Power amplifiers on field-effect transistors. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers

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The use of field-effect transistors in a power amplifier can significantly improve the sound quality with a general simplification of the circuit. The transfer characteristic of field-effect transistors is close to linear or quadratic, so there are practically no even harmonics in the output signal spectrum, in addition, there is a rapid decrease in the amplitude of higher harmonics (as in tube amplifiers). This allows the use of shallow negative feedback in field-effect transistor amplifiers or completely abandon it. After conquering the expanses of "home" Hi-Fi, field-effect transistors began to attack car audio. The published schemes were originally intended for home systems, but maybe someone will dare to apply the ideas embodied in them in a car ...

Rice. 1 (click to enlarge)

This scheme is already considered a classic. In it, the output stage, operating in AB mode, is made on MIS transistors, and the preliminary stages are on bipolar ones. The amplifier provides quite high performance, but to further improve the sound quality, bipolar transistors should be completely excluded from the circuit (next picture).

Fig. 2

After all the reserves for improving the sound quality have been exhausted, only one thing remains - a single-cycle output stage in a "pure" class A. The current consumed by the preliminary stages from a higher voltage source in both this and the previous circuit is minimal.

Fig. 3

The output stage with a transformer is a complete analogue of lamp circuits. It's a snack ... The integrated current source CR039 sets the operating mode of the output stage.

Fig. 4

However, a broadband output transformer is a rather complicated assembly to manufacture. An elegant solution - a current source in the drain circuit - was proposed by Pass Laboratories.

Rice. 5 (click to enlarge)

Subsequently, the amplifier was finalized - the OOS circuits for direct and alternating current were separated, which made it possible to somewhat reduce the harmonic coefficient at maximum power. This was achieved at the cost of a significant reduction in the sensitivity of the amplifier. The part numbers for this option are given in parentheses. However, in any case, to realize the potential of this amplifier, a preamplifier with an unusually high output voltage is required. The success of this amplifier prompted designers to develop single-ended amplifiers of the class A. An interesting scheme was proposed by Andrea Ciffoli. To reduce the supply voltage, he replaced the current source in the drain circuit with a choke. The quality indicators of the amplifier are mainly determined by the design of this inductor. Its inductance is 0,5H with a DC resistance of not more than 0,5 Ohm, it is made on a ferromagnetic magnetic core. The amplifier is powered by a car battery.

Author: A. Shikhatov; Publication: bluesmobile.com/shikhman

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