High quality transistor UMZCH. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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High quality transistor UMZCH. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers

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The characteristic transistor sound (dry, harsh, opaque) is not necessarily inherent in transistor amplifiers. Indeed, most industrial developments of transistor UMZCH with a harmonic coefficient of less than 0,05% and a frequency band of 20 ... 20000 Hz sound far from the best, while requiring a significant increase in higher frequencies.

As an example of a successful development, one can cite an amplifier [1], developed at the dawn of the development of transformerless UMZCH circuitry. The amplifier contains only one common-emitter (CE) voltage amplification stage and has a distortion of about 2% at an output power of 2 W. However, at higher frequencies it sounds quite clear, transparent detailed does not require their rise.

Paradoxically, tube amps with 2% distortion subjectively sound better than transistor amps with 0,002% harmonics. This is explained by the fact that the spectrum of harmonics in tube amplifiers is much narrower and only of a low order, not higher than the third, while in transistor ones it is up to the eleventh order.

A very important advantage of high-power lamps is that the carrier dissipation time and turn-on delay are equal to zero when the control voltage is applied. In addition, the output characteristics of the triode are ideal for the output stage, which, as you know, operates on a complex load (per impedance). A static induction field-effect transistor (SIT) has characteristics close to those of a triode when a negative voltage is applied to the gate. However, bipolar transistors are still the most accessible to radio amateurs.

Let us briefly consider the main causes of distortion in transistor amplifiers.

Distortion occurs in the output stage. Transient distortion of the first kind (step type) is due to the strongly pronounced S-shaped form of the transmission characteristic of emitter followers. The way to reduce this kind of distortion is to increase the quiescent current and the depth of the OOS.

Crosstalk distortion of the second kind occurs due to the time delays of the signal caused by the switching process, and leads to distortion in the zero-crossing region. These distortions arise due to a rather long resorption time not of the main carriers of the base, but because during this time, there is practically no feedback, the preliminary stages develop full amplification, which leads to impulse surges up to the supply voltage. This type of distortion can be reduced by using high power output transistors with a unity gain cutoff frequency of 5 MHz or more. Increasing the OOS in this case does not help.

The main characteristics of the amplifier:

Sensitivity, V ...... 0,7
Input impedance, kOhm.......5,6
Rated output power at a load of 4 ohms, W.......60
Coefficient of non-linear distortion, %......0,02
Bandwidth of effectively amplified frequencies, Hz at the level of 0 - 0,5 dB.......3...80 000
by level 0 - 3 dB.......1...120 000
without chain R1.C2.......1...550 000
without R1.C2 chain and OOS chain ....... up to 85
OOS depth, dB.......32
Signal-to-noise ratio, dB.......96

Dynamic intermodulation distortion (TIM) occurs at signal edges where the slew rate exceeds the maximum allowed at the amplifier output. The main cause of these distortions is the overload of the input stages. To eliminate specific phase distortions, the bandwidth of the amplifier must be at least 250 kHz, which corresponds to an output signal slew rate of about 50 V/µs. To reduce this kind of distortion, you need an amplifier with an operating frequency range without feedback up to 25 kHz or more. The depth of the OOS should not be more than 20 ... 30 dB. The spectrum of the signal fed to the power amplifier should be limited, for example, by using a passive filter with a cutoff frequency of about 100 kHz.

The next type of distortion is due to the nonlinearity of the current transfer coefficient of the output transistors h21e-f(Ik). And since RBX=h21e-Ki (for a cascade with a common collector) is the load of a voltage amplifier with a large output impedance, its gain also changes several times during the period of the output signal, which ultimately causes a nonlinearity in the amplitude characteristic of the amplifier as a whole. To reduce distortions of this kind, it is necessary to reduce the output impedance of the voltage amplifier or increase the input impedance of the output stage, performing it according to a three-stage Darlington circuit, which is undesirable due to an increase in switching time and, as a result, an increase in switching distortion. More details about other types of distortions can be found in [6].

The development of the proposed amplifier (Fig. 1) is based on the concepts presented in [2] and [3].

(click to enlarge)

Circuit solutions are borrowed from [4] and [5]. The amplifier is powered by a rectifier with an ungrounded midpoint, which eliminates the failure of the loudspeaker from the constant component of the output stage. An important advantage of an inverting amplifier is the complete absence of a common-mode component in the input differential stage. Unlike a non-inverting amplifier, this stage does not cause distortion caused by parasitic modulation of the current source voltage on transistor VT2 and the collector-emitter voltage of transistors VT1, VT3. In addition, this solution has good noise immunity in terms of power supply, characteristic clicks do not occur when the power is turned on and off.

The signal pickup from the differential stage is symmetrical, i.e. VT3, VT7, VT8 - OE-OK-OB; VT1, VT4, VT8 - OB-OK-OE. This allows for maximum gain and high common-mode rejection (CMRR). The load of the voltage amplifier on transistors VT7, VT8 with emitter connections is the current generator on the transistor VT11. Stabilization of the output resistance is made using resistors R17, R18. The bias to the output stage is supplied from a voltage generator on transistors VT9, VT10. The quiescent current of the output transistors is set within 50 -100 mA by selecting the resistor R21. Transistor VT14 (VT15) detects the emitter current VT16 (VT17) and turning off (cutoff) the output transistors is prevented, thus eliminating the possibility of switching distortion. Protection of output transistors against current overload is performed using diodes VD2