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Electronic Devices Guide

Linear and Limiting Amplifier Types

The purpose of amplifier usage in various situations is to amplify a signal that is attenuated and is reduced or originally small. Amplifiers are classified into different categories. One of these categories is linear and limiting amplifiers. The differences and features between the two are described below.

Generally, linear amplifiers amplify the input signal at a fixed ratio and output it, so the input and output level are proportional. Conversely, the output level of the limiting amplifier is fixed and does not change, even to an input of signal exceeding a certain level. Figure 1 shows the input and output level characteristics of an amplifier that can be used as a linear amplifier or as a limiting amplifier. With this amplifier, the voltage-gain is 10 times (=20dB), accordingly in the area ①, the output of 1V is obtained for the input level 0. 1V, and the output of 2V is obtained for the input of 0. 2V, thus acting as a linear amplifier. On the other hand, in the area ②, the output is 3. 5V in all this range, whereas it ideally should be 5V and 7V for an input of 0. 5V and of 0. 7V, respectively. In other words, in the area ②, it acts as a limiting amplifier.

Amplifier I/O Characteristics
Figure 1 Amplifier I/O Characteristics

Figure 2 shows the waveform of the limiting amplifier. In this example, it's output level is limited at 3.5V. Limiting amplifiers are employed in front of devices with limited input level or digital processors. As limiting amplifiers compress the distortion of waveform such as ringing and overshooting, they are often intentionally employed for effect of waveform shaping (Figure 3).

Behaivor of Limiting Amplifier
Figure 2 Behaivor of Limiting Amplifier
Wavefrom shaping by Limiting Amplifier
Figure 3 Wavefrom shaping by Limiting Amplifier

On the other hand, the linear amplifier amplifies to the signals precisely proportional, as shown in Figure 3. The linearity index indicates the precision of the signal amplification. To achieve the intended linearity, the amplifier shall be employed only in the range where the curve of the amplification overlays the ideal line (Figure 4).

Recent years, the optical communication adopts multi-level signals such as PAM. The multi-level signal with amplifiers with poor linearity distorts the output waveform (Figure 5). To amplify the PAM signal as intended, linear amplifiers with excellent linearity shall be employed.

Linear Amplifier Operation
Figure 4 Linear Amplifier Operation
Amplifier Linearity and Waveform
Figure 5 Amplifier Linearity and Waveform

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