In this article we will explore the voltage follower, which is a good example of an op-amp circuit that is simple but very useful.
General purpose operational amplifier (op-amp) applications
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Transimpedance Amplifiers: Operational Amplifier-Based Current-Voltage Signal Converters
Operational amplifiers are versatile, user-friendly components that have been incorporated into a wide variety of circuits and applications. One reason for the popularity of op amps is their ability to combine simplicity and performance: op amp circuits are a valuable addition to many types of systems, but they are not difficult to design and often require few external components.
Operational Amplifier Voltage Followers
The most basic form of a voltage follower, also known as a unity gain buffer, is shown in the figure below.
As you can see, the only component necessary is the op-amp itself (however, you do need a decoupling capacitor for the IC’s power supply)
The voltage follower produces an output signal equal in amplitude to the input signal. Because the input signal is applied to the non-inverting input, no inversion occurs. Therefore, the voltage follower is an irreversible buffer.
The unit gain operation of the voltage follower is through non-positive feedback. The input signal is applied to the non-inverting input of the op-amp and the output is connected directly to the inverting input.
If the op-amp is used as an open-loop amplifier (i.e., without negative feedback), a small increase in the input voltage results in a large increase in the output voltage, since the op-amp has a very high gain.
The negative feedback connection produces a compensating effect: it returns the increased output voltage to the negative part of the differential input stage and, therefore, the output voltage is reduced. The overall effect of negative feedback in a voltage follower is to stabilize the output voltage at a value that is equal to the voltage at the non-inverting input.
When the input signal is relative to the dynamic performance of the op-amp, we do not notice this reconciliation action. We simply observe the same output signal as the input signal. However, when we apply a fast transition to the voltage follower, the stabilizing effect is obvious.
The following figure gives three examples of what this settling behavior might look like.
Reasons to use a voltage follower
Voltage followers do not increase or decrease the amplitude of the input signal, nor do they filter out high frequency noise. Therefore, you may be wondering why such a circuit is so useful. It is true that a voltage follower does not intentionally change the amplitude or frequency characteristics of the input signal, but it does allow us to improve the impedance relationship.
When we send a voltage signal from one subcircuit to another, we must consider the output impedance of the source subcircuit and the input impedance of the load subcircuit.
The output impedance of the source and the input impedance of the load form an a divider so that the voltage transfer depends on the ratio of the input impedance to the output impedance. Effective voltage transfer requires a source circuit with low output impedance and a load circuit with high input impedance.
The voltage follower has low output impedance and very high input impedance, which makes it a simple and effective way to solve the impedance relationship problem. If a high output impedance subcircuit must transmit the signal to a low input impedance subcircuit, a voltage follower placed between these two subcircuits will ensure that the full voltage is delivered to the load.
An example of a simple but important voltage following application is the circuit shown below.
A reference voltage (VREF) can be generated using a resistive voltage divider, but the output impedance of the circuit will not be very low, especially if a higher value resistor is used to reduce current consumption. The voltage follower is not negatively affected by the output impedance of the voltage divider, and it generates a low output impedance reference voltage for the other components in the system.
Voltage follower stability
In general, you can rely on a voltage follower to do what it is named for, that is, to create an output signal that follows the input signal. However, there is a serious failure mode that every circuit designer needs to be aware of. The problem here is that stability voltage followers, like other types of op-amp circuits, are susceptible to oscillations.
Oscillations in negative feedback amplifiers are related to phase shifts, which change negative feedback to positive feedback. You might think that voltage followers would not have stability problems because the circuit is not overall amplified, but in fact, voltage followers are more susceptible to oscillation than circuits with higher gain (for more information on this interesting but somewhat complex topic, see the AAC article Gain Margin and Phase Margin .)
In most cases, all you need to do to prevent oscillations in a voltage follower is to choose an op-amp that is described as “unity gain stable”. These op amps are internally compensated in such a way that they produce stable operation even when the device is used in a voltage follower configuration.
High Current Voltage Followers
The low output impedance of a voltage follower makes it a good circuit for driving current into a low impedance load, but keep in mind that most op amps are not designed to provide large output currents.
You can create a high-current version of a voltage follower using the configuration shown below (see this article for more information)
A voltage follower is a unit gain, irreversible buffer that requires only an operational amplifier (and decoupling capacitor).
Voltage followers have high input impedance and low output impedance which is the essence of their buffering action. They reinforce a signal, thus allowing a high impedance source to drive a low impedance load.
A high current unity gain driver can be created by incorporating an external transistor into the voltage follower configuration.