What kind of diode used in an amplifier?

Lots of kinds of diodes are used in amplifiers.

In the power supply section, diodes may be used in groups of two or four, as rectifiers to convert AC (from the mains supply, via a transformer) into DC for the amplifier. These diodes are usually rated at several hundred volts, and several amps. They are classified as rectifier diodes. A typical example is 1N5404.

In the amplifier circuitry, diodes may be used to provide a known (approximately) voltage drop where required by the design of the circuit. They can also be used for temperature feedback, although for direct temperature compensation a transistor is more common. These diodes are generally small-signal types like 1N4148.

In logic switching, diodes are often used for simple OR-type functions. Look on Wikipedia for "diode OR". These are small-signal types.

If the amplifier has a switching power supply, or is a Class D amplifier, high speed switching diodes are used extensively. They are used for rectification (though they must be high-speed devices because of the high switching frequency), suppression and clamping in the Class D output stage (ditto), and miscellaneous functions in the control circuitry. These are high-speed, high-current diodes.