Differential Amplifier Symbol
Definition: The Differential Amplifier symbol represents an analogue circuit block that amplifies the voltage difference between its non-inverting input (IN+) and inverting input (IN−) while rejecting voltages that are common to both inputs, characterised by its differential gain A_d and Common-Mode Rejection Ratio (CMRR), and represented in schematics as a triangle block with two inputs and one output per IEC 60617-05 and IEEE 315.
Also known as: diff amp, difference amplifier, subtractor amplifier, long-tailed pair, instrumentation amplifier input stage.
What the Differential Amplifier symbol means
The Differential Amplifier symbol denotes a circuit or functional block whose output voltage is proportional to the difference between its two input voltages: V_out = A_d × (V_IN+ − V_IN−). The amplifier rejects common-mode signals—voltages present equally on both inputs—which is quantified by the Common-Mode Rejection Ratio (CMRR, typically 60–120 dB for precision op-amp based designs). This property makes differential amplifiers essential for measuring small signals in the presence of noise, ground loops, or electromagnetic interference.
In circuit diagrams the differential amplifier block appears in sensor signal conditioning chains (strain gauges, thermocouples, ECG electrodes), motor current sensing, and long-distance differential signal transmission (RS-422, RS-485). The symbol's three pins (IN+, IN−, OUT) cleanly separate the signal subtraction function from any common-mode noise on the signal lines.
How to identify the Differential Amplifier symbol
The Differential Amplifier symbol is either a triangle (following the op-amp convention) with two inputs on the left side—IN+ (non-inverting, upper) and IN− (inverting, lower, often marked with a minus sign or bubble)—and one output on the right apex, or a rectangular functional block labelled 'DIFF AMP' or '−' with the same three-pin arrangement. The '+' label on the non-inverting input and '−' on the inverting input are the key identifying marks. Power supply pins (V+ and V−) may be shown or omitted depending on schematic detail level.
Function in a circuit
An ideal differential amplifier produces V_out = A_d × (V+ − V−), where A_d is the differential voltage gain. A practical differential amplifier built from four resistors and one op-amp has a differential gain of R2/R1 (with matched resistor pairs). Key performance parameters are CMRR (common-mode rejection ratio), input impedance, gain accuracy, and offset voltage. An instrumentation amplifier (INA) is a precision differential amplifier with very high input impedance and precise gain set by a single resistor.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-05 represents amplifiers as triangles or labelled rectangular blocks. A differential amplifier is drawn as a triangle with '+' and '−' inputs, equivalent to the standard op-amp symbol but with emphasis on the subtraction function. |
|---|---|
| ANSI/IEEE 315 | IEEE 315 / ANSI Y32.2 uses the same triangle-with-two-inputs symbol for operational and differential amplifiers. The non-inverting (+) input is typically shown at the upper terminal and the inverting (−) at the lower. |
| Key difference | IEC 60617-05 and IEEE 315 use essentially identical triangle symbols for the differential amplifier. The primary practical difference in schematics is whether the symbol is drawn as a generic triangle (op-amp style) or as a labelled rectangular block (functional-block style used in block diagrams). |
Terminals / pins
| Pin | Name |
|---|---|
| in_pos | IN+ |
| in_neg | IN- |
| out | OUT |
Typical values
Differential gain A_d: 1–1000 V/V (set by feedback resistors). CMRR: 60 dB (discrete transistor pair) to 120 dB (precision INA). Input impedance: 10 kΩ–1 GΩ (INA type). Offset voltage V_os: 1 µV–5 mV. Common ICs: INA128, INA826, AD8221 (instrumentation amplifiers); LM324, TL071 (op-amp based difference amp).
Where the Differential Amplifier symbol is used
- Strain gauge and load cell signal conditioning bridges where the differential signal is a few millivolts riding on a large common-mode supply voltage.
- ECG and biomedical amplifiers measuring microvolt-level biopotentials in the presence of 50/60 Hz power-line interference.
- Motor phase current sensing using a shunt resistor in the motor power path, where the shunt voltage rides on a high common-mode bus voltage.
- RS-422/RS-485 line receiver circuits extracting differential data signals from a twisted pair in the presence of common-mode noise.
- Thermocouple amplifiers sensing small thermoelectric voltages riding on ambient temperature baseline.
- Battery cell voltage monitoring in series battery stacks where each cell's voltage is measured differentially across a high common-mode voltage.
