1 The power amplifier uses field effect tubes or bipolar transistors (hereinafter referred to as transistors) as the pre-stage and post-stage to amplify the signal. And the power amplifier using the field effect tube, the replayed music is warm and smooth, full of charm. Amplifiers made with transistors also have their advantages. Let's compare a field-effect transistor with a transistor. 1. Field effect tubes mainly include junction field effect tubes (JFET) and insulated gate field effect tubes (IGFET). The substrate (B) and the source analysis (S) of the insulated gate field effect tube are connected together, and its three poles are the gate (G), the drain (D) and the source (S). The transistor is divided into NPN and PNP tubes, and its three poles are the base (b), collector (c), and emitter (e). The G, D, and S poles of the FET have similar functions to the b, c, and e poles of the transistor. The difference between insulated gate effect transistors and junction field effect transistors is that their conductive mechanisms and current control principles are fundamentally different. Junction transistors use the width of the depletion region to change the width of the conductive channel to control the drain current The gate field effect transistor uses the electric field effect on the surface of the semiconductor and the amount of electrical induced charge to change the conductive channel to control the current. The difference in their properties makes junction field effect transistors often used in the input stage (front stage) of the power amplifier, and insulated gate field effect transistors are used in the last stage (output stage) of the power amplifier.
2. The internal current of the bipolar transistor is formed by two carriers, which is controlled by the current. The field effect transistor is a voltage control device, the gate (G) basically does not take current, and the base of the transistor always takes a certain current, so when only a very small amount of current is allowed from the signal source, the field should be selected Effect tube. When a certain amount of current is allowed to be selected, a transistor is used for amplification to obtain a higher voltage amplification factor than the field effect tube.
3. The field effect tube uses multiple electrons to conduct electricity (multiple: electrons are the majority carriers, referred to as multiples), while the transistor uses both multiples and minority (holes are minority carriers, referred to as minoritys), Because the concentration of the minority carrier is susceptible to external conditions such as temperature and radiation, it is more appropriate to use field effect transistors under the conditions of severe environmental changes.
4. The power amplifier circuit is a weak current system with high sensitivity, it is easy to accept the influence of some irregular signals from the outside and inside, that is, when the input terminal of the amplifier is short-circuited, there are still some irregular voltages or The current change output can be noticed by using an oscilloscope or a speaker. This is the noise or interference voltage of the power amplifier. The influence of noise is usually expressed by the noise coefficient Nf, the unit is decibel (dB), the smaller the Nf, the better, Nf = input signal noise ratio / output signal noise ratio, the transistor has three sources of noise: ⑴ thermal noise: due to the current Irregular thermal motion of the electrons is generated by the bulk resistance in the semiconductor tube; ⑵ Shot noise: the current in the so-called triode is only an average value. In fact, the number of carriers injected into the base region through the emission junction is The instants are different, which causes an irregular flow of emitter current or collector current, resulting in shot noise; ⑶ flutter noise: the reason for the transistor's flutter noise is not yet clear, but it is assumed that the carriers are in the crystal The generation and recombination of the surface is related to the semiconductor material itself and the level of technology. The noise of the field effect tube is only generated by the movement of the carrier, so the Nf of the field effect tube is smaller than that of the transistor.
Not only does the amplifier amplify the noise at its input, but the amplifier itself also has noise, so the signal-to-noise ratio at its output must be less than the signal-to-noise ratio at the input. End signal to noise ratio, Nf is greater, so the field effect tube or low noise transistor is usually used in the front stage of the low noise amplifier.
5. The drain and source of the field effect tube can be interchanged, and the gate voltage of the depletion insulated gate tube can be positive or negative, and the flexibility is stronger than that of the transistor. However, in audio power amplifiers, field effect tubes mostly appear as N-channel / P-channel pairs, and transistors also appear as PNP / NPN pairs. However, field effect transistors are more fragile in amateur applications, and the cost is higher.
From the comparison between the above FET and transistor, it is not difficult to find that the FET has the advantages of high input impedance, low noise, low power consumption, high thermal stability, and strong radiation resistance. Therefore, the overall performance of the FET should be Better than transistors, in many excellent power amplifiers, field effect transistors have been more commonly used. It is also not uncommon for those who use transistor-based power amplifiers to achieve good voice. Based on the characteristics of the two types of pipes, engineers take their respective advantages and design a combined pipe. The effect is of course better than them.
2. The internal current of the bipolar transistor is formed by two carriers, which is controlled by the current. The field effect transistor is a voltage control device, the gate (G) basically does not take current, and the base of the transistor always takes a certain current, so when only a very small amount of current is allowed from the signal source, the field should be selected Effect tube. When a certain amount of current is allowed to be selected, a transistor is used for amplification to obtain a higher voltage amplification factor than the field effect tube.
3. The field effect tube uses multiple electrons to conduct electricity (multiple: electrons are the majority carriers, referred to as multiples), while the transistor uses both multiples and minority (holes are minority carriers, referred to as minoritys), Because the concentration of the minority carrier is susceptible to external conditions such as temperature and radiation, it is more appropriate to use field effect transistors under the conditions of severe environmental changes.
4. The power amplifier circuit is a weak current system with high sensitivity, it is easy to accept the influence of some irregular signals from the outside and inside, that is, when the input terminal of the amplifier is short-circuited, there are still some irregular voltages or The current change output can be noticed by using an oscilloscope or a speaker. This is the noise or interference voltage of the power amplifier. The influence of noise is usually expressed by the noise coefficient Nf, the unit is decibel (dB), the smaller the Nf, the better, Nf = input signal noise ratio / output signal noise ratio, the transistor has three sources of noise: ⑴ thermal noise: due to the current Irregular thermal motion of the electrons is generated by the bulk resistance in the semiconductor tube; ⑵ Shot noise: the current in the so-called triode is only an average value. In fact, the number of carriers injected into the base region through the emission junction is The instants are different, which causes an irregular flow of emitter current or collector current, resulting in shot noise; ⑶ flutter noise: the reason for the transistor's flutter noise is not yet clear, but it is assumed that the carriers are in the crystal The generation and recombination of the surface is related to the semiconductor material itself and the level of technology. The noise of the field effect tube is only generated by the movement of the carrier, so the Nf of the field effect tube is smaller than that of the transistor.
Not only does the amplifier amplify the noise at its input, but the amplifier itself also has noise, so the signal-to-noise ratio at its output must be less than the signal-to-noise ratio at the input. End signal to noise ratio, Nf is greater, so the field effect tube or low noise transistor is usually used in the front stage of the low noise amplifier.
5. The drain and source of the field effect tube can be interchanged, and the gate voltage of the depletion insulated gate tube can be positive or negative, and the flexibility is stronger than that of the transistor. However, in audio power amplifiers, field effect tubes mostly appear as N-channel / P-channel pairs, and transistors also appear as PNP / NPN pairs. However, field effect transistors are more fragile in amateur applications, and the cost is higher.
From the comparison between the above FET and transistor, it is not difficult to find that the FET has the advantages of high input impedance, low noise, low power consumption, high thermal stability, and strong radiation resistance. Therefore, the overall performance of the FET should be Better than transistors, in many excellent power amplifiers, field effect transistors have been more commonly used. It is also not uncommon for those who use transistor-based power amplifiers to achieve good voice. Based on the characteristics of the two types of pipes, engineers take their respective advantages and design a combined pipe. The effect is of course better than them.
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