1. MALVINO
Electronic
PRINCIPLES
SIXTH EDITION

2. Negative Feedback
Chapter 19

3. 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.

4. 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)

5. 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.

6. 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.

7. 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).

8. 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).

9. 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.

10. The noninverting circuit is a VCVS amplifier.1 B = R2 R1
vout
vin R2
The feedback fraction B = v2
vout v2 R1
ACL =
AOL
1 +AOLB 1 B @
The term AOLB is called the loop gain
and is normally much greater than 1.

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

12. Input impedance of the noninverting amplifier
vout
vin R2
zin(CL) = (1+AOLB)Rin RCM 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

13. Output impedance of the noninverting amplifier
vout
vin R2
zout(CL) =
Rout
1 + AOLB R1
(Rout = 75 W for a 741)
Rout = the open-loop output resistance of the op amp

14. 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%

15. THD of the noninverting amplifier
vout
vin R2
THDCL =
THDOL
1 + AOLB R1

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

17. The VCIS amplifier 1 R1 transconductance: gm = iout = gm vin vin
R2 is the load R2
iout
iout =
vin
R1 +
AOL
R1 + R2 @ R1 R1
zin(CL) = (1+AOLB)Rin
zout(CL) = (1+AOL)R1

18. The ICIS amplifier iin RL iout Ai = AOL(R1+R2) RL+AOLR1 R2 R1 @ + 1 R2
zin(CL) = R2
1+AOLB
R1+R2 R1
B =
zout(CL) = (1+AOL)R1

19. 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+AOLB)f2(OL)
The gain-bandwidth product is constant:
ACLf2(CL) = funity

20. Slew-rate distortion Negative feedback has no effect.
The op amp is not acting in the linear mode so the feedback doesn’t 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.