Practical skills | Performance indicators of triode amplifier circuits-EEWORLD

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In electronic circuit systems, there are many types of amplifier circuits (devices), which can be divided into low-frequency amplifiers, high-frequency amplifiers, and radio frequency amplifiers; there are Class A amplifiers, Class B amplifiers, and so on.

The RF part of wireless communication equipment includes low noise amplifier, intermediate frequency amplifier, buffer amplifier, driver amplifier, power amplifier, etc. The amplifier circuit is shown in Figure 1.

figure 1

Theoretically, the requirements and considerations in designing different amplifiers are very different, but their essential characteristics are the same: one is the control and conversion of energy, and they all amplify the power of the signal.

Only when the transistors in the amplifier circuit operate in the amplification region can the output and input of the transistor amplifier circuit maintain a linear relationship, that is, the circuit does not produce distortion.

Amplification without distortion is meaningful. For triode amplifier circuits, amplification factor, passband, input and output resistance, linearity, output power and efficiency are usually used to measure.

Amplification factor is an important indicator for measuring triode amplifier circuits. The value of amplification factor is the ratio of the output of the circuit to the input.

figure 2

A dot on a letter indicates a phasor, which has both amplitude and phase values. However, some materials do not use dots to identify phasors.

Due to the presence of reactive elements such as capacitors, inductors and junction capacitance of semiconductor devices in the amplifier circuit, when the input signal frequency is low or high, the value of the circuit amplification factor will decrease and cause signal phase shift.

This means that the same amplifier circuit has different amplification capabilities for signals of different frequencies. Under normal circumstances, an amplifier circuit is only suitable for amplifying signals within a certain frequency range.

But people always hope that the amplifier circuit has approximately equal amplification ability for signals in a specific frequency range, such as the high-frequency amplifier circuit of a television and the low-noise receiving amplifier circuit of a mobile phone.

Bandwidth is used to measure the ability of an amplifier circuit to amplify signals of different frequencies.

The bandwidth of an amplifier refers to the passband of the amplifier, that is, the frequency range in which the amplifier circuit can maintain stable operation. The concept of passband here is actually the same as the concept of passband in the filter section.

It is equal to the difference between the highest and lowest frequencies at which a device or circuit can operate normally. Therefore, two frequency values are needed to define bandwidth.

FIG3 is a schematic diagram showing the amplitude-frequency characteristics of an amplifier, where Am is the maximum gain of the amplifier, usually referred to as the intermediate frequency gain.

image 3

For an amplifier circuit, the maximum power that can be obtained on the load without distortion of the output signal is called the maximum output power of the amplifier circuit.

The DC power consumed by an amplifier is the product of the operating voltage and the operating current.

Conversion efficiency is an extremely important indicator for measuring amplifiers (especially power amplifiers). The amplifier converts the DC power of the power supply into AC signal power output. Only a part of the DC power is converted into useful signal power and obtained by the load, and the other part is consumed by the amplifier itself and parasitic elements in the circuit.

There are two ways to define the efficiency of an amplifier.

Generally speaking, it is always hoped that the input resistance of the amplifier circuit is large and the output resistance is small. However, if the amplifier circuit is expected to obtain a larger input current, the input resistance should be appropriately smaller;

If you want to provide a larger current to the load, the output resistance should be appropriately larger. Therefore, the size of the input resistance and output resistance of the amplifier circuit should be designed according to actual needs.

Reference address：Practical skills | Performance indicators of triode amplifier circuits

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