Table of Contents

Overview
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The Folded Cascode amplifier is an advanced analog circuit topology that improves upon traditional cascode designs, offering better high-frequency performance and voltage headroom.

Problem with Standard Cascode
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Miller Effect Issue:

At high transconductance (\(G_m\)), the Miller Effect causes:

$$ C_{miller} = C_{gd} \times (1 + A_v) $$

This increased capacitance reduces high-frequency gain.

Folded Cascode Solution
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Key Advantages
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CharacteristicStandard CascodeFolded Cascode
Voltage HeadroomLimitedImproved
Output SwingRestrictedExtended
High-Freq PerformanceMiller limitedBetter
ComplexitySimpleMore complex

Circuit Operation
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Structure:

        VDD
         |
    [PMOS Cascode] ← IREF2 (bias)
         |
    +----+----+
    |         |
  [Input]   [Output]
    |         |
    [NMOS Cascode]
         |
        VSS

Design Features
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1. Lower On-Resistance
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Folded structure reduces transistor \(R_{on}\) compared to stacked cascodes.

2. Improved Headroom
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  • Near VDD: PMOS cascode provides margin
  • Near VSS: Extended output swing range

3. High Output Impedance
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$$ R_{out} = g_{m} \cdot r_{o1} \cdot r_{o2} $$

Cascode tail current source (M9-M10) enhances output impedance over single-transistor designs.

4. Power Supply Rejection
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PMOS transistors connected to bias circuit IREF2 improve supply rejection.

5. Negative Feedback
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Connecting Vout to Vin1 implements negative feedback for stability.

Transistor Functions
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TransistorsFunction
M1-M2Input differential pair
M3-M4PMOS current mirror
M5-M6Cascode devices
M7-M8Output stage
M9-M10Cascode tail current source
M11Bias generation

Trade-offs
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AdvantageDisadvantage
Better high-freq responseIncreased complexity
Improved headroomHigher power consumption
Higher gainMore transistors
Better linearityLarger area

Applications
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  • High-speed operational amplifiers
  • Low-voltage analog design
  • ADC/DAC front-ends
  • Sensor interfaces
  • RF circuits