What is a Current Follower Circuit?

A current follower circuit, also known as a current buffer or current amplifier, is an electronic circuit that acts as a current-controlled current source. Its primary function is to provide a high-impedance input and a low-impedance output while maintaining a constant current gain. In other words, the output current follows the input current with a fixed ratio, typically unity (1:1).

The current follower circuit is designed to isolate the input stage from the output stage, preventing loading effects and ensuring that the input signal is not affected by the load connected to the output. This is particularly useful when dealing with low-current signals or when driving loads that require a high current.

Basic Principles of a Current Follower

The basic principle behind a current follower is the use of active devices, such as transistors or operational amplifiers (op-amps), to control the current flow. The input signal is applied to the active device, which in turn regulates the output current based on the input current.

Current Mirror Configuration

One common implementation of a current follower is the current mirror configuration. In this setup, two transistors (usually BJTs or MOSFETs) are connected in a way that the output current is a scaled version of the input current. The scaling factor is determined by the ratio of the transistors’ sizes or the ratio of the resistors used in the circuit.

Here’s a simple current mirror circuit using BJTs:

       +VCC
        |
        |
        +---------+
        |         |
        |     Q1  |
        |    /    |
  Iin --|-- B     |
        |    \    |
        |     E   |
        |         |
        +---------+
        |
        |
        +---------+
        |         |
        |     Q2  |
        |    /    |
        +-- B     |
        |    \    |
        |     E   |--+---- Iout
        |         |  |
        +---------+  |
                     |
                    GND

In this circuit, Q1 and Q2 are matched transistors. The input current Iin flows through Q1, setting up a base-emitter voltage (VBE) that is shared by Q2. As a result, the output current Iout is a scaled version of Iin, with the scaling factor determined by the ratio of the transistors’ emitter areas.

Op-Amp Based Current Follower

Another common implementation of a current follower utilizes an operational amplifier (op-amp) in a negative feedback configuration. The op-amp adjusts its output voltage to maintain the input current equal to the output current.

Here’s a basic op-amp based current follower circuit:

       +VCC
        |
        |
        |    +---+
  Iin --|----| + |
        |    |   |
        |    | - |
        |    +---+
        |     |
        |     |
        |     |
        |     +-----+---- Iout
        |           |
        |          +-+
        |          | |
        |          | | RL
        |          | |
        |          +-+
        |           |
       GND         GND

In this circuit, the op-amp’s non-inverting input (+) is connected to the input current source Iin, while the inverting input (-) is connected to the output. The op-amp adjusts its output voltage to maintain the same current at both inputs, effectively making Iout equal to Iin.

Applications of Current Followers

Current followers find applications in various areas of analog and mixed-signal electronics. Some common use cases include:

1. Current Amplification: Current followers can be used to amplify small currents while maintaining a high input impedance and a low output impedance. This is particularly useful in sensor applications or when dealing with low-current signals.

2. Current Scaling: By adjusting the scaling factor of a current follower, it is possible to scale the input current to a desired level. This can be used for current matching or for interfacing different parts of a circuit that operate at different current levels.

3. Current Summing: Multiple current followers can be connected in parallel to sum their input currents. This is useful in analog computing or in circuits that require the addition of multiple current signals.

4. Load Isolation: Current followers provide a high input impedance, which helps isolate the input stage from the load. This prevents loading effects and ensures that the input signal is not affected by the connected load.

5. Biasing Circuits: Current followers can be used to generate stable and accurate bias currents for other parts of a circuit. By setting a reference current at the input, the current follower can provide a constant current source for biasing purposes.

Advantages and Limitations

Current followers offer several advantages over other current-control techniques:

• High input impedance, which minimizes loading effects on the input signal
• Low output impedance, allowing the circuit to drive loads effectively
• Accurate current scaling and current mirroring
• Simple and compact circuit implementation

However, current followers also have some limitations:

• Limited bandwidth due to the presence of parasitic capacitances and the finite bandwidth of active devices
• Sensitivity to device mismatches and variations in temperature and process parameters
• Potential for noise and offset errors, especially in high-precision applications
• Limited voltage compliance range, as the active devices must remain in the active region for proper operation

Frequently Asked Questions (FAQ)

1. Q: What is the difference between a current follower and a voltage follower?
   A: A current follower is designed to maintain a constant current gain between the input and output, while a voltage follower (also known as a voltage buffer) maintains a constant voltage gain of unity. Current followers deal with current signals, while voltage followers deal with voltage signals.

2. Q: Can a current follower be used as a current amplifier?
   A: Yes, a current follower can be used as a current amplifier by adjusting the scaling factor between the input and output currents. By using appropriate ratios of transistor sizes or resistor values, the output current can be made larger or smaller than the input current.

3. Q: What is the input impedance of an ideal current follower?
   A: An ideal current follower has an infinite input impedance, meaning it draws no current from the input signal source. In practice, the input impedance is finite but still very high, typically in the range of megaohms or gigaohms.

4. Q: How does a current follower differ from a current mirror?
   A: A current mirror is a specific implementation of a current follower that uses matched transistors to scale the input current. The term “current follower” is more general and can refer to any circuit that provides a current-controlled current source, including op-amp based configurations.

5. Q: What are the main limitations of current followers?
   A: The main limitations of current followers include limited bandwidth, sensitivity to device mismatches and variations, potential for noise and offset errors, and a limited voltage compliance range. Careful design and device selection can help mitigate these limitations.

Conclusion

Current follower circuits are essential building blocks in analog and mixed-signal electronics. They provide a means to control and amplify current signals while maintaining a high input impedance and a low output impedance. By using active devices such as transistors or op-amps, current followers can scale, sum, and isolate current signals, making them valuable in a wide range of applications.

Understanding the basic principles and implementations of current followers, such as current mirrors and op-amp based configurations, is crucial for designing and analyzing analog circuits. While current followers have some limitations, their advantages in terms of simplicity, accuracy, and load isolation make them indispensable in many electronic systems.

As with any electronic design, careful consideration of the specific requirements, trade-offs, and limitations is necessary when implementing current followers. By leveraging the properties of current followers and addressing their limitations, designers can create robust and efficient analog circuits for various applications, from sensor interfaces to biasing networks.