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MALVINO & BATES

MALVINO & BATES. ELECTRONIC PRINCIPLES. SEVENTH EDITION. Chapter. 19. Negative Feedback. Topics Covered in Chapter 19. Four types of negative feedback VCVS voltage gain Other VCVS equations The ICVS amplifier The VCIS amplifier The ICIS amplifier Bandwidth. Amplifier possibilities.

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MALVINO & BATES

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  1. MALVINO & BATES ELECTRONIC PRINCIPLES SEVENTH EDITION

  2. Chapter 19 Negative Feedback

  3. Topics Covered in Chapter 19 • Four types of negative feedback • VCVS voltage gain • Other VCVS equations • The ICVS amplifier • The VCIS amplifier • The ICIS amplifier • Bandwidth

  4. Amplifier possibilities • The input can be a voltage. • The input can be a current. • The output can be a voltage. • The output can be a current. • The total number of combinations is four.

  5. Four types of negative feedback • Voltage-controlled voltage source (VCVS) (ideal voltage amplifier) • Current-controlled voltage source (ICVS) (transresistance amplifier) • Voltage-controlled current source (VCIS) (transconductance amplifier) • Current-controlled current source (ICIS) (ideal current amplifier)

  6. Converters • VCIS and ICVS amplifiers can be viewed as converters. • The VCIS is often called a voltage-to-current converter. • The ICVS is often called a current-to-voltage converter.

  7. VCVS equivalent circuit Low zout High zin vin vout AVvin The input impedance is very high and the output impedance is very low. It approaches the ideal voltage amplifier.

  8. ICVS equivalent circuit Low zout iin Low zin vout rmiin The input impedance is very low and the output impedance is very low. It is well suited for converting a current into a voltage (rm is the transresistance).

  9. VCIS equivalent circuit iout High zout High zin vin gmvin The input impedance is very high and the output impedance is very high. It is well suited for converting a voltage into a current (gm is the transconductance).

  10. ICIS equivalent circuit iin iout High zout Low zin Aiiin The input impedance is very low and the output impedance is very high. It approaches the ideal current amplifier.

  11. VCVS voltage gain • Loop gain is the voltage gain of the forward and feedback paths • Loop gain is very large • Closed-loop voltage gain is ultrastable • Gain depends on characteristics of external resistors

  12. Rf R1 1 = +1 B v2 The feedback fraction B = vout 1 @ B AVOL AV(CL) = 1 +AVOLB The noninverting circuit is a VCVS amplifier. vout vin Rf v2 R1 The term AVOLB is called the loop gain and is normally much greater than 1.

  13. The loop gain is usually very large which provides: • Gain stability • Low distortion • Low offsets • Near ideal input impedance • Near ideal output impedance

  14. zin(CL) = (1+AVOLB)Rin RCM Input impedance of the noninverting amplifier vout vin Rf R1 (RCM > 100 MW for a 741) Rin = the open-loop input resistance of the op amp RCM = the common-mode input resistance of the op amp

  15. VCVS negative feedback • Has a curative effect on the flaws of an amplifier • Stabilizes voltage gain • Increases input impedance • Decreases output impedance • Decreases harmonic distortion

  16. zout(CL) = Rout 1 + AVOLB Output impedance of the noninverting amplifier vout vin Rf R1 (Rout = 75 W for a 741) Rout = the open-loop output resistance of the op amp

  17. Distortion • A sine wave has only one frequency called the fundamental. • An amplifier with distortion adds energy at new frequencies called harmonics. • Total harmonic distortion (THD) is the percentage of harmonic voltage in the output signal. • THD = (Total harmonic voltage/fundamental voltage) x 100%

  18. THDOL THDCL = 1 + AVOLB THD of the noninverting amplifier vout vin Rf R1

  19. ICVS amp characteristics • An ICVS is a transresistance amplifier • Equivalent to a current-to-voltage converter • Input impedance is zero • Input current produces a precise value of output voltage

  20. Rout R zout(CL) = zin(CL) = 1 + AVOL AVOL 1 + AVOL @ iin R vout = iin R 1 +AVOL The ICVS amplifier R R is the transresistance vout iin

  21. VCIS amp characteristics • The VCIS is a transconductance amplifier • Equivalent to a voltage-to-currentconverter • Ideally has infiniteinput impedance • Input voltage produces a precise value of output current

  22. R2is the load vin R1 + R2 @ R1 AVOL iout = gm vin vin 1 iout = transconductance: gm = zin(CL) = (1+AVOLB)Rin zout(CL) = (1+AVOL)R1 R1 R1 + The VCIS amplifier vin R2 iout R1

  23. ICIS amp characteristics • Approaches the perfect current amplifier • Input impedance is zero • Output impedance is infinite

  24. Ai = R2 @ R2 + 1 R1 AVOL(R1+R2) R1 zin(CL) = B = 1+AVOLB R1+R2 RL+AVOLR1 The ICIS amplifier iin RL iout R2 R1 zout(CL) = (1+AVOL)R1

  25. VCVS bandwidth • Negative feedback increases the bandwidth of an amplifier. • Less voltage is fed back at the higher frequencies due to rolloff which effectively increases the input signal. • An equation for closed-loop bandwidth: f2(CL) = (1+AVOLB)f2(OL) • The gain-bandwidth product is constant: AV(CL)f2(CL) = funity

  26. Slew-rate distortion • Negative feedback has no effect. • The op amp is not acting in the linear mode so the feedback does not help. • Even though the small-signal bandwidth might be adequate, the power bandwidth might not. • Independent calculations of both bandwidths are required to ensure adequate performance.

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