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The Flipped Voltage Follower is a low voltage voltage follower that has many applications. Much of this is taken from Flipped Voltage Follower A Useful Cell.

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  1. 1. Basic operation
  2. 2. Flipped Voltage Follower Current Sensor (FVFCS)
  3. 3. Differential Flipped Voltage Follower (DFVF)
  4. 4. Flipped Voltage Follower Differenetial Pair (FVFDP)

Attach:fvf.png | Basic FVF

§1.  Basic operation

The basic FVF cell is a modification of the source follower. It has the following properties:

  • The current through M1 is constant.
    • This give a more precise copy of the voltage than a traditional source follower can provide.
    • Or it can be said that the voltage gain is truly 1
  • It can operate at very low voltages (Vdd = 1.5V is a common for these circuits)
  • It is able to source much more current than a simple source follower
  • It provides a very low impedance output node. r_{out} = 1/(g_{m1} g_{m2} r_{o1})
  • In all analysis done here, body effect and channel length modulation are neglected
  • Applications
    • Current conveyor
    • Multiplier
    • In a translinear loop for multiplicative arithmetic. Fundamental operation: \sqrt{ I_1 I_2}

§2.  Flipped Voltage Follower Current Sensor (FVFCS)

Attach:fvfcs.png | FVFCS The FVF can provide a current sensing cell.

  • The input node has very low impedance, thus the voltage at the input node is nearly constant.
  • Large currents can be sourced at the input node
  • This configuration has 2 possible outputs
    • With all transistors biased in saturation the output is a linear copy of the input current with Ib added.
      • The added Ib can easily be removed in the next stage
    • If the sensing transistor (the diode connected one) is biased near the linear region a current gain can be achieved
      • In this configuration the FVFCS acts as a class AB amplifier, because Ib is always present, but the peak output current is much higher than that.
  • Minimum supply voltage: V_{ddmin} = V_T + 2 V_{DS}
  • Applications
    • Current mirror with very low input impedance
      • "basic implementation of the FVFCS, has the lowest input resistance as well as the lowest input voltage requirements reported to date"
      • Input impedance: r_i = \frac1{g_{m1} g_{m2} r_{o1}} ~ 20-100Ω
      • The low input voltage means it can still mirror for voltages within VDSsat from the rail
      • A cascode is easily added to the output stage to increase output impedance
    • Current sensor - can sink large currents with a nearly constant input voltage

§3.  Differential Flipped Voltage Follower (DFVF)

Attach:dfvf.png | DFVF This structure is a non-linear Class-AB amplifier, because the quiescent point is Ib and not zero, but the maximum output current is much larger than the quiescent current.

  • For large signals this structure is non-linear, however it can be treated as linear for small signal-analysis
  • When V+=V- then Iout = Ib
    • As V_+ - V_- = V_{id} > 0 the output grows by the square law
    • The node shared between the three transistors has a nearly constant voltage
  • Output may also be taken as a voltage from node shared between the current source and the positive input transistor
  • Minimum supply voltage: V_{ddmin} = V_T + 2 V_{DS}

§4.  Flipped Voltage Follower Differenetial Pair (FVFDP)

The final structure is a full differential pair.

  • Once again this is a Class-AB structure.
  • Vcm should be set to the common mode value of V_{id}=V_+ - V_-.
    • The paper Flipped Voltage Follower A Useful Cell provides some useful circuits for doing this.
    • If Vcm is not set to the input common mode, the two output currents become skewed from each other.

Attach:fvfdp.png | FVFDP

  • Minimum supply voltage: V_{ddmin} = V_T + 2 V_{DS}
  • Applications
    • As input and output stage on many opamps, OTA's, etc.
    • Multiplier

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Last edited by DrLock. Originally by DrLock.