# Transistor low frequency amplifier

 Transistor low-frequency amplifiers are mainly used to amplify low-frequency small-signal voltages, with frequencies ranging from tens of hertz to one hundred kilohertz
 First, the bias circuit of the transistor
 In order for the amplifier to achieve linear amplification, the transistor must not only have a suitable static operating point, but must also stabilize the operating point. Due to the influence of temperature on the tube parameters Î², Icbo, Ube, they are finally reflected in the change of Ic. In order to eliminate this effect, we stabilize the static operating point through the negative feedback of the DC or voltage biased by the transistor. The two kinds of bias circuits and working point stability principle are as follows
Table I, Bias circuit of transistor amplifier
Circuit
Working point stability principle
Calculation formula
Negative current feedback
 Set temperature T â†‘, DC negative feedback process As a result, Ic remains unchanged
U = (1 / 3-1 / 5) Ec
Re = (1 / 3-1 / 5) Ec / Ic
Rb = Rb1 // Rb2
=== (2-5) Re
Ub = Rb1Ec / (Rb1 + Rb2)
Voltage negative feedback
 Set temperature T, DC negative feedback process As a result, Ic remains unchanged
Rb = Î² (Ec-Ube) / Ic-Î²Rc
Ic = Ec / (Rc + Rb / Î²)
According to experience, usually take
Rb / Rc = (2-10)
 Second, the three circuit forms of the amplifier The amplifier is a three-terminal circuit, one of which must be the common â€œgroundâ€ terminal of the input and output. It is a common circuit, which is connected to the base and is called a common circuit. These three have different performances, see the table below
Three circuit forms and their performance comparison
 3. Graphic method
 The so-called graphical method is to use the input and output characteristic curves of the transistor to analyze the performance of the amplifier by drawing. The graphical method can intuitively and comprehensively indicate the working process of the transistor amplification, and can calculate some performance indicators of the amplifier. Examples to illustrate the graphical process of graphical methods, Example: Knowing the parameters and input voltage Ui = 15sinÏ‰t (millivolt) in the circuit shown in the figure below requires graphically determining the static operating point parameters Ibq, Icq, and Iceq of the circuit, and calculating the voltage and current amplification factors Ku and Kio.
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Posted at 2006-04-16 18:54 â€¢ 241 times read
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