What are the operating principles of a wideband amplifier?

Basic concept

A wideband amplifier is an electronic circuit capable of amplifying signals over a wide frequency range. Unlike narrowband amplifiers, the gain of wideband amplifiers remains relatively stable over a relatively wide frequency range.

Working principle

Transistor selection and characteristic utilization

Broadband amplifiers usually use transistors with high frequency characteristics (such as high frequency bipolar transistors or field effect transistors) as amplification elements. Taking the field effect transistor (FET) as an example, the FET has the characteristic of high input impedance, which makes it in the broadband amplifier circuit can reduce the load influence on the previous circuit, so as to better receive and amplify the input signal. At high frequencies, some characteristics of the transistor (such as electrode capacitance, cut-off frequency, etc.) will affect the amplification performance. For wideband amplifiers, transistors with higher cutoff frequency will be selected, and the adverse effects of factors such as electrode capacitance can be reduced by reasonable circuit design.

Circuit structure and frequency compensation

Common emitter common base (CE-CB) or common source common gate (CS-CG) structure

In wideband amplifiers, common emitter - common base (for bipolar transistors) or common source - common gate (for field-effect transistors) cascade structures are often used. In the case of the common-fire common-base structure, the common-fire stage provides a larger voltage gain, and the common-base stage has better high-frequency characteristics (such as lower input capacitance and higher cutoff frequency). The output signal of the common-emitter stage is coupled directly to the input of the common-base stage, and the high cutoff frequency characteristic of the common-base stage can extend the bandwidth of the entire circuit. This structure can effectively improve the high frequency response ability of the amplifier while ensuring a certain voltage gain, so as to achieve broadband amplification.

Frequency compensation technique

In order to further extend the bandwidth of the amplifier, frequency compensation technology is also used. One of the common methods is to use capacitance compensation. For example, a suitable compensation capacitor is added to the interstage connection of the amplifier. When the signal frequency increases, the capacitive reactance of the compensation capacitor decreases, which can provide an additional signal path, thereby improving the gain characteristics of the amplifier in the high frequency band, making the gain of the amplifier more stable in a wide frequency range.

The application of negative feedback

Negative feedback technology is widely used in broadband amplifiers. By introducing a negative feedback network between the output and input of the amplifier, the performance of the amplifier can be effectively improved. Negative feedback can reduce the gain sensitivity of the amplifier, making the gain of the amplifier more stable over a wide frequency range. For example, when the frequency of the input signal changes, the output of the amplifier does not experience large gain fluctuations due to negative feedback. In addition, negative feedback can also improve the linearity of the amplifier, reduce noise and distortion, which is very important for processing signals of different frequencies and amplitudes in broadband amplification.