Introduction to high frequency circuits
The high-frequency circuit is a radio circuit, but it does not involve microwave circuits. (Microwaves are used to process circuits above one gigahertz, starting with physics electromagnetic fields, very different from our common circuits), for radio wave emission, Receive, modulate, demodulate, amplify, etc.
Digital circuits process digital signals. Digital signals only have high and low signals. For example, digital circuits in CMOS process have a working range of 0-3.3 volts, 0-0.8 volts is considered low, 2.4-3.3 volts is considered high, and other voltages It is considered invalid, and all electrical signals are divided into a sequence of high and low levels. It is suitable for high-speed processing, high-precision processing, and computer interface, and is directly processed by a computer.
Analog circuits do not distinguish levels, all continuous signals are processed together (the macroscopic physical quantities of nature are continuous) for power, amplification, filtering, and so on.
The analog circuit and the high-frequency circuit are actually very close, except that the circuit operates at a high frequency, and the physical characteristics of many components are changed, and the processing method and the problem to be processed are different.
High frequency circuit performance index
High-frequency small-signal amplification has two forms of resonant amplification and wideband amplification. The performance indicators mainly include the following items.
1. Gain
High-frequency circuits, like low-frequency circuits, have indicators of voltage gain and power gain. For the resonant amplifying circuit, it means that at the resonant frequency f0, for the wideband amplifying circuit, it means a bubble around a certain frequency.
2. Passband
Similar to the low-frequency circuit concept, for the resonant amplifier circuit, the passband refers to the difference between the two corresponding frequencies whose normalized amplitude drops to 0.707 with respect to the resonant frequency f0; for the wideband amplifying circuit, the corresponding with respect to a certain frequency definition.
3. Selectivity
The selectivity is mainly for the resonant amplifying circuit, and the characterization circuit selects the ability of the useful signal to suppress the unwanted signal, which is usually measured by the rectangular coefficient and the suppression ratio, and is based on the resonant characteristic curve of the circuit.
4, noise figure
When the amplifier circuit works, irregular movement of carriers occurs due to various reasons, and noise is formed inside the circuit, which affects the signal quality. This effect is usually described by the ratio of the signal power Ps to the noise power Pn (referred to as the signal to noise ratio). The noise figure is defined as the ratio of the input signal to noise ratio to the output signal to noise ratio.
5, stability
The stability of the high-frequency amplifier circuit refers to the stability of the main performance of the working state or condition. For example, changes in ambient temperature or fluctuations in the supply voltage can affect the DC operating state of the amplifying circuit; the parameters of the circuit components also change, resulting in a change in the gain of the amplifying circuit, a shift in the center frequency, and a distortion in the resonant curve. Even self-excited and completely unable to work.
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