What is a Current Follower?
A current follower, also known as a current buffer, is an electronic circuit that produces an output current that follows or mirrors an input current. It acts as a current amplifier with a gain of approximately unity, meaning the output current is essentially the same as the input current.
The main purpose of a current follower is to provide a high impedance input and a low impedance output, effectively isolating the input signal from the load. This allows the circuit to drive loads that would otherwise affect the performance of the input source.
Basic Current Follower Circuit
The most basic form of a current follower circuit consists of a single transistor and a resistor. The schematic for a basic NPN transistor-based current follower is shown below:
| Component | Description |
|---|---|
| Q1 | NPN Transistor |
| R1 | Emitter Resistor |
| Iin | Input Current |
| Iout | Output Current |
The input current (Iin) is applied to the base of the transistor (Q1), while the output current (Iout) is taken from the emitter. The emitter resistor (R1) sets the maximum output current and helps stabilize the circuit.
In this configuration, the transistor operates in the active region, and the base-emitter voltage (VBE) remains relatively constant at around 0.7 V for silicon transistors. The output current is given by:
Iout = (Iin – IB) ≈ Iin
Where IB is the small base current required to maintain the transistor in the active region. Since IB is much smaller than Iin, the output current closely follows the input current.
Current Follower Characteristics
Input Impedance
One of the key characteristics of a current follower is its high input impedance. This is because the input signal is applied to the base of the transistor, which draws very little current. The input impedance (Zin) can be approximated by:
Zin ≈ β × re
Where β is the transistor’s current gain (typically 50-200) and re is the intrinsic emitter resistance, given by:
re = VT / IE
Here, VT is the thermal voltage (approximately 26 mV at room temperature), and IE is the emitter current.
Output Impedance
Another important characteristic of a current follower is its low output impedance. This allows the circuit to drive loads without affecting the output current. The output impedance (Zout) is approximately equal to the reciprocal of the transistor’s transconductance (gm):
Zout ≈ 1 / gm
Where gm is given by:
gm = IE / VT
Bandwidth
The bandwidth of a current follower is determined by the transistor’s frequency response and the circuit’s parasitic capacitances. The upper cutoff frequency (fT) is given by:
fT ≈ gm / (2π × (Cπ + Cμ))
Where Cπ is the base-emitter capacitance, and Cμ is the collector-base capacitance.
Current Follower Applications
Current followers find applications in various electronic circuits, including:
- Current sources
- Current mirrors
- Active loads
- Biasing circuits
- Signal conditioning circuits
Current Sources
A current follower can be used as a constant current source by applying a fixed input current. This is useful in biasing circuits, LED drivers, and other applications requiring a stable current.
Current Mirrors
Current mirrors are circuits that replicate a reference current. They are commonly used in analog integrated circuits for biasing, loading, and level-shifting. A simple current mirror can be constructed using two matched transistors, with one acting as the reference and the other as the output.
Active Loads
In analog circuits, current followers can be used as active loads to provide high AC resistance while maintaining a low DC voltage drop. This is particularly useful in amplifier stages, where a high load impedance is desired to achieve high gain.
Biasing Circuits
Current followers are often employed in biasing circuits to establish stable operating points for transistors. By providing a constant current, they help maintain the desired DC operating conditions, even in the presence of temperature variations or device mismatches.
Signal Conditioning Circuits
In signal conditioning applications, current followers can be used to convert voltage signals to current signals or to provide isolation between stages. This is useful in systems where long cable runs or ground loop issues are a concern.
Current Follower Variations
Several variations of the basic current follower circuit exist, each with its own advantages and trade-offs.
Cascode Current Follower
A cascode current follower employs an additional transistor in series with the main transistor to increase the output impedance and improve the circuit’s frequency response. The schematic for a cascode current follower is shown below:
| Component | Description |
|---|---|
| Q1, Q2 | NPN Transistors |
| R1 | Emitter Resistor |
| Iin | Input Current |
| Iout | Output Current |
The cascode transistor (Q2) operates in the common-base configuration, providing a high impedance load for Q1. This reduces the effect of the collector-base capacitance (Cμ) and extends the circuit’s bandwidth.
Wilson Current Mirror
The Wilson current mirror is a modified current follower that improves the output current accuracy and reduces the effect of transistor mismatches. It consists of three transistors and two resistors, as shown in the schematic below:
| Component | Description |
|---|---|
| Q1, Q2, Q3 | NPN Transistors |
| R1, R2 | Emitter Resistors |
| Iref | Reference Current |
| Iout | Output Current |
The Wilson current mirror operates by creating a feedback loop that forces the collector currents of Q1 and Q2 to be equal. This minimizes the impact of base current errors and improves the current mirroring accuracy.
Widlar Current Source
The Widlar current source is a variant of the current follower that allows for the generation of small output currents using a larger reference current. It employs an additional resistor in the emitter path of the output transistor to create a voltage drop, reducing the output current. The schematic for a Widlar current source is shown below:
| Component | Description |
|---|---|
| Q1, Q2 | NPN Transistors |
| R1, R2 | Emitter Resistors |
| Iref | Reference Current |
| Iout | Output Current |
By selecting appropriate values for R1 and R2, the output current can be made much smaller than the reference current. This is useful in low-power applications or when a large current ratio is required.
Current Follower Design Considerations
When designing a current follower circuit, several factors must be considered to ensure optimal performance.
Transistor Selection
The choice of transistor is critical in a current follower design. Key parameters to consider include:
- Current gain (β)
- Transition frequency (fT)
- Collector-emitter saturation voltage (VCE(sat))
- Package type and thermal characteristics
In general, high-β transistors with a high fT are preferred for better current mirroring accuracy and wider bandwidth. Low VCE(sat) is desirable for low voltage drop and improved efficiency.
Resistor Selection
The selection of resistors in a current follower circuit depends on the desired output current and the maximum power dissipation. The emitter resistor value can be calculated using Ohm’s law:
R = V / I
Where V is the desired voltage drop across the resistor, and I is the maximum output current.
The power rating of the resistor should be chosen to handle the maximum expected power dissipation, given by:
P = I^2 × R
Thermal Considerations
Current follower circuits can be affected by temperature variations, primarily due to changes in the transistor’s base-emitter voltage (VBE) and current gain (β). To minimize thermal effects, consider the following:
- Use transistors with matched temperature coefficients
- Employ temperature compensation techniques, such as VBE multipliers or current mirrors with thermal feedback
- Provide adequate heat sinking and ventilation for power transistors
Layout and Parasitics
Proper circuit layout is essential to minimize the impact of parasitic elements, such as stray capacitances and inductances. Some guidelines for optimal layout include:
- Keep trace lengths short, especially for high-frequency signals
- Use ground planes to reduce ground impedance and minimize ground loops
- Separate power and signal grounds to avoid coupling noise
- Use decoupling capacitors close to the transistors to bypass high-frequency noise
Frequently Asked Questions (FAQ)
1. What is the difference between a current follower and a voltage follower?
A current follower is designed to mirror an input current at its output, while a voltage follower (also known as a unity-gain buffer) is designed to reproduce an input voltage at its output. Current followers provide high input impedance and low output impedance, while voltage followers provide high input impedance and low output impedance for voltage signals.
2. Can a current follower be used with PNP transistors?
Yes, a current follower can be implemented using PNP transistors. The circuit topology remains the same, but the polarity of the input current and the power supply voltages must be reversed. In a PNP current follower, the input current is applied to the emitter, and the output current is taken from the collector.
3. How does the transistor’s current gain affect the performance of a current follower?
The transistor’s current gain (β) directly impacts the accuracy of the current mirroring in a current follower. A higher β results in better current matching between the input and output. However, very high β transistors can be more susceptible to thermal runaway and may have a lower transition frequency (fT), limiting the circuit’s bandwidth.
4. What is the purpose of the emitter resistor in a current follower circuit?
The emitter resistor in a current follower circuit serves two main purposes:
- It sets the maximum output current by limiting the voltage drop across the transistor’s base-emitter junction.
- It helps stabilize the circuit by providing negative feedback, which reduces the impact of transistor parameter variations.
5. Can a current follower be used to amplify current signals?
While a current follower has a current gain of approximately unity, it can be used as a building block for current amplifiers. By combining multiple current followers or using them in conjunction with other circuit techniques, such as current mirrors or differential amplifiers, it is possible to create current amplifiers with a wide range of gains and bandwidths.
Conclusion
Current followers are essential building blocks in analog electronic circuits, providing a means to mirror currents, isolate stages, and drive loads. By understanding the basic principles and characteristics of current followers, designers can effectively utilize them in a variety of applications, from biasing circuits to signal conditioning systems.
This article has covered the definition, basic circuit topology, key characteristics, and applications of current followers. It has also explored several variations of the basic circuit, including cascode current followers, Wilson current mirrors, and Widlar current sources.
When designing current follower circuits, careful consideration must be given to transistor selection, resistor sizing, thermal management, and layout techniques. By following best practices and understanding the trade-offs involved, designers can create robust and reliable current follower-based circuits.
As with any engineering discipline, the design of current followers requires a combination of theoretical knowledge and practical experience. Continued learning and experimentation are essential to mastering the art of analog circuit design.
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