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楼主 |
发表于 2012-3-28 17:10
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回复 173# Gautau
国外 OTL大师 对串叠式有相同的觀点。
Cascode Amplifiers
Cascode amplifiers are another method of using two triodes in a single stage of gain. In this case, the plate of the first tube is used to drive the cathode of the second (top) tube, as shown in Figure 3. The top tube is arranged as a constant current source, which has the effect of acting like the circuit of Figure 1 with a very large plate resistor and a very high plate voltage. In this arrangement, the gain and linearity are substantially improved over basic triode amplifiers. The gain can approach the amplification factor of the tube squared, which means that gain is nearly that of a pentode circuit, but with low distortion typical of triode amplifiers.
Advantages
High gain.
Lower noise (for the amount of gain developed).
Good high frequency bandwidth. This varies with stray capacitance and the internal inter-electrode capacitance of the tubes used.
Low parts count (for the amount of resulting gain). This means a simpler circuit design and an overall less expensive device.
Disadvantages
High output impedance. One of the results of this is that certain tubes become unacceptable for cascode use, such as the 12AX7. |
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