.s), will vary in value with both time and change in temperature and will have manufacturing tolerances.
The gain of the amplifier may therefore vary with
time, with change in ambient temperature, and when
a component has to be replaced by another of the
Same type. (manufacturing tolerances)
Any fluctuations in the power supply may also cause
the gain of the amplifier to alter.
1、what is feedback?
2、For many applications a more or less constant gain iS necessary，and this can be obtained if negative feedback (n.f.b.) iS applied to the amplifier, at the expense, however,of a reduction in gain.
Openloop circuit 、gain without n.f.b、A
Closeloop circuit、 gain with n.f.b、Af 、F(feedback factor)
1+AF (feedback depth)
1、feedback make our circuit stabilization:
Quiescent operation point;
Stability of gain: current gain, voltage gain;
Decrease non-linearity distortion
Change input and output impedance
Reduces amplitude／frequency distortion
2、generate oscillation (oscilloscope)
If the Input point---feedback point at the same terminal:
And same polarity==positive feedback
And different polarity==nagtive feedback
If the input point---feedback point at the different terminal:
And same polarity==negative feedback
And different polarity==positive feedback
If the output point---sampling point at the same terminal:
If the output point---sampling point at the different terminal:
If the feedback point--input point at the same terminal:
If the feedback point--input point at the different terminal:
An n.f.b. amplifier has a fraction of its output signal feed back into its input terminals in antiphase with the input signal.
voltage-voltage feedback, (series-voltage feedback)
voltage-current feedback, (series-current feedback)
current-current feedback. (parallel-current feedback)
current-voltage feedback. (parallel-voltage feedback)
A voltage amplifier has a voltage gain of 100 before n.f.b.is applied.Calculate its voltage gain if 3/100 of the output voltage is fed back to the input in antiphase with the input signal
The effective load resistance RL(eff) seen by the transistor in an emitter follower circuit iS the resultant of the emitter resistor R3 in parallel with the external load resistance RL.The voltage gain of an emitter follower is
When voltage-current feedback is applied to an amplifier the fed-back voltage is proportional to the current-flowing in the load.
An amplifier of the type shown in Fig.4.8b has the following data:R2=4.7 kΩ,R3=1 kΩand gm=5 mS.Calculate the voltage gain of the circuit.
Calculate the current gain of the circuit shown in Fig.4.12a if the current gain of the transistor is 120.
The feedback factor is (Ans.)
Current gain with feedback
4.3.1. stability of gain
4.3.2.input and output impedance
4.3.4. non-linearity distortion
Input point--feedback point--sample point--output point amplifier the fed-back voltage is proportional to the current-flowing in the load.Conclusion
An amplifier has a voltage gain of 1000.If 3／100 of the output voltage IS applied as negative feedback,calculate the change in overall gain if the gain before feedback falls by 50％.
Gain with feedback,inherent gain being 1000.
New gain with feedback,inherent gain having been reduced to 500.
Therefore amplifier the fed-back voltage is proportional to the current-flowing in the load.
Change in gain
Thus a 50％fall in the inherent gain of the amplifier results in only a 3.13％change in the overall gain.
If the loop gain is much larger than unity then equations(4.1)and(4.8)become
It is evident that, if , the gain of the amplifier iS merely a function of the feedback circuit and iS quite independent of the characteristics of the amplifier itself.Any changes in the performance of the amplifier will not now affect the overall gain.
Taking antilogl0 of sides
Change in gain=50-32.7=17.3 dB (Ans.)
From equation(4.12),the reduction in harmonic distortion is