Operational Amplifier: Op-Amp as Differential Amplifier or Op-Amp as subtractor (With Examples)

Summary

This video from ALL ABOUT ELECTRONICS explains how to design a differential amplifier, or subtractor, using an op-amp. It covers the combination of inverting and non-inverting configurations, the use of resistors to control gain, and the application of the superposition theorem to calculate output voltage. Practical examples illustrate how to handle input voltages and bias limits, highlighting the importance of voltage dividers to equalize gains. The video also discusses real-world applications with sensors, the limitations of input impedance, and solutions like buffers or instrumentation amplifiers, making it a comprehensive guide for both learning and practical circuit design.

Takeaways

• 😀 An op-amp can function as a differential amplifier, amplifying the difference between its inverting and non-inverting inputs.
• 😀 Open-loop op-amp configuration has very high gain, which can cause output saturation, hence feedback is required.
• 😀 Combining inverting and non-inverting configurations allows the design of a true differential amplifier.
• 😀 The superposition theorem is used to calculate the output by considering one input voltage at a time and summing their effects.
• 😀 Output voltage of a differential amplifier is given by Vout = (R2/R1) * (V2 - V1) when gains are made equal for both inputs.
• 😀 Voltage dividers can be used to adjust the input at the non-inverting terminal to achieve equal gain.
• 😀 Differential amplifiers are useful in sensor applications, such as amplifying the difference from a balanced bridge circuit connected to an RTD sensor.
• 😀 Low input impedance of a differential amplifier may load the source, which can be mitigated using input buffers.
• 😀 Instrumentation amplifiers are preferred over differential amplifiers for very small sensor signals due to their high input impedance and high gain.
• 😀 Output voltage must always be checked against the op-amp bias voltages to prevent saturation.
• 😀 Practical examples demonstrate calculating output voltage by considering each input individually and combining them, emphasizing the role of resistor ratios and voltage dividers.
• 😀 Differential amplifiers can be used to perform subtraction operations between two voltages in electronic circuits.

Q & A

What is the primary function of a differential amplifier?

A differential amplifier amplifies the difference between two input voltages applied at the inverting and non-inverting terminals of an op-amp.

Why should an op-amp not be used in open-loop configuration for amplification?

In open-loop configuration, the gain of the op-amp is extremely high, which can easily saturate the output, making it impractical for precise amplification.

How can the gain of a differential amplifier be controlled?

The gain can be controlled by adjusting the resistor values, specifically R1 and R2, in the amplifier circuit.

What method is used to calculate the output when multiple voltage sources are applied to a differential amplifier?

The superposition theorem is used, which involves considering one voltage source at a time while setting the others to zero and then summing the individual contributions.

How is equal gain for both inputs achieved in a differential amplifier?

Equal gain is achieved by using a voltage divider for the non-inverting input so that the effective gain applied to both inputs becomes the same, ensuring the output is proportional to the difference of the two inputs.

What is the output equation of a differential amplifier using resistors R1 and R2?

The output voltage Vout is given by Vout = (R2 / R1) * (V2 - V1), where V1 and V2 are the input voltages at the inverting and non-inverting terminals, respectively.

In practical applications with sensors, why might a differential amplifier require a buffer?

A buffer is required because the input impedance of a differential amplifier is low, which could load the sensor source and affect voltage measurements. Buffers provide high input impedance to prevent this.

What is the main advantage of using an instrumentation amplifier over a standard differential amplifier?

An instrumentation amplifier provides high gain with very high input impedance, which is ideal for amplifying small sensor signals without loading the source.

How does biasing voltage affect the output of a differential amplifier?

The output voltage cannot exceed the op-amp's biasing voltages. If the calculated output is higher than the supply voltages, the output will saturate at the positive or negative supply limit.

Can you give an example calculation of a differential amplifier output using superposition?

For example, with V1 = 2V, V2 = 3V, R2/R1 = 5, and bias ±15V: Vout1 = -10V (from V1 alone), Vout2 = 16V (from V2 alone), so Vout = 16 - 10 = 6V.

Where can differential amplifiers be used in sensor-based circuits?

They can be used with bridge circuits, such as an RTD sensor setup, to amplify the difference in voltage caused by changes in temperature or other measured quantities.

Why is superposition important for analyzing differential amplifier circuits?

Superposition allows us to analyze the contribution of each input voltage separately and then combine them, simplifying the calculation of the output voltage in complex circuits.