![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]](https://image.slidesharecdn.com/webinarnegativefeedbackinanalogicdesign02apr2020-200412155557/85/Negitive-Feedback-in-Analog-IC-Design-02-April-2020-25-320.jpg)
Negitive Feedback in Analog IC Design 02 April 2020
- 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).
- 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