Question

Define a difference amplifier circuit and its use in subtracting signals.

Answer 1

A difference amplifier circuit, often referred to as a differential amplifier or subtractor, is an electronic circuit used to amplify the voltage difference between two input signals while rejecting any common-mode signal component (signals that appear in phase on both inputs). The primary purpose of a difference amplifier is to subtract or amplify the voltage difference between the two input signals.

Difference amplifiers are commonly used in various applications, including instrumentation, signal processing, audio equipment, and communication systems. Here's how a difference amplifier works and its use in subtracting signals:

Circuit Configuration:
A basic difference amplifier consists of two input terminals, labeled as "Non-Inverting Input" (+) and "Inverting Input" (-), and an output terminal. The circuit typically utilizes operational amplifiers (op-amps) as the core component due to their high gain and differential amplification capabilities. The schematic diagram of a basic difference amplifier circuit is as follows:

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         +Vcc       R2
           │         │
           │         │
           ▼         ▼
Vin+ ───────┤       ├───► Vout
           │  Op-Amp │
Vin- ───────┤       ├───►
           │         │
           ▲         ▲
           │         │
          R1        R3
           │         │
           │         │
         GND        GND


Operation:
The voltage difference between the two input signals (Vin+) and (Vin-) is amplified and appears at the output (Vout). The gain of the amplifier is determined by the resistor values (R2 and R3) and is given by the formula:

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Gain (A) = -R3/R2


The negative sign indicates that the output is inverted with respect to the input voltage difference. The resistor (R1) is typically used for biasing and stability purposes.

Use in Subtracting Signals:
One of the key applications of a difference amplifier is subtracting two input signals. When two signals with similar frequency components are applied to the non-inverting (+) and inverting (-) inputs, the amplifier amplifies the voltage difference while canceling out any common-mode signals.

For example, consider two signals, A and B, where you want to subtract signal B from signal A. By connecting signal A to the non-inverting input and signal B to the inverting input of the difference amplifier, the output will be the amplified difference between the two signals, effectively subtracting signal B from signal A. Any common-mode noise or interference that is present in both signals will be rejected, as the amplifier responds only to the voltage difference between the inputs.

In summary, a difference amplifier circuit is a versatile tool for subtracting or amplifying the voltage difference between two input signals, finding applications in various fields where accurate signal subtraction or amplification is required while rejecting common-mode noise.