Amplifier Non-Linearity Leads to Output Signal Distortions

Key Takeaways

• A practical amplifier circuit comprises non-linear active devices such as bipolar transistors and field-effect transistors. The non-linear devices present in an amplifier introduce distortions into the output signal compared to ideal linear devices. 

• Clipping distortions in the output signal results from improper bias to the amplifier. It is the direct consequence of biasing a nonlinear device in the amplifier. 

• Weak distortion can be classified into either 𝝱 distortion or gm distortion.

High fidelity amplifiers give low distortions

Nonlinearity is a phrase in mathematics used to describe situations where the graph plotted between two quantities or parameters is not a straight line. In electronics, we describe the nonlinear behavior of circuits when the input-output relationship is not direct. 

Nonlinearity is a fundamental parameter in the design of amplifiers.  In nonlinear amplifiers, the output signal is affected by various types of distortions. The amplifier nonlinearity is directly related to output signal distortions. In ideal amplifiers, signals are amplified without any change in their waveshape and we say the amplifier is of good fidelity. We often see the term Hi-Fi, meaning high fidelity, written on audio amplifiers. This is an indication of low distortions in the output signal.  

Let's discuss the nonlinearities in an amplifier and the distortions caused by them. 

 Amplifier Nonlinearity

A practical amplifier circuit is comprised of non-linear active devices such as bipolar transistors and field-effect transistors. The non-linear devices present in an amplifier introduce distortions into the output signal compared to ideal linear devices. The effects of nonlinearities of the active devices used in amplifiers compared to the linear devices are:

1. Input and output offsets- The input and output offsets establish the need for a level shift in the signals to compensate for the offset. The biasing given to the active devices is the offset given to shift the input signals and output signals.
2. A limited output range- The output range of non-linear devices is limited. The input-output relationship in non-linear devices is curved, not a straight line. Clipping distortion in amplifiers is an example of a limited output range of non-linear active devices.
3. Curved input-output relationship- The output signal deviates from the expected signal when the linear transfer function is taken into account. The curved input-output relationship causes weak distortions in amplifiers. Harmonics and intermodulation are also results from the non-linear characteristics of the devices present in an amplifier. 

The various distortions due to amplifier nonlinearity are briefed in the following section.

Harmonic Distortion and Phase Distortion

In amplifier output signals, high-order frequency components other than the fundamental frequency of the output signal may be present due to component non-linearity. The fundamental frequency components of the output signal increase linearly with the input amplitude, whereas the higher-order frequency components, say second-order or third-order, increase quadratically or cubically with input signal amplitude. This type of distortion, caused by higher-order frequency components, is generally called harmonic distortion. There can also be DC shifts in the output signal due to second-order non-linearity, and this distortion affects the phase of the output signal. Phase distortion in amplifiers increases with an increase in frequency. 

Intermodulation

Intermodulation is another measure of distortion present in nonlinear amplifiers. When several input signals of different frequencies are applied to a non-linear amplifier, there will be spurious frequency components present in the amplifier output signal. These components can be a mix of various frequency components in the input signals.  

Clipping Distortion

Clipping distortion in the output signal results from improper bias to the amplifier. It is the direct consequence of biasing a nonlinear device in the amplifier. When the amplifier bias is too small, the output signal may get clipped. The bias of a non-linear device should be considered both in the current domain and the voltage domain. The bias voltage and current should be greater than the expected maximum peak of the output current and output voltage. 

Weak Distortion

Weak distortion is due to the weak non-linearity of the input-output relation of the devices in the amplifier. The weak distortion introduces higher-order terms in the output, as long as the input signal is not zero. The distortion is termed as ‘weak’,  as the higher-order terms are relatively small in high-performance amplifiers. The magnitude of the higher-order components is very small compared to the input signal. Weak distortion can be classified into two types:

1. 𝝱 distortion- This type of distortion is seen in current-driven bipolar transistor circuits. 

2. gm distortion- This type of distortion is caused by the dependency of collector current on the transconductance (gm) of the transistor. In bipolar transistors, the low impedance and input capacitance yield gm distortion.

In addition to non-linear circuit components, some of the topologies of transistor amplifiers are inherently non-linear, such as class AB, B, C, and D. In such amplifiers, filtering and compensation schemes are used to overcome the distortions caused by non-linearities. 

Amplifier nonlinearity affects the performance of electronic circuits, so it is important to operate the amplifier in the linear region of active devices. While fixing the Q-point, designers should prioritize maintaining the linearity of an active device throughout the amplifier operation.

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