The webinar discusses the topics of negative feedback and its importance across the Analog IC design spectrum. In the talk, we discuss about the variations of feedback (Shunt and Series combinations) and their usage. It has applications in many control circuit design for power management, reference designs, regulator design, noise reduction in the system, gain desensitization and PLL design among many other systems. And the end of the talk, the audience is expected to understand the need for the feedback and its applications

1. Negative Feedback in
Analog IC Design
G S Javed, PhD
Analog Design Manager
Founder, King Consultants Education
drgs.javed@gmail.com
02 – April - 2020
Webinar
@ConsultantsKing

2. History of Feedback
• Man has been using feedback
mechanism inherently from
beginning of time.
• Parents of toddlers use them to
the maximum, and toddlers use
them to become aware.
• It is also used as “positive
reinforcement” in dogs during
their training.
• James Watt patented a form of
governor in 1788 to control the
speed of his steam engine.

3. History of Feedback
• James Clerk Maxwell in 1868 described "component
motions" associated with these governors that lead to a
decrease in a disturbance or the amplitude of an
oscillation.
• The term "feedback" was well established by the 1920s, in
reference to a means of boosting the gain of an electronic
amplifier.
• Negative feedback for amplifiers was invented in 1927 by
Harold Black to stabilize the gain and correct the distortion
of amplifiers used in long distance telephone networks.

4. Outline
• General Feedback Structure
• Negative Feedback Properties
• Sense and Return Circuits
• Applications
– Bias
– References
– Regulators
– PLL
• Summary
The material in this webinar are partially adapted from the
Lecture Notes of Prof. GY Wei, Div. EAS, Harvard University

5. General Feedback Structure
A = Forward Gain
β = Feedback factor (FB gain)

6. Finding Loop Gain
The negative sign comes from the fact that we are applying negative feedback

7. Negative Feedback Properties
Negative feedback takes a sample of the output signal and applies it to
the input to get several desirable properties. In amplifiers, negative
feedback can be applied to get the following properties
• Desensitized gain – gain less sensitive to circuit component
variations
• Reduce nonlinear distortion – output proportional to input
(constant gain independent of signal level)
• Reduce effect of noise
• Control input and output impedances – by applying appropriate
feedback topologies
• Extend bandwidth of amplifier
Literally, all the Analog IC design jobs have the above requirements

8. Let’s see two properties
• These two properties are extremely common
in Analog low frequency and high frequency
designs
• Gain Desensitivity
• Bandwidth Extension

9. Gain Desensitivity
Feedback can be used to desensitize the closed-loop gain to variations in the
basic amplifier.
This result shows the effects of variations in A on Af is mitigated by
the feedback amount.
1+Aβ is also called the desensitivity amount
If Af is large, the
desensitivity is large

10. Bandwidth Extension
Notice that the mid-band gain
reduces by (1+AMβ) while the 3-dB
roll-off frequency increases by
(1+AMβ)

11. Sense and Return Circuits

12. Sense and Return Mechanisms
• Adding a feedback loop consists of sensing the output signal and
returning (a fraction) of the result to the summing node at the
input.
• Given the inputs and outputs can be either voltages or currents,
there are four types of feedback:
– voltage-voltage (V-V), voltage-current (V-I),
current-voltage (I-V), and current-current (I-I)

13. Examples of FOUR Amplifiers
Sense and Return Circuits
V
V
I
V
V
I
I
IThese
amplifiers
alone do not
have good
performance
Augmented by additional amplifier stages or different configurations (e.g., cascoding).

14. Circuit Examples
Voltage
Return
Current
Return

15. Voltage – Voltage
(Series – Shunt Feedback)
Feedback can be constructed out of capacitors
Find the open-loop gain
– At low frequency, cap loads negligible
– Break feedback and zero out input to feedback Break the
loop here
Find the loop gain, closed-loop gain, and
closed-loop Rout
V
V
increases input resistance and
reduces output resistance

16. Current – Voltage
(Series – Series Feedback)
Source degeneration (with RF) is a form of current-voltage FB
– Voltage across the resistor is the feedback voltage
VF subtracts from Vin to reduce Vgs of the nMOS
– Without the feedback R, iout/vin = gm → A = gm
– RF is the feedback circuit that senses the output current
and subtracts a voltage from the input
VF = βIout → β = RF
V
I
Increases both input and output resistance

17. Voltage – Current
(Shunt – Shunt Feedback)
Transimpedance gain stage is through M2 (a common-base
amplifier) and feedback is thru the capacitor divider and M1
(transconductor).
V
I
increases input resistance and reduces output resistance

18. Current – Current
(Shunt – Series Feedback)
RS and RF constitute the FB circuit
RS should be small and RF large
– The same steps can be taken to solve for
A, Aβ, Af, Rif, and Rof
Remember that both A and β circuits are
current controlled current sources
A current-current FB circuit is used for current amplifiers
For the β circuit – input resistance should be low and
output resistance be high
I
I