Example
In a Wheatstone bridge strain-gauge circuit, the bridge output is a differential voltage of 1–10 mV representing mechanical strain, riding on a common-mode voltage of 2.5 V (half the excitation voltage). An INA128 instrumentation amplifier (differential amplifier symbol with IN+, IN−, OUT, and a gain resistor R_G) amplifies the differential signal by 100× (gain set by R_G = 499 Ω) to 100 mV–1 V, while rejecting the 2.5 V common-mode to within the ADC's input range.
Key facts
- A Differential Amplifier amplifies the voltage difference V_IN+ − V_IN− by a gain factor A_d and rejects voltages common to both inputs; its output is V_out = A_d × (V_IN+ − V_IN−).
- The three circuit pins are IN+ (non-inverting input), IN− (inverting input), and OUT (differential output); power supply pins V+ and V− are usually present but often omitted from functional diagrams.
- Common-Mode Rejection Ratio (CMRR) quantifies the amplifier's ability to reject common-mode noise: CMRR = 20 × log10(A_d / A_cm) in dB; typical values are 80–120 dB for precision instrumentation amplifiers.
- IEC 60617-05 and IEEE 315 / ANSI Y32.2 both represent the differential amplifier as a triangle with '+' and '−' labelled inputs, or as a labelled rectangular block in system-level diagrams.
- An instrumentation amplifier (INA) is a precision differential amplifier with high input impedance and gain set by a single external resistor; common ICs include the INA128, AD8221, and INA826.
- A simple difference amplifier built with one op-amp and four matched resistors (R1, R2, R3, R4 with R1=R3, R2=R4) produces gain A_d = R2/R1 and finite input impedance R1.
- Differential amplifiers are fundamental to RS-422 and RS-485 serial communication receivers, where the receiver must extract data from a ±0.2 V minimum differential signal in the presence of common-mode voltages ranging from −7 V to +12 V.
Frequently asked questions
What does the differential amplifier symbol look like?
The differential amplifier symbol is a triangle with two inputs on the left—IN+ (non-inverting, upper, marked '+') and IN− (inverting, lower, marked '−')—and one output at the right apex. Alternatively it is drawn as a labelled rectangle inscribed 'DIFF AMP' with the same three-pin arrangement. Power supply pins (V+, V−) may be shown on the top and bottom edges.
What does a differential amplifier do?
A differential amplifier amplifies the voltage difference between its two input terminals (V_IN+ − V_IN−) by a differential gain A_d, while rejecting voltages that appear equally on both inputs (common-mode voltages). This makes it ideal for measuring small signals in electrically noisy environments.
What is CMRR and why does it matter?
CMRR (Common-Mode Rejection Ratio) is the ratio of differential gain to common-mode gain, expressed in dB: CMRR = 20 × log10(A_d / A_cm). A CMRR of 80 dB means the amplifier amplifies differential signals 10,000 times more than common-mode signals, allowing a 1 mV differential signal to be measured accurately in the presence of several volts of common-mode noise.
What is the difference between a differential amplifier and an instrumentation amplifier?
A basic difference amplifier uses one op-amp and four resistors, giving moderate input impedance (limited by the input resistors). An instrumentation amplifier (INA) uses three op-amps and provides very high input impedance, precisely settable gain (via one external resistor), and much higher CMRR. The INA is preferred for precision sensor applications such as strain gauge and biomedical measurements.
What are the pins of the differential amplifier block?
The three primary pins are IN+ (non-inverting input), IN− (inverting input), and OUT (the amplified difference output). Full circuit implementations also have V+ and V− supply pins. On an instrumentation amplifier IC, additional pins include the gain-setting resistor terminals and a REF (output reference voltage) pin.
What standard defines the differential amplifier symbol?
The differential amplifier is represented per IEC 60617-05 (semiconductor and analogue circuit symbols) and IEEE 315 / ANSI Y32.2. Both use a triangle with '+' and '−' labelled inputs for the op-amp or differential amplifier block; functional block diagrams use a labelled rectangle.
How is a simple difference amplifier built with an op-amp?
A standard difference amplifier uses four resistors and one op-amp: R1 in series with IN−, R2 from that junction to the op-amp output (feedback), R3 in series with IN+, and R4 from that junction to ground. With R1=R3 and R2=R4, the gain is A_d = R2/R1 and the circuit rejects common-mode signals. Resistor mismatch degrades CMRR, so precision 0.1% tolerance resistors are used in high-accuracy applications.
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