Field Effect Transistors and Bipolar Transistors in Audio

1. Power amplifiers use field effect tubes or bipolar transistors (hereinafter referred to as transistors) as the front and back stages to amplify the signal. The power amplifier made of field effect tubes can reproduce warm and smooth music with full charm. Amplifiers made of transistors also have their advantages. Let's compare field effect tubes and transistors. 1. Field effect tubes mainly include junction field effect tubes (JFETs) and insulated gate field effect tubes (IGFETs). The substrate (B) of the insulated gate field effect tube is connected to the source (S), and its three poles are the gate (G), drain (D) and source (S). Transistors are 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 field effect tube have similar functions to the b, c, and e poles of the transistor. The difference between insulated gate field effect transistors and junction field effect transistors is that their conductive mechanisms and current control principles are fundamentally different. Junction transistors use the change in the width of the depletion region to change the width of the conductive channel in order to control the drain current, while insulated gate field effect transistors use the electric field effect on the surface of the semiconductor and the amount of inductive 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 (prestage) of power amplifiers, while insulated gate field effect transistors are used in the final stage (output stage) of power amplifiers.

2. Bipolar transistors The internal current is formed by two types of carriers, and it is controlled by current. Field effect transistors are voltage-controlled devices. The gate (G) basically does not take current, while the base of the transistor always takes a certain amount of current. Therefore, when only a very small amount of current is allowed to be taken from the signal source, field effect transistors should be selected . When a certain amount of current is allowed to be taken, a transistor is selected for amplification, which can obtain a higher voltage amplification factor than field effect transistors

. 3. Field effect tubes use majority carriers to conduct electricity (majority carriers: electrons are majority carriers, referred to as majority carriers), while transistors use both majority carriers and minority carriers (holes are minority carriers, referred to as minority carriers). Since the concentration of minority carriers is easily affected by external conditions such as temperature and radiation, it is more appropriate to use field effect tubes under conditions where the environment changes drastically .

4. The power amplifier circuit is a weak current system with high sensitivity. It is easily affected by some irregular external and internal signals. That is, when the input end of the amplifier is short-circuited, there are still some irregular voltage or current changes at the output end, which can be detected using an oscilloscope or speaker. This is the noise or interference voltage of the power amplifier. The impact of noise is usually expressed by the noise factor Nf, in decibels (dB). The smaller the Nf, the better. Nf = input signal-to-noise ratio / output signal-to-noise ratio. There are three sources of noise in transistors: ⑴ Thermal noise: due to the irregular thermal motion of carriers, it is sometimes generated through the body 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 emitter junction is different at each instant, which causes an irregular flow of emitter current or collector current, generating shot noise; ⑶ Flutter noise: The reason why transistors generate flutter noise is not very clear now, but it is assumed to be caused by the generation and recombination of carriers on the crystal surface, so it is related to the semiconductor material itself and the process level. The noise of field effect tubes is only generated by the movement of carriers, so the Nf of field effect tubes is smaller than that of transistors.

The amplifier not only amplifies the noise at its input, but also has noise in the amplifier itself, so the signal-to-noise ratio at its output must be smaller than the signal-to-noise ratio at the input. The greater the noise of the amplifier itself, the smaller its output signal-to-noise ratio is than its input signal-to-noise ratio, and the larger Nf is. Therefore, field effect tubes or low-noise transistors are usually used in the front stage of low-noise amplifiers. 5. The drain and source of field effect tubes can be interchanged, and the gate voltage of depletion-type insulated gate tubes can be positive or negative, which is more flexible than transistors. However, in audio power amplifiers, field effect tubes mostly appear

in N -channel/P-channel pairs, and transistors also appear in PNP/NPN pairs. However, field effect tubes are relatively fragile in amateur applications and have higher costs. From the above comparison of field effect tubes and transistors, it is not difficult to find that field effect tubes have the advantages of high input impedance, low noise, low power consumption, high thermal stability, and strong radiation resistance. Therefore, the overall performance of field effect tubes is better than that of transistors. In many excellent power amplifiers, field effect tubes have been widely used. It is also not uncommon to see power amplifiers that use transistors to achieve good sound. Engineers have designed a combination tube based on the characteristics of the two tubes, taking their respective advantages. The effect is of course better than the two.