3H amplifier on a digital microcircuit. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur
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When developing a simple amplifier for TDS-22 stereo telephones, which could be installed, say, in a home-made player, a small-sized VHF-FM receiver, a hearing aid, the author managed to assemble an amplifier on a widespread 564LN2 chip (see figure).
The maximum amplitude of the undistorted signal at the load of the proposed amplifier without any circuitry tweaks is almost equal to the supply voltage. Many listeners commented on the good sound even with average quality stereo phones. This is probably due to the use of a single-cycle power amplifier operating in class A in the final stage. The microcircuit can be replaced with K561LN2.
A little more about the design. CMOS inverters DD1.1, DD1.2 and transistor VT1 form a sensitive three-stage inverting amplifier, covered through a resistor R3 deep negative feedback (OOS) DC voltage. This resistor shifts the operating point of the first stage, and behind it the rest, to the linear sections of the transfer characteristics. Also, this resistor forms an AC voltage feedback, leading to a linearization of the frequency response up to 100 kHz [1].
The frequency-dependent feedback (R4C2 circuit) was selected subjectively, according to the fidelity of the sound of TDS-22 miniature stereo phones when listening to jazz compositions and choral singing from the output of a high-quality CD player.
The VD1R2R5C3 circuit provides the efficiency of an amplifier operating in class A with the so-called "floating operating point" [2]. The current consumption in the absence of a signal is 2...3 mA. If the economic factor is not so important, this chain can be excluded. Then the current will be equal to 10 ... 20 mA and does not depend on the volume of the signal.
By the way, the nominal input signal voltage is 40 mV. If desired, this value can be reduced to 10 mV by reducing the resistance of the resistor R1 to 12 kOhm, but at the same time, as in amplifiers based on conventional inverting op-amps, the parameters will deteriorate somewhat.
The frequency response of the amplifier can be changed by selecting the values of the parts of the R4C2 chain.
In the final stage, it is permissible to install a transistor of the GT108, GT109 series with a base current transfer coefficient of at least 50. Instead of telephones, the amplifier allows the connection of a dynamic head with a voice coil with a resistance of 50 Ohms - then everyone around can listen to music recordings.
When mounting parts, it is necessary to follow the rules for working with microcircuits of the CMOS structure to protect them from breakdown by static electricity. The inputs of all unused inverters should be connected to one of the outputs of the power supply.
Establishing an amplifier is reduced to selecting a resistor R2 until a symmetrical two-sided signal limiting at the load is obtained with an increase in the input signal with a frequency of 1 kHz above 50 mV (rms).
The amplifier works stably at a supply voltage of 2 ... 4,5 V.
Literature
- Voskoboynikov M. Digital microcircuits in LF devices. - Radio, 1981, No. 7-8, p. 37.
- Muravin V. Hearing aids: Sat: "To help the radio amateur", vol. 93, p. 42-59. - M.: DOSAAF, 1986.
Author: A. Ladyka, St. Petersburg
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