PTAT Current Source Symbol
Definition: The PTAT Current Source symbol represents a Proportional To Absolute Temperature current source — an analog circuit block whose output current increases linearly with absolute temperature (in kelvins) — depicted in schematic diagrams as a rectangular functional block labelled 'PTAT' and 'I ∝ T' with VCC, GND, and IOUT pins, used in bandgap references, temperature sensors, and bias circuits.
Also known as: proportional to absolute temperature source, PTAT circuit, PTAT bias, temperature-proportional current source, bandgap PTAT cell.
What the PTAT Current Source symbol means
The PTAT Current Source symbol denotes a functional analog building block that produces an output current directly proportional to the ambient temperature expressed in kelvins (K). Because absolute temperature in kelvins is always positive and increases in a predictable, near-linear fashion with heating, PTAT sources provide a stable, predictable bias current for analog circuits across temperature.
PTAT current sources are foundational to bandgap voltage references, integrated temperature sensors (such as the AD590 and LM135), ADC bias networks, and oscillator frequency-compensation circuits. The symbol communicates to schematic readers that the current delivered at IOUT is a designed function of temperature, not a fixed constant, and that this temperature dependence is intentional and exploited in the surrounding circuit.
How to identify the PTAT Current Source symbol
The PTAT Current Source symbol is drawn as a rectangle with rounded corners containing the text label 'PTAT' in the upper portion and 'I ∝ T' (current proportional to temperature) in the lower portion, separated by a horizontal line. Three pins extend from the block: VCC (power supply positive, on the left upper edge), GND (power supply ground, on the left lower edge), and IOUT (current output, on the right centre edge). This functional-block style follows IEEE 315-1975 rectangular block notation for analog sub-circuits.
Function in a circuit
A PTAT current source exploits the temperature-dependent base-emitter voltage difference between two bipolar transistors operating at different current densities. The difference in Vbe between the two transistors is ΔVBE = (kT/q) × ln(N), where k is Boltzmann's constant, T is absolute temperature in kelvins, q is the electron charge, and N is the emitter area ratio. An op-amp or feedback network forces this ΔVBE across a resistor, producing a current I = ΔVBE / R = (kT/q) × ln(N) / R that is linearly proportional to absolute temperature T. This current is mirrored or buffered to produce IOUT.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617 does not define a dedicated symbol for PTAT current sources. The functional block is represented using the IEC 60617-13 general amplifier/functional block rectangle with appropriate text annotation. |
|---|---|
| ANSI/IEEE 315 | IEEE 315-1975 (ANSI Y32.2) defines general rectangular functional block symbols used to represent PTAT current sources in analog IC and system-level schematics. No dedicated IEEE symbol exists; text annotation ('PTAT') identifies the block function. |
| Key difference | Both IEC and ANSI/IEEE use an identical rectangular block notation for the PTAT current source; neither standard defines a unique dedicated glyph. The block is distinguished solely by its 'PTAT' and 'I ∝ T' text label. |
Terminals / pins
| Pin | Name |
|---|---|
| vcc | VCC |
| gnd | GND |
| iout | IOUT |
Typical values
Output current temperature coefficient: typically +1 µA/K to +10 µA/K depending on design. Example: AD590 produces 1 µA/K (298 µA at 25 °C = 298 K). Supply voltage: typically 2–40 V. Accuracy: ±0.5 °C to ±3 °C for calibrated devices. Operating range: -55 °C to +150 °C for industrial grade.
Where the PTAT Current Source symbol is used
- Bandgap voltage references, where a PTAT current combined with a complementary CTAT (complementary to absolute temperature) current produces a temperature-stable reference voltage
- Integrated temperature sensors (e.g. AD590, LM135/235/335) that produce a current directly readable in kelvins
- Bias current generation in ADCs and op-amps to compensate for temperature-dependent transistor parameters
- Oscillator and VCO frequency compensation to counteract temperature drift of RC or LC timing components
- CMOS bandgap reference cells in microcontrollers and FPGAs for internal voltage reference generation
- Thermal shutdown circuits in power ICs that trigger protection when die temperature exceeds a threshold
Example
In a precision bandgap voltage reference IC schematic, the PTAT block generates a current of 298 nA at 25 °C (298 K) from its IOUT pin, which is summed with an equal CTAT current through a resistor; the combination produces a 1.2 V output (the silicon bandgap voltage) that remains stable to within ±1 mV across a -40 °C to +125 °C operating range.
Key facts
- PTAT stands for Proportional To Absolute Temperature; the output current of a PTAT source increases linearly with temperature in kelvins (K), described by I = (kT/q) × ln(N) / R.
- The PTAT current source symbol is a rectangular functional block labelled 'PTAT' and 'I ∝ T' with three pins: VCC (power supply), GND (ground), and IOUT (current output), following IEEE 315-1975 block notation.
- PTAT sources exploit the difference in base-emitter voltages (ΔVBE) of two bipolar transistors operated at different current densities — a fundamental property of the pn junction described by the Shockley diode equation.
- Combining a PTAT current with a CTAT (Complementary To Absolute Temperature) component produces a temperature-independent bandgap reference, the basis of all precision voltage references since the 1970s.
- The AD590 is a classic two-terminal PTAT current source producing exactly 1 µA/K; at 25 °C (298.15 K) it outputs 298.15 µA, making it directly readable as a temperature in kelvins.
- PTAT current sources are integrated into virtually all modern precision analog ICs — op-amps, ADCs, DACs, and microcontrollers — as part of their internal bandgap reference circuits.
- Temperature coefficient of a PTAT source is positive: current increases approximately 3.3 nA/K per kelvin for a 1 µA/K design, contrasting with CTAT sources whose current decreases with temperature.
Frequently asked questions
What does PTAT mean?
PTAT stands for Proportional To Absolute Temperature. A PTAT current source produces an output current that increases linearly with temperature expressed in kelvins (K), following the relationship I = (kT/q) × ln(N) / R, where k is Boltzmann's constant, T is absolute temperature, q is the electron charge, and N is the transistor emitter-area ratio.
What does the PTAT current source symbol look like?
The PTAT current source symbol is a rectangle labelled 'PTAT' in the upper half and 'I ∝ T' in the lower half, with three pins: VCC and GND on the left side (power supply) and IOUT on the right side (current output). It follows the IEEE 315-1975 rectangular functional-block convention.
What is the difference between a PTAT and a CTAT current source?
A PTAT (Proportional To Absolute Temperature) source produces a current that increases with temperature — its temperature coefficient is positive. A CTAT (Complementary To Absolute Temperature) source produces a current that decreases with temperature — its coefficient is negative. Combining equal PTAT and CTAT components cancels the temperature dependence, producing a stable bandgap voltage reference.
Where is a PTAT current source used in a circuit?
PTAT current sources are used in bandgap voltage references (combined with CTAT to produce temperature-stable 1.2 V), in temperature sensor ICs (AD590, LM135) that output current proportional to temperature, in bias circuits for analog ICs to compensate temperature drift, and in oscillator frequency-compensation networks.
What standard defines the PTAT symbol?
No single international standard defines a dedicated symbol for the PTAT current source. IEEE 315-1975 (ANSI Y32.2) and IEC 60617-13 define the general rectangular functional-block symbol used. The block is identified by its 'PTAT' text annotation rather than a unique graphical element.
What are the pins of a PTAT current source block?
The PTAT current source schematic block has three pins: VCC (positive power supply input), GND (ground / negative supply), and IOUT (the temperature-proportional current output). The IOUT pin provides a current sink or source whose magnitude is proportional to the ambient absolute temperature.
What is the output of a PTAT source at 25 °C?
For a standard 1 µA/K PTAT design (such as the AD590 IC), the output at 25 °C (298.15 K) is approximately 298 µA. A 10 µA/K design would produce approximately 2.98 mA at the same temperature. The exact value depends on the design's chosen temperature coefficient and calibration.
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