Operation
All amplifier circuits are classified by "class of operation" as A, B, AB and C etc. See amplifier classes. Some significantly different circuit topologies exist compared to transistor designs.
- The grid (where the input signal is presented) needs to be biased substantially negative with respect to the cathode This makes it extremely difficult to direct couple the output of one valve to the input of a following valve as is normally done in transistor designs.
- Valve stages are coupled with components rated to withstand several hundred volts, typically a capacitor, occasionally a coupling transformer. The phase shifts introduced by coupling networks can become problematic in circuits that have feedback.
- There is no valve analog of the complementary devices widely used in "totem pole" output stages of silicon circuits. Push-pull valve topologies therefore typically require a transformer.
- The very high output impedance of valves (compared with transistors) usually requires matching transformers to drive low impedance loads such as loudspeakers or cutting lathe heads. The transformer is used as the load, in place of the resistor usually used in small-signal and driver stages. The reflected impedance of the transformer primary at the frequencies in use is much higher than the DC resistance of the windings, often kilohms. High performance transformers are however severe engineering compromises, are expensive, and in operation are far from ideal. Output transformers dramatically increase the cost of a valve amplifier circuit compared to a direct-coupled transistor alternative. However in both tube and solid state amps matching output transformers are required for public address applications where low-loss high impedance/high voltage lines are used to connect multiple distant loudspeakers.
- The open loop linearity of valves, especially triodes, makes it possible to use little or no negative feedback in circuits whilst retaining acceptable or even excellent distortion performance (especially for small-signal circuits).
Read more about this topic: Valve Amplifier
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