19. An Example

20. Differential Pair
V
I
V
Veff = Vgs – Vt = 200mV
Vt = 450 mV
Consider a differential pair with the following design:
Due to variations in the PVT,
1. Vt increases by 10%
2. µnCox and R decrease by 10%
How much Vb change (Voltage Output) to ensure
a. A constant drain current in the matched differential pair Q2-3 (V-I)
b. A constant voltage drop across resistors R (V-V)
c. A constant gain

21. Differential Pair - Solved
V
I
V
Gain
Feedback
Σ
Input Outputerror
Make error = 0
Vb
Due to variations in the PVT,
1. Vt increases by 10%
2. µnCox and R decrease by 10%
Veff = Vgs – Vt = 200mV
Vt = 450 mV

22. Applications
1. Bias
2. References, BGR and
3. Regulators, LDO
4. PLLs

23. Bias, Regulators and References
In an Analog IC, many sub circuits work together to generate all
of the various DC voltages and currents
Bias circuit – generates voltage to keep transistors near a desired operating point.
Reference circuit – generates a voltage and/or current of known fixed absolute value.
Regulator circuit – improves the quality of a dc voltage or current, usually decreasing the noise
Generate and/or Improve a DC signal

24. 1. Basic Constant gm circuit
The transconductance gm is probably the most
important parameters in an analog amplifier,
it needs to be stabilized.
KVL in the loop of Q13, Q15 and RB,
With,
On simplification, we get
gm13 is stabilized, as the ratios of the currents are mainly dependent on geometry

25. 2. Voltage Reference
A voltage reference supplies a fixed DC voltage of known amplitude that does not
change with temperature. This can be combined with an accurate resistance to
provide a stable DC current.
The most popular approach is to cancel the negative temperature dependence of a PN
junction with a positive temperature dependence from a PTAT (proportional to
absolute temperature) circuit. PTAT is usually realized by amplifying the voltage
difference of two forward-biased base-emitter (or Diode) junctions.
Voltage references realized this way is called “Bandgap” voltage references.
[J.Mu, TCAS II, 2017]

26. Circuits for Band Gap Reference (BGR)
One of the most common circuits on any IC.
The amplifier in the feedback loop keeps the
collector voltages of Q1 and Q2 equal. R3=R4
ensures same IC and VCE.
V
V
V
Current CMOS Implementations
Vref = VBE2 + VR1
VBE2 = VBE1+ VR1
V
V
Current References

27. 3. Voltage Regulators
A regulator’s main purpose is to produce a voltage which has low noise and
from which some current may be drawn. They are common when a critical
analog circuit must operate from the same power supply voltage as other
circuits.
As digital circuits are major sources of power supply noise, regulators are common
in today’s mixed analog-digital IC.
The regulated voltage is generally lower than the regulator’s supply voltage.

28. Voltage Regulation
Pass Transistor source the load current.
The impedance (1/gm) divided by Opamp
gain (A), 1/A.gm affects the regulator’s load
current at low frequency.
Replace with resistive divider for
variable-output regulators
V
V
Goal: Keep Vref = Vreg
BGR
VDD – Vreg = Drop Out Voltage, VDO
If the value of VDO, can be kept low,
It essentially becomes an Low Drop out Regulator or LDO

29. Low Drop Out Regulators (LDO)
Why PMOS ?
Keep Veff small and stay close to VDD
Goal: Keep Vref = Vreg
Pass Transistor (PMOS) source the load current.
V
V
BGR
1. It is critical sub-block in power management design.
2. It becomes necessary to isolate the sensitive analog blocks from the
supply noisy digital blocks in large System on Chips and Microprocessor
ICs.

30. 4. Phase Locked Loops
Input
Phase
Output
Phase
Error in Phase
Goals:
Constant Phase
Fixed Frequency
Output
Phase &
Frequency
Input
Phase &
Frequency
Error in Phase
and/or Frequency

31. Controlling the VCO output
with negative feedback
1. Negative Feedback Loop: if the “loop gain” is sufficiently high, the circuits
minimizes the error.
2. The PD produces repetitive pulses at its output, modulating the VCO frequency
and generating large sidebands.
3. Interpose a low-pass filter between the PD and the VCO to suppress these
pulses
V V V
V
V
Error signal Error signal
Error signal

32. Take Away
• In this webinar, we learnt about negative feedback and its real life
examples.
• The properties of amplifiers’ operation where –ve FB has a role to
play.
• Generally, it is used in a loop configuration for enhance stabilization
and/or reduction of noise (unwanted parameter).
• Understood its role in most common circuits in Analog IC Design like
Biasing circuits, references (BGR), regulators, LDO and PLLs.
• Realize that most jobs in Analog IC design are due to negative
feedback.
Never take negative feedback in life too in a wrong way, it is probably to stabilize
you or reduce the noise within and/or around you.

33. Kindly give your feedback about the
Webinar here.
It is easy to remember.
http://tiny.cc/FB_DrGSJ

34. References
• Lecture Notes, G. S. Javed, King Consultants Education
• Lecture Notes, G. Y. Wei, Div AES, Harvard University
• B. Razavi, Design of Analog CMOS IC, UCLA

35. Contact Me
@ConsultantsKing
http://tiny.cc/KingConsultants
King Consultants Education