Applications of Op-Amps in Electronics | ECExplain

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Concept overview

An operational amplifier is a high-gain differential voltage amplifier with very high input impedance and very low output impedance. Its open-loop gain is typically 100 dB or more, making it unsuitable for direct use without feedback. When negative feedback is applied through external resistors or capacitors, the closed-loop gain becomes predictable and the circuit behaviour is set almost entirely by the passive components. The LM741, TL071, LM358, and Texas Instruments OPA2134 are common general-purpose and audio op-amp ICs.

Real-world applications

INA128 Instrumentation Amplifier for ECG Front End

Biomedical Electronics / Healthcare

ECG machines and patient monitors use instrumentation amplifiers such as the Texas Instruments INA128 to amplify the millivolt-level differential signal between body surface electrodes while rejecting the large common-mode 50 Hz or 60 Hz interference from mains wiring. The INA128 achieves a CMRR of 120 dB. A single external resistor RG sets the gain from 1 to 10000. The amplified signal is then passed to a notch filter and ADC for digital processing.

Active Anti-Aliasing Filter before ADC

Industrial Instrumentation / DAQ

Data acquisition systems using high-speed ADCs such as the ADS8688 from Texas Instruments place an active Sallen-Key low-pass filter built around op-amps such as the OPA2134 before each ADC input. The filter attenuates signal components above half the sampling frequency to prevent aliasing. A second-order Butterworth response is commonly chosen because it provides flat passband gain with a monotonically decreasing stopband.

Current-to-Voltage Converter in Photodiode Receivers

Optical Communications / Consumer Electronics

In optical fibre receivers and barcode scanners, a photodiode generates a current proportional to incident light intensity. A transimpedance amplifier built around an op-amp such as the OPA657 converts this current to a voltage: Vout = -Iph * Rf. The feedback resistor Rf sets the transimpedance gain. A small feedback capacitor Cf stabilises the circuit by limiting bandwidth and preventing oscillation caused by the photodiode capacitance.

How it works in practice

In an inverting amplifier configuration, the non-inverting input is tied to ground, and the input signal is applied through resistor R1 to the inverting input. Feedback resistor Rf connects from output back to the inverting input. Because the op-amp drives its output to maintain zero differential voltage between its inputs, the inverting input is a virtual ground. The output voltage is Vout = -(Rf/R1) * Vin. The gain is set solely by the resistor ratio, independent of the open-loop gain, as long as the open-loop gain is much larger than the closed-loop gain. For a summing amplifier, multiple input resistors are connected to the inverting input. For a difference amplifier, resistors are placed on both inputs with matched ratios. For an integrator, Rf is replaced by a capacitor C: Vout = -(1/R1C) integral(Vin dt). The bandwidth of the closed-loop amplifier is limited by the gain-bandwidth product (GBW) of the op-amp: if an OPA2134 has a GBW of 8 MHz and the closed-loop gain is 100, the usable bandwidth is 80 kHz.

Examples

Wheatstone Bridge Amplifier for Strain Gauge

A strain gauge on a structural beam forms one arm of a Wheatstone bridge. At zero strain, the bridge is balanced and the differential output is zero. Under load, the gauge resistance changes by a few ohms, producing a millivolt differential. An INA126 instrumentation amplifier with Rg = 4.7 kohm sets a gain of 212. The amplified signal drives a 12-bit ADC. The system resolves strain changes corresponding to microstrains, suitable for structural health monitoring in bridges and aircraft frames.

Wien Bridge Oscillator for Audio Test Signal

A Wien bridge oscillator uses an op-amp with positive feedback through an RC network and an incandescent bulb as a nonlinear element for amplitude stabilisation. The oscillation frequency is f = 1 / (2 pi R C). With R = 10 kohm and C = 10 nF, the frequency is approximately 1590 Hz. This circuit is the basis of the HP 200A audio oscillator, the first product sold by Hewlett-Packard. Modern versions replace the bulb with an AGC circuit using a JFET or analog multiplier for lower distortion.

Future scope

Fully differential op-amps such as the Texas Instruments THS4521 are replacing single-ended op-amps in ADC driver stages for high-speed converters above 100 MSPS, because the differential signal path cancels even-order distortion and improves dynamic range. In CMOS image sensors, column-level op-amp circuits perform correlated double sampling to suppress reset noise below 1 electron RMS in sensors used in smartphone cameras and scientific CCD replacements. For implantable biomedical devices, chopper-stabilised op-amps operating at supply voltages below 1 V with sub-microwatt power consumption are a research focus in groups at MIT and IMEC to extend battery life in neural recording implants.