Voltage Followers Act as Near-Ideal Voltage Sources

Key Takeaways

• The voltage follower has three ideal characteristics: an infinite input impedance, a zero output impedance, and a gain of one. Practical models can closely approximate this functionality.

• Arguably, the most common voltage follower implementation is that of an op-amp that ties the output pin back to the inverting differential input.

• Voltage followers (and their dual current followers) can also use single transistors (BJTs, FETs) of varying topologies.

A voltage follower uses high input impedance, low output impedance, and a gain of 1 to match the output voltage to the input.

A voltage follower is among the first circuits many students discover when learning op-amp applications. Its simplicity belies its usefulness: the ability to isolate source from load ensures optimal performance at both ends. Additional functionality relating to driving outputs for performance benefits or noise protection further endear voltage follower circuits to designers. While there are many approaches to voltage follower circuits from a component and topology standpoint, all implementations focus on a characteristic low-output, high-input impedance plus unity gain to match the output voltage to that of the input.

Voltage Follower Applications

Voltage Follower Op-Amp Characteristics

A voltage follower comprises the simplest negative feedback op-amp network possible, needing only a connection between the output and negative differential input pins. Unlike most op-amp configurations, a voltage follower’s gain is one and neither amplifies nor attenuates the input signal – the signal simply passes through without alteration. The voltage follower’s name indicates its role as a unity-gain amplifier: the voltage out “follows” the voltage in. The ideal feedback resistance is zero because there is no resistor (or, indeed, any component) associated with the feedback pathway beside the intrinsic parasitics of the materials. It’s possible to show the gain calculation for the voltage follower trivially when combined with the ideal infinite input impedance:

Purely for amplification, a voltage follower may seem redundant if designers could replace the network with the voltage input line. However, op-amps offer more benefits than just amplification. As mentioned, designers treat ideal op-amps as infinite input impedance devices (the practical input impedance is an appreciably high value in the MΩs to TΩs), meaning the current draw on the input is extremely low, and the source signal experiences minimal drawdown. Connecting a low-impedance load directly to the source incurs incredible power losses that can reduce signal parameters, cause inefficient power delivery, and produce excess thermal dissipation that ages board and component materials. 

Along the same lines, the op-amp's output impedance is also crucial. As a reversal of the input impedance, the output impedance is ideally zero (practically nonzero, but extremely minuscule), which is also optimal when calculating power draw: the load sees an impedance approaching zero, allowing maximum current draw. In other words, optimal circuit performance requires a maximum impedance on the input to ensure minimal voltage drop and power dissipation alongside a minimum output impedance for maximum power transfer. For this reason, voltage followers are known as buffer circuits or amplifiers due to their ability to isolate the source and load. Buffer circuits also see use in digital circuits for driving loads above the capabilities of the standard logic family used in manufacturing.

Building Followers From Single Transistor Topologies

While op-amps are commonly associated with voltage followers, single-transistor voltage followers are another possibility. Consider first the common collector/emitter follower BJT topology so named for its near-ubiquitous role as a voltage follower circuit: the common collector uses the base as the input, the emitter as the output, and the collector ties to a net common to both input and output, such as a voltage rail, ground, etc. In this arrangement, the emitter monitors the voltage drop from the input to the output, adjusting the current driven to the emitter resistor to keep the input and output voltage equal (i.e., no potential difference). The output voltage can thus follow the input voltage from values between the minimum base-emitter voltage drop up to the power supply rail. Like the op-amp, the BJT voltage follower possesses a high input impedance and a low output impedance that effectively bridges the feedback connection and drastically reduces power consumption as a load or source. 

A common-drain model, or source follower, is the equivalent FET mode to the common collector/emitter follower. Here, the gate acts as the input, the source is the output, and the drain at some voltage (power supply rail or ground) is common to the other two terminals. Like with the emitter follower, the output voltage at the source controls the current sent to the source resistor to match the input voltage. The common drain possesses the requisite buffer characteristics: a high input impedance, a low output impedance, and near unity gain, making it another excellent option for transforming a poor voltage source into near-ideal functionality. 

The dual of the voltage follower circuit is the current buffer/follower circuit, and true to contrasting form, it utilizes a low input impedance and high output impedance to match the output current to the input current. The high output impedance keeps the current draw from the source optimally low, while the low input impedance maximizes the current delivered to the load. Designers can use alternate single transistor topologies – common gate for BJTs, common base/grounded base for FETs – to incorporate current following as necessary.

Single Transistor Follower Topologies

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A voltage follower may seem like a trivial application on the surface, but in-depth evaluation makes their usefulness readily apparent. Due to their low cost and ease of integration from a routing and packaging standpoint, designers should apply voltage followers liberally between sources and loads (or anywhere the circuit requires isolation) to maximize performance. Cadence’s PCB Design and Analysis Software suite gives design teams a comprehensive simulation environment to measure signal and power integrity. These results can feed into OrCAD PCB Designer, keeping design stages in lock-step with fast and easy-to-use tools.

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