# Current Mirrors: A 1:1 Current Source

### Key Takeaways

• Current mirrors use various transistor network configurations to deliver an output current with equal magnitude and opposite direction to the input.

• Two fundamental modes of current mirror operation exist–a current sink and a current source.

• Current mirrors must prioritize gain, output resistance, and voltage for optimal output current characteristics.

Current mirrors reproduce input current magnitude with reverse polarity.

Current mirrors are load-independent circuits (both on the input and output) used for accurate current biasing and to simulate ideal references. It achieves this loading condition with a theoretically infinite resistance on the output (so loading cannot draw down the current) and a perfect conductance on the input such that the mirror doesn’t cause any loading and draw down the current. Practical current mirrors must deal with a high yet finite output resistance, near-perfect conductance, and frequency limitations arising from parasitic reactance. These factors contribute to a percent error loss for total current reflection; the goal of the circuit designer is to weigh the different topologies and balancing methods to minimize these effects.

Common Current Mirror Variations

 Buffered Wilson Widlar Uses an extra bipolar junction transistor (BJT) between the input and output to reduce gain error. It has the side effect of making the current mirror’s frequency response poorer. An extra BJT (or two) creates a gain closer to an ideal current sink/source. Ensures output current is very near input current and possesses a high output impedance. Eliminates a degeneration resistor to generate small currents with moderate resistances.

## The Basics of Current Mirrors: Design and Motivation

Consider a current mirror as an amplifier with a gain of -1; the magnitude conserves while the direction flips. Designers can construct a current mirror from two transistors of equal parameters (values and operating conditions) and the same base-collector/gate-drain junction voltage (for BJTs and MOSFETs, respectively) and obtain the same collector/drain current on both transistors. Fundamentally, there are two current mirror cases to consider:

• A current source on the input of the current mirror produces a current sink; input and output current flow into the device.

• A current sink on the input of the current mirror creates a current source; input and output current flow out of the device.

One of the biggest impediments to a current mirror is the inherent nature of the common transistor packages. BJTs are current-controlled current sources, but β (the proportionality between the base current and collector current in forward-active mode) varies with temperature and between devices. MOSFETs, on the other hand, are voltage-controlled current sources. Despite the lack of a discrete, readily implementable current amplifier, both BJTs and MOSFETs perform admirably in the role, as feedback and the current input and output support a voltage conversion before and after amplification.

Achieving this conversion is simpler than expected: design only has to diode-connect the BJT or MOSFET by shorting the base-collector or gate-drain junction, respectively, while grounding the emitter/source pin. With a resistor on the input voltage to the collector or drain, the current out of the input will be some rail or signal voltage less the junction voltage divided by the resistance. Perhaps unsurprisingly for a circuit with mirror in its name, the output greatly resembles a flipped input: the emitter/source connects to ground, the base-collector/gate-drain voltage connects to the base or source of the second transistor, and the output is the current out of the collector or drain.

## Defining Current Mirror Characteristics and Advanced Models

The basic current mirror topology described is not without faults. It’s easiest to start with the defining mirror characteristics to understand the limitations of this general model and variants:

• Gain - For BJTs, the base current subtracts from the output total twice (once for each transistor), making the BJT current mirror fall short of the ideal -1 gain goal. As voltage-controlled devices, MOSFETs do not suffer from this issue; it is possible to compensate for this offset using a BJT design.

• Output resistance - Output resistance must approximate infinity relative to the output voltage to keep the collector-base/gate-drain voltage at zero for an independent output voltage and resistance. This condition strengthens the circuit against temperature fluctuations and circuit perturbations. Engineers can introduce degeneration (negative feedback) by adding an equivalent resistor to each emitter or source. For BJTs, designers must compensate for the voltage drop with an additional resistor between the bases.

• Output voltage - The voltage on the base/gate of the second transistor must remain zero to prevent forward biasing. The minimum voltage to achieve this state is known as the compliance voltage.

Some additional considerations are for MOSFET current mirrors. The drain voltage is a function of the gate-source and drain-gate junction voltage. However, the network analysis is reducible by recognizing that the drain-gate voltage must be zero for current mirror operations. Solving for the gate-source voltage shows an identical relationship between it and the input current; the drain-gate voltage on the output side and the output current are much the same. The basic current mirror is complete if the MOSFET characteristics match (channel length, width, etc.). Note that output resistance and compliance voltage equations will change between the two transistor types, but the general design tips are similar.

More advanced current mirror options exist that improve the performance or characteristics of the simple current mirrors:

• Buffered feedback - A third BJT between the input and output transistors significantly reduces gain error by tying its base to the collector of the input BJT and the emitter to the input and output base-base connection. However, the frequency response is poorer.

• Wilson - A third BJT combines with the basic current mirror; the base of the output transistor and collector of the input transistor tie together, and the output’s emitter no longer connects to ground (the input and “middle” transistor still share this trait). The middle transistor’s collector ties to the output transistor’s emitter, while the base of the input and middle transistors also connect here.

• Full Wilson - A fourth BJT adds to the Wilson model on the input side, as described above.

• Widlar - The Widlar current source eliminates the degeneration resistor on the input side; the effect moderates the circuit's resistor values.

## Simulate for a Smooth Production With Cadence

Current mirrors have many implementations, but the basic functionality remains the same. By carefully balancing input and output circuit relationships, engineers can create an output current that is suitably insulated from the voltage and possesses the necessary gain. Proper circuit modeling ensures these circuit outputs act as a reliable current source. Cadence’s PCB Design and Analysis Software suite gives electronic design teams extensive simulation to characterize any circuit imaginable before entering production. Alongside the user-friendly and powerful OrCAD PCB Designer, ECAD is faster than ever.

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