Voltage Follower: Working and Use

A voltage follower is a versatile circuit that plays a crucial role in various analog electronics applications by offering high input impedance, low output impedance, and improved signal integrity.

What Is A Voltage Follower

A voltage follower, also known as a unity-gain buffer or non-inverting buffer amplifier, is an electronic circuit that provides a high-impedance input and a low-impedance output. 

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• High Input Impedance

 The voltage follower acts like a high resistance for the input signal. This means it draws very little current from the source providing the signal. This is important because some signal sources, like sensors, can be easily affected if too much current is drawn from them.

• Low Output Impedance

The voltage follower acts like a very low resistance for the output signal. This means it can provide a strong signal that can drive other circuits without being weakened.

Benefits of using a Voltage Follower

• Isolation: The high input impedance helps isolate the source of the signal from the rest of the circuit. This can be important to prevent unwanted interactions between components.

• Buffering: The low output impedance allows the voltage follower to drive multiple circuits without the output signal weakening. This makes it useful for distributing a signal to several destinations.

• Signal Integrity: By minimizing current draw from the source and providing a strong output, the voltage follower helps maintain the integrity of the signal.

The Purposes of A Voltage Follower

• Protecting Signal Sources

Some signal sources, especially sensors, can be easily affected if too much current is drawn from them. The high input impedance of the voltage follower prevents this by acting like a high resistance, minimizing current draw and protecting the source.

• Driving Multiple Loads

The low output impedance allows the voltage follower to provide a strong and clear signal to multiple circuits (loads) without the signal weakening. This is useful for distributing a signal to various destinations within a larger circuit.

• Signal Integrity

By minimizing current draw from the source and providing a strong output signal, the voltage follower helps maintain the integrity of the original signal. This is important for ensuring accurate signal processing in various applications.

How Does A Voltage Follower Work

A voltage follower achieves its high input impedance and low output impedance through the magic of negative feedback within an operational amplifier (op-amp).

Components:

• Operational Amplifier (Op-Amp): The heart of the voltage follower circuit. It's a high-gain differential amplifier that amplifies the difference between its two input voltages.

• Resistor (Optional): In some voltage follower configurations, a resistor might be used to set the output gain slightly below 1 (unity gain).

Working Principle

1. Input Signal:  An input voltage signal is applied to the non-inverting input terminal (positive input) of the op-amp.

2. Negative Feedback: The inverting input terminal (negative input) of the op-amp is connected directly to the output of the voltage follower circuit. This creates a negative feedback loop.

3. Op-Amp's Response:  Since the inverting and non-inverting inputs are initially at different voltages (due to the input signal), the op-amp will try to amplify the difference (negative feedback) to drive its two inputs towards the same voltage.

4. Output Adjustment:  The op-amp adjusts its output voltage to match the voltage at the non-inverting input (the input signal).  Due to the very high gain of the op-amp, even a small difference between the inputs will cause a large change in the output. This large change is then fed back to the inverting input, rapidly bringing it closer to the voltage at the non-inverting input.

5. Equilibrium Reached:  This process continues until the voltage at the inverting input (output voltage) becomes virtually identical to the voltage at the non-inverting input (input signal). At this point, the negative feedback loop stabilizes, and the output voltage remains almost exactly the same as the input voltage.

Why Does An Op Amp Have A High Input Impedance and A Low Output Impedance

Operational amplifiers (op-amps) are designed to have a very high input impedance and a very low output impedance for two main reasons:

1. Minimizing Impact on the Input Signal

High Input Impedance: An op-amp acts like a very high resistance when connected to the input signal source. This means it draws minimal current from the source. This is crucial because some signal sources, especially sensors, can be easily affected if too much current is drawn from them.

• Imagine the input signal source is a delicate water fountain. A high input impedance is like a tiny cup collecting water – it takes a minimal amount without affecting the overall flow.

2. Maximizing Signal Strength at the Output

Low Output Impedance: An op-amp acts like a very low resistance when delivering the amplified signal to the output. This allows the op-amp to drive various loads (other circuits) effectively without weakening the signal strength.

• Think of the output as a water pump. A low output impedance is like a powerful pump that can push water (signal) with high pressure (strength) to different destinations (loads).

How Does The Op-Amp Achieve This

Op-amps achieve these characteristics through their internal design and the principle of negative feedback:

1. Differential Input Stage: 

Most op-amps have a differential input stage that compares the voltage at the positive input (non-inverting) with the voltage at the negative input (inverting).

2. High Gain: 

Op-amps are designed with very high open-loop gain. This means even a small difference between the two input voltages gets amplified significantly by the op-amp.

3. Negative Feedback Loop: 

In many op-amp configurations, the output voltage is fed back to the inverting input terminal. This creates a negative feedback loop.

FAQs

Why would we need a voltage buffer?
A voltage buffer amplifier is employed to move a voltage between two circuits: one with a low input impedance level and the other with a high output impedance level.
To what extent does the voltage follower give voltage gain?
A voltage follower circuit has a gain of one (unity) and does not magnify the incoming signal since the output value is equal to the input voltage.
What is a flipped voltage follower?
A popular improved buffer cell for low-power and/or low-voltage operation is the "flipped voltage follower (FVF)". 

Conclusion

In conclusion, the ability of voltage followers to provide high input impedance and low output impedance makes them valuable for impedance matching, signal buffering, active filter design, and various other applications where signal integrity and efficient signal transmission are crucial.