Mod3_1_Introduction_to_Feedback_Amplifiers.pdf
- 1. Feedback Concept Depending on the relative polarity of the signal being fed back into a circuit, feedback is classified as Positive feedback. Negative feedback. Negative feedback results in decreased voltage gain, for which a number of circuit features are improved . Positive feedback drives a circuit into oscillation as in various types of oscillator circuits.
- 2. Negative Feedback Properties • Since Negative feedback reduces the transfer gain, why it is used? – The answer is that many desirable characteristics are obtained for the price of gain reduction. • A negative feedback Desensitizes the gain of an amplifier -Make the gain less sensitive to circuit and temperature variations Reduces nonlinear distortion -Make the output proportional to input Reduces the effect of noise -Minimize the contribution of output unwanted signals Controls the input and output impedances -Raise or lower the input and output impedances Extends the bandwidth of the amplifier -Increases the bandwidth
- 3. Feedback – a historical perspective • All Op-amp circuits employ negative feedback. • In a CE amplifier, introducing an emitter resistor RE, improves the stability but decreases the gain. This is due to a negative feedback. Negative Feedback Applications
- 4. Basic configuration of a feed-back amplifier • Si the primary input. • S the error signal. • A open loop gain of the amplifier. • So the output. • Feedback factor • Sfb Feedback signal General Feedback Structure
- 5. Basic configuration of a feed-back amplifier • The -ve sign at the summing node () indicates that Sfb is subtracted from the input, which in turn becomes the new input to the amplifier, which is amplified and delivered to the load, while a part of it is fed back.
- 6. Ideal Closed Loop Signal Gain • So = AS → (1); where S is the error signal. • Sfb = So → (2) • At the summing node we have S = Si − Sfb;
- 7. Ideal Closed Loop Signal Gain • (1) →So = A(Si − Sfb) • So = ASi – ASfb • So = ASi – ASo from (2) • Rearranging - i.e. – So to one side and Si to the other • So+A So = ASi • (1+A )So = ASi ; • The closed loop gain Af = So/Si = A/(1+A) • Af ≈ 1/ as A >> 1 ( For Negative Feedback) • Define A loop gain • 1+A desensitivity factor.
- 8. Properties of Negative feedback • Gain Desensitivity: Defines how the change in open loop gain (A) affects the change in closed loop gain (Af). Af = A/(1+A) → (1) • Assume is constant, taking differentials on both sides → (2) • Dividing (2) by (1) • The percentage change in gain Af is smaller than the percentage change in A by the factor (1+A). Hence the name desensitivity factor.
- 9. Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 1 Properties of Feedback Circuits • Bandwidth Modification: • Suppose the feedforward amplifier above has a one- pole transfer function • A0 is the low-frequency gain and ω0 is the 3-dB bandwidth • Transfer function of the closed-loop system is
- 10. Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 2 Properties of Feedback Circuits • Bandwidth Modification: • The closed-loop gain at low frequencies is reduced by a factor of , and the 3-dB bandwidth is increased by the same factor, revealing a pole at ssssss • If A is large enough, closed-loop gain remains approximately equal to 1/β even if A experiences substantial variations • At high frequencies, A drops so that βA is comparable to unity and closed-loop gain falls below 1/β
- 11. Copyright © 2017 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education. 3 Effect of Feedback on Noise • Feedback does not improve noise performance of circuits • In Fig. (a), the open-loop amplifier A1 is characterized by only an input-referred noise voltage and the feedback network is assumed to be noiseless • We have (Vin – βVout + Vn)A1 = Vout, and hence • Circuit can be modified as in Fig. (b), input-referred noise is still Vn
- 12. Properties of Negative feedback • Reduction in Nonlinear Distortion: