Current Mirror Symbol
Definition: The Current Mirror symbol represents a two-terminal active circuit block — drawn as a rectangular functional block with a reference current input (I_ref) and a mirrored current output (I_out) — used in analog and mixed-signal schematics to denote a circuit that forces I_out to equal (or be a precise ratio of) I_ref regardless of the load voltage, typically implemented using matched BJT (bipolar junction transistor) or MOSFET transistor pairs, as described in analog IC design practice under IEC 60617 active block conventions and ANSI/IEEE 315.
Also known as: current mirror circuit, BJT current mirror, MOSFET current mirror, Wilson current mirror, cascode current mirror, bias current source, I_ref I_out symbol.
What the Current Mirror symbol means
The current mirror symbol denotes a circuit that 'mirrors' (copies) a reference current to one or more output branches. The reference current (I_ref) is established by an external resistor, a precision current source, or a bandgap reference, and the output current (I_out) is forced to equal I_ref (for a unity-gain mirror) or a multiple of it (for a gain mirror) by the transistor matching inside the mirror circuit. The output current is largely independent of the voltage at the output terminal — the current mirror behaves as a high-impedance current source.
Current mirrors are fundamental building blocks in analog integrated circuits (op-amps, ADCs, DACs, bias circuits) because they allow precise current copying and scaling without using resistors (which are large and poorly matched in IC processes). The simplest implementation is two matched NPN BJTs with their bases and one collector tied together (the reference side) and the second collector as the output. More advanced topologies — Wilson, Widlar, cascode — improve output impedance and reduce the systematic error caused by finite transistor beta (hFE).
How to identify the Current Mirror symbol
As a functional block symbol, the current mirror is drawn as a rectangle labelled 'Current Mirror' with two terminals: I_ref (the reference current input, entering from the left or bottom) and I_out (the mirrored current output, exiting from the right or top). Arrow symbols or current-flow indicators may be drawn on the terminal lines showing current direction. In a detailed transistor-level schematic the current mirror is instead drawn as two matched BJT or MOSFET symbols with their base (or gate) nodes connected together and a diode-connected transistor on the reference side.
Function in a circuit
A current mirror forces I_out to track I_ref by exploiting the voltage-current characteristics of matched transistors. In a simple BJT current mirror, both transistors have identical VBE (base-emitter voltage) because they are fabricated identically and at the same temperature; since IC is exponentially related to VBE by the Ebers-Moll equation, identical VBE produces identical collector currents. The reference transistor is diode-connected (base shorted to collector), so it sets VBE = VBE(ref). The mirror transistor, sharing the same VBE, produces IC_out = IC_ref × (W/L ratio), making the output current independent of the output voltage as long as the transistor remains in its active region (collector voltage > VCE_sat for BJT, or VDS > VGS − Vth for MOSFET).
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-13 (analogue elements) provides the general framework for active functional blocks in schematics. There is no dedicated current mirror symbol in IEC 60617; the circuit is represented either as a functional block rectangle or at the transistor level using standard BJT/MOSFET symbols per IEC 60617-05. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2 / IEEE 315-1975 does not define a specific current mirror symbol; it is drawn as a functional block rectangle following the general convention for active circuits, or at the transistor level using standard BJT/MOSFET symbols per IEEE 315. |
| Key difference | No dedicated current mirror symbol exists in either IEC 60617 or ANSI/IEEE 315; both standards represent it as a labelled functional block or at the transistor level. The functional block representation is identical in both standard frameworks. |
Terminals / pins
| Pin | Name |
|---|---|
| ref | I_ref |
| out | I_out |
Typical values
Output current range: nanoamperes to amperes (depending on technology and transistor size). Output impedance: 10 kΩ–10 MΩ (simple mirror); > 100 MΩ (cascode or regulated cascode mirror). Current accuracy (mismatch): 0.1%–2% for well-matched IC transistors. Compliance voltage (minimum voltage across I_out): VCE_sat ≈ 0.2 V (BJT); VDS_sat ≈ VGS − Vth (MOSFET). Temperature coefficient of output current: typically −0.3% to −0.5%/°C (I_ref set by resistor) or near-zero ppm/°C (I_ref from bandgap). Supply voltage: determined by the transistor type and the application.
Where the Current Mirror symbol is used
- Op-amp bias circuit — the internal bias currents of an operational amplifier (tail currents, collector loads, input stage bias) are generated and distributed using a current mirror network referenced to a single resistor, ensuring all bias currents track each other over temperature.
- Active load in differential pairs — PMOS or PNP current mirrors are used as the active collector/drain load of a differential amplifier input stage (classic op-amp front end), converting the differential current into a single-ended output voltage with very high gain.
- DAC (Digital-to-Analog Converter) current steering — current-mode DACs use an array of binary-weighted current mirrors to sum precise binary fractions of a reference current into an output current proportional to the digital input code.
- LED driver current matching — multiple-output LED driver ICs use current mirrors to guarantee that each output channel sinks exactly the same current, ensuring uniform brightness across LED arrays regardless of forward voltage variations.
- Bandgap reference output buffering — a current mirror copies the bandgap reference current to multiple output branches in a mixed-signal IC, distributing a stable, temperature-independent bias current to ADCs, DACs, PLLs, and oscillators.
- Transimpedance amplifier (TIA) bias — photodiode transimpedance amplifiers use a current mirror to supply a precise bias current to the photodetector, setting the operating point without a resistor that would increase noise.
Example
In an op-amp input stage schematic, a PMOS current mirror (M3/M4) is drawn as the active load of the differential pair (M1/M2). The reference branch — M3 diode-connected to the left input transistor drain — sets VGS for M3. M4 shares the same gate voltage (VGS) and mirrors the current from the right input transistor drain to the output node. Any differential input signal unbalances the drain currents of M1 and M2; the current mirror converts this differential current imbalance into a single-ended voltage at the output, providing the high first-stage voltage gain of the op-amp.
Key facts
- The Current Mirror symbol represents a circuit that forces I_out to equal (or be a fixed ratio of) I_ref, behaving as a high-impedance current source whose output current is set by the reference and is independent of the load voltage.
- The simplest BJT current mirror uses two matched NPN transistors: the reference transistor is diode-connected (base = collector) to set VBE; the mirror transistor shares the same VBE and produces IC_out ≈ IC_ref.
- Pins on this symbol: I_ref (reference current input, x=14 y=40) and I_out (mirrored current output, x=36 y=40).
- The systematic error of a simple BJT current mirror is caused by the base current drawn by both transistors (the hFE error); the output current is actually I_ref × hFE/(hFE + 2), approximately 2/hFE lower than ideal.
- The Wilson current mirror and cascode current mirror topologies achieve significantly higher output impedance (> 100 MΩ vs ~10 kΩ for simple mirror) and lower systematic error, at the cost of requiring one additional transistor and one extra VBE of compliance voltage.
- MOSFET current mirrors use the relationship ID = (k/2)(VGS − Vth)² to mirror current; they are preferred in CMOS ICs because zero gate current eliminates the beta-error problem that affects BJT mirrors.
- A current mirror is not a single component — it is a circuit topology; when drawn as a functional block in a schematic it abstracts two or more matched transistors plus their interconnections into a single symbol for clarity.
- Current mirrors can be scaled to produce I_out = N × I_ref by using N parallel transistors on the output side (for integer multiples) or by adjusting transistor W/L ratios in MOSFET mirrors (for non-integer ratios).
Frequently asked questions
What does the current mirror symbol look like in a schematic?
The current mirror symbol is typically a rectangular block labelled 'Current Mirror' with two terminals: I_ref (reference current input) and I_out (mirrored current output), often with current-direction arrows. In a transistor-level schematic it is drawn as two matched BJT or MOSFET symbols with their base/gate nodes connected and a diode-connection on the reference transistor.
What does a current mirror do in a circuit?
A current mirror copies a reference current (I_ref) to its output (I_out) with high accuracy, regardless of the voltage at the output terminal. It behaves as a current-controlled current source: I_out tracks I_ref (and any changes in I_ref) while presenting a high impedance to the load. This makes it ideal for distributing precise bias currents throughout an IC.
What transistors are used in a current mirror?
Current mirrors are most commonly built with matched BJT transistors (NPN or PNP pairs) or matched MOSFET transistors (NMOS or PMOS pairs). In integrated circuits the transistors are fabricated simultaneously on the same silicon die to achieve very close matching of VBE (BJT) or VGS (MOSFET). MOSFET mirrors are preferred in modern CMOS ICs because they have zero gate current and therefore no beta-error.
What is the beta error in a BJT current mirror?
The beta error (or hFE error) in a simple BJT current mirror arises because the reference transistor's base current is drawn from the reference terminal, reducing the current that reaches the collector by a factor of 2/hFE. The output current is approximately I_ref × hFE/(hFE + 2). For hFE = 100, the error is about 2%. Wilson and cascode mirror topologies reduce this error by more than an order of magnitude.
What is the difference between a simple and a Wilson current mirror?
A simple two-transistor current mirror has relatively low output impedance (~VT/IC, typically 10–50 kΩ) and a 2/hFE systematic error. The Wilson current mirror (three transistors) uses negative feedback to cancel the beta error, reducing systematic error to approximately 2/hFE², and achieves output impedance approximately hFE/2 times higher than the simple mirror — making the output current much less sensitive to load voltage variations.
Where are current mirrors used in analog ICs?
Current mirrors are used throughout analog ICs: as active loads in op-amp differential input stages (maximising voltage gain), as bias current distribution networks (supplying matched currents to multiple circuit blocks), in current-mode DACs (generating precise binary-weighted output currents), and in LED driver ICs (ensuring matched current to each LED channel for uniform brightness).
What standard defines the current mirror symbol?
No dedicated current mirror symbol exists in IEC 60617 or ANSI Y32.2 / IEEE 315. Both standards use a general rectangular functional block convention for active circuit blocks not individually defined. At the transistor level, IEC 60617-05 and ANSI/IEEE 315-1975 define the BJT and MOSFET symbols used to draw the current mirror's internal transistors.
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