Voltage Follower

A voltage follower outputs exactly what it sees at its input, but with near-zero output impedance and near-infinite input impedance. ADC driver stages and sensor buffer circuits use it to stop a high-impedance source from being loaded down by a low-impedance measurement circuit.

Core Concept

The voltage follower is the non-inverting amplifier taken to its limit. The output pin connects directly to the inverting input, with no resistors in the feedback path. This makes the feedback factor β = 1, so the closed-loop gain Acl = 1 + Rf/R1 becomes exactly 1. The output voltage tracks the input with no amplification and no phase shift.

The value is in impedance transformation. A piezoelectric sensor or pH electrode might have a source impedance of 10 MΩ. Connecting it directly to a 1 kΩ load would drop most of the voltage across the source resistance. A TL071 voltage follower presents its 10^12 Ω JFET input impedance to the sensor and drives the load from its near-zero output impedance.

The slew rate of the op-amp limits how fast the output can follow a fast input. For TL071 with SR = 13 V/µs, a 10 V step will take a minimum of 10/13 µs = 0.77 µs to settle. For large-signal, high-frequency following, choose an op-amp like the LM318 with SR = 70 V/µs.

Key Equations

Closed-loop gain: Acl = 1. Vout = Vin.

Input resistance (ideal): Rin = infinity. For TL071 (JFET input): ~10^12 Ω.

Output resistance (closed loop): Rout_cl = Rout_ol / (1 + A*β). With open-loop Rout = 75 Ω and Aβ >> 1, closed-loop Rout is a fraction of an ohm.

Bandwidth: BW = GBP / 1 = GBP. For TL071, BW = 3 MHz at unity gain. For LM741, BW = 1 MHz.

Settling time for a step input is dominated by slew rate: t_slew = ΔV / SR

Given:
  Op-amp: TL071
  SR = 13 V/µs = 13 × 10^6 V/s
  Input step: Vin jumps from 0 V to 8 V instantaneously
  Find: minimum time for output to follow the step

Why this formula:
  Slew rate limits the rate of change of output voltage.
  The output cannot change faster than SR volts per second.

Formula:
  t_slew = ΔV / SR

Substitution:
  ΔV = 8 - 0 = 8 V
  SR = 13 × 10^6 V/s
  t_slew = 8 / (13 × 10^6)

Calculation:
  t_slew = 8 / 13,000,000
  t_slew = 6.15 × 10^-7 s

Final Answer:
  Minimum slewing time = 0.615 µs
  The output follows the 8 V step in about 0.62 µs.

Exam Tip: GATE asks about bandwidth of a voltage follower. Because Acl = 1, bandwidth equals the GBP directly. Many students compute GBP/Acl and substitute Acl = 0 or Acl = 2 by mistake. Also, the voltage follower has the widest bandwidth of any closed-loop configuration using the same op-amp. Slew rate and bandwidth are different limits: SR governs large-signal response, GBP governs small-signal bandwidth.

Key Properties

• Gain is exactly 1 (0 dB); output equals input in both magnitude and phase.
• Input impedance is the op-amp's own input impedance; for TL071 this is approximately 10^12 Ω, making it ideal for high-impedance sensors.
• Closed-loop output impedance drops to a fraction of an ohm, allowing the follower to drive low-impedance loads directly.
• Bandwidth equals the op-amp's GBP; TL071 gives 3 MHz at unity gain, LM741 gives 1 MHz.
• Slew rate still limits large-signal performance; TL071 at 13 V/µs outperforms LM741 at 0.5 V/µs for fast signals.
• No external resistors are needed; this reduces component count and eliminates resistor noise from the signal path.

Quick Revision

• Acl = 1; Vout = Vin.
• Output connects directly to inverting input; no resistors needed.
• BW = GBP (widest bandwidth possible for that op-amp).
• Rin is very high; Rout is very low after feedback.
• Large signal settling limited by SR = ΔV/t.
• Used as impedance buffer between high-impedance source and low-impedance load.
• TL071 preferred over LM741 for sensor buffering due to JFET inputs and higher SR.
• Exam trap: students say the voltage follower has infinite bandwidth because gain is 1. It does not. Its -3 dB point is the GBP value, which is finite, and SR separately limits large-signal response.