Voltage Shunt Negative Feedback in Operational Amplifiers

1
Voltage Shunt Negative Feedback

• CEC

2
Contents

• Comparison of Negative Feedback Topologies.
• Inverting Amplifier.
• Concept of Virtual Ground.
• Voltage Shunt Feedback.
• Gain and Bandwidth with Feedback.
• Input and Output Impedances with Feedback.
• Noise and Distortion with Feedback.
• Total Output Offset Voltage with Feedback.
• I/V Converter and Inverter.

3
Comparison
4
Inverting Amplifier
Negative Feedback
Feedback Resistor
a
Virtual Ground
Inverting Amplifier Good Example of a Voltage
Shunt Feedback Amplifier.
R1 and Rf determine Amplifier Gain.
5
Inverting Amplifier

• Input voltage drives the inverting terminal.
• Noninverting terminal grounded.
• A fraction of the amplified and inverted output
  fedback to the inverting input through a feedback
  resistor.
• An increase in output signal results in increased
  feedback , causes a decrease in output signal.

6
Virtual Ground Concept
Virtual Ground
V2
I in
V1
V1 V2 0V
7
Virtual Ground vs Physical Ground
Virtual Ground

• At a physical ground, voltage 0., current ? 0.
• Physical ground can sink large currents, junction
  where return currents meet, merge return
  currents from all branches in the circuit.
• At a virtual ground, voltage 0, current 0.
• OPAMP input impedance very high, ideally no
  current flow into the OPAMP input terminals.

V2
Very high input impedance
Physical Ground
8
Inverting Amplifier as Voltage Shunt Feedback
Amplifier

Voltage Shunt Negative Feedback Configuration
V2
V2
A portion of the output voltage fedback in
parallel and out of phase with the input signal.
Feedback Resistor
9
Voltage Shunt Feedback

Block Diagram
Feedback Circuit
10
Voltage Shunt Feedback
11
Closed Loop Voltage Gain
Apply KCL at node V2,
Open loop Gain A very high, Gain Af reduces with
negative feedback.
-
Negative sign indicates phase shift between input
and output.
1
12
Bandwidth with Feedback

• Gain Bandwidth product a figure of merit of an
  amplifier.
• Gain Bandwidth product of a single break
  frequency OPAMP to remain constant.
• Gain decreases with negative feedback by a factor
  1 Aß.
• Hence Bandwidth must increase by the same amount,
  ie 1 Aß.
• Closed loop bandwidth for inverting amplifier
  less than that of noninverting amplifier.

13
Closed Loop Bandwidth with Feedback
Unity Gain Bandwidth (UGB) is the bandwidth when
amplifier gain is unity.
Frequency Response
Gain Bandwidth Product shall be constant.
14
Input Resistance with Feedback
Thevenins equivalent circuit
Input Resistance with feedback
Input Resistance decreases with feedback.
Feedback Resistor Rf split into two Miller
components
15
Output Resistance with Feedback
vo
-
Output Resistance decreases with feedback
Rf gtgt Ro , Ro 0, hence io ia
Source Shorted
Thevenins equivalent circuit
16
Total Output Offset Voltage with Feedback

• Function of Gain when temperature and power
  supply voltages are fixed
• Ratio of total output offset voltage without
  feedback to (1Aß).
• VooT

17
Noise with Feedback

• Noise fed back out of phase with input noise and
  added along with signal.
• Noise cancellation with negative feedback.
• Negative feedback reduces amplifier gain, hence
  the impact of noise.
• Noise with feedback Nf

18
Distortion with Feedback

• Distortion reduced with negative feedback.
• Negative feedback reduces amplifier gain, hence
  also the distortion present in the signal.
• Distortion with feedback Df .

19
Current to Voltage Converter

iin
Hence Vo -IinRf
Vin and R1 can be replaced by a current source iin
Vin and R1 replaced by a current source
I/V Converter
20
Inverter
R1 Rf
When R1 Rf, Vo -Vin
Inverter is a special case of inverting amplifier
with Gain -1.
1800 phase difference between input and output.
21
References

• OPAMPS and Linear Integrated Circuits, Ramakanth
  A Gayakwad, 4th Edition, Pearson Education.
• Your prescribed text books.

22
Thank You