Lecture 13 High-Gain Differential Amplifier Design

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Lecture 13High-Gain Differential Amplifier
Design

• Woodward Yang
• School of Engineering and Applied Sciences
• Harvard University
• woody_at_eecs.harvard.edu

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Overview

• Background
• This lecture investigates different topologies
  (and their characteristics) that can be used to
  implement differential amplifiers with extremely
  high gain. We will again be using cascoding.

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Review of Amplifier Characteristics

• Lets review some of the characteristics of the
  different (single-ended) amplifier topologies
  that weve looked at so far.
• We will augment this table when we look at the
  frequency response characteristics of these
  amplifiers

Amplifier Type Rin Rout Av Ai
Common-source/emitter High High High High
Common-gate/base Low High High 1
Common-drain/collector High Low lt 1 High
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Multi-Stage Amplifiers (Cascading)

• We can cascade different types of amplifiers to
  get desired overall characteristics. Often want
• High input impedance
• High gain
• Low output impedance
• Mix and match cascades of different types of
  amplifiers to get desired result

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Common-Emitter Emitter-Follower Cascade

• A common configuration (for discrete BJT
  amplifier design) is a common-emitter
  emitter-follower (common-collector) cascade
• CE stage has high voltage gain and high input
  impedance
• CC stage has low output impedance to drive
  various load conditions
• CC stage also presents a high impedance load to
  the CE amplifier which enables high voltage gain
  for the CE stage

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Common-Source Source-Follower Cascade

• Similarly, cascade a common-source amplifier with
  a source-follower.

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Building Op Amps

• Op amps are an important component of modern CMOS
  ICs. They used to designed as general purpose
  amplifiers that can meet a variety of
  requirements. The main target was extremely high
  gain (gt1e5), high input impedance and low output
  impedance (like an ideal amplifier). This was
  done (to some extent) at the expense of different
  aspects of performance (e.g., speed, output
  voltage range, power, etc.). Designs these days
  are much more tailored to have (good enough)
  performance w.r.t. the specific needs of
  particular applications. Within an IC, often use
  Operational Transconductance Amplifiers (OTA).
• Some performance parameters of op amps
• Gain and Bandwidth
• Want as large as possible
• Output Swing
• Maximize w.r.t. power supply (but supply
  shrinking in modern processes)
• Linearity
• Combat non-linearity with feedback
• Noise and Offset
• Can minimize by trading off other parameters
• Supply Rejection
• Strong dependence on current source output
  resistance

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Simple One-Stage Op Amps

• Two differential pair amplifiers that we have
  already seen can be used as op amps. The
  low-frequency, small-signal gain of both is
  gmN(roNroP). The capacitive loads (CL) usually
  determine their bandwidth.

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Cascoded Amplifier

• Use cascoding to increase load resistance
• Cascode both the active loads and the
  differential pair
• Higher effective load resistance
• Higher ro for the differential pair
• Reduces Miller effect (will see later)
• However, there are some limitations
• Reduced output swing (must keep all devices in
  saturation)
• What is the output dynamic range?
• How might one increase the output swing range for
  vo?

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Use High-Swing Cascodes

• We can use the high-swing cascode circuit as a
  load to achieve higher output range in a
  single-ended output telescopic amp

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Cascode Op Amps

• Amplifiers that use cascoding are often called
  telescopic cascode amps. While gain increases,
  the output range of these devices are limited.
• Connecting in unity-gain feedback configuration
  results in significant reduction of output range

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DC Biasing for High-Gain Amplifiers

• One of the challenges of using cascodes for high
  gain is appropriately setting the DC biasing for
  the circuit. Lets look at an example
• What is the raitio of ILOAD vs. ITAIL?

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DC Biasing Contd

• Strategy for setting up DC bias
• All transistors should be saturation
• Set VBNC so that differential input pair in
  saturation
• Want to set it to the edge with sufficient
  saturation margin (300mV)
• Set VBP so that ILOAD ITAIL/2
• Set VBPC so that pMOS currnet source loads are
  close to edge of saturation
• Need to set VBP and VBPC carefully to keep
  devices in saturation and the DC common mode of
  the output nodes to be in the middle of the
  output swing range
• This can be challenging to do due to the high
  output resistance at the output.
• Would be nice if there was a way to automatically
  set the biasing

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Common-Mode Feedback Biasing

• Use an amplifier to set the pMOS current source
  with respect to some desired output common-mode
  voltage (VREF).

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CM FB Biasing

• Heres how it works
• Use large resistors to find the average
  (common-mode) output voltage
• An amplifier compares VREF to VOUT,CM and sets
  VBP such that VOUT,CM VREF
• Lets understand how it works
• What happens to VBP if VREF increases?
• What happens to VBP if VOUT,CM increases?

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Folded Cascode Circuit

• In order to alleviate some of the drawbacks of
  telescopic op amps (limited output range), a
  folded cascode can be used
• M1 is common-source transconductance amp and M2
  is common-gate transimpedance amp
• Advantage is M2 no longer stacks on top of M1
• Possible for either pMOS or nMOS cascodes
• The output resistance for cascode and folded
  cascode are roughly equivalent (gmro2)

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Folded Cascode Amplifier

• Turn a differential telescopic cascode amplifier
  into a folded cascode amplifier

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Full circuit Implementation of Folded Cascode
Amplifier

• Reference current sources are set
• A version with nMOS differential pair inputs also
  possible (flip upside down)
• What sets output common mode?
• Depends on relative output resistances looking up
  and down
• Can vary with process and reference current
  mismatches

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Gain of a Folded-Cascode Amplifier

• Calculate gain using the differential
  half-circuit. Gain can be calculated as GmRout
  where Gm is the short-circuit transconductance of
  the overall circuit and Rout is the output
  resistance.
• Short out Vout to ground and solve for Iout/Vin
  Gm
• Solve for the output resistance

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Common-Mode Feedback

• Use feedback to set the output common mode of a
  folded cascode amplifier, called common-mode
  feedback
• Sense the average (common-mode) voltage at the
  output, compare to a desired reference voltage
  (Vref), and use it to set the current source
• For Vin0, feedback sets IFBIREF2IREF1/2 and
  common-mode voltage Vref

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Two-Stage Op Amps

• In order to implement amplifiers with high gain
  and high swing, we must resort to two-stage
  amplifier designs
• First stage used to generate high gain
• Second stage to generate high swing
• Use any high-gain first stage and high-swing
  second stage
• two simple examples (differential and
  single-ended output amplifiers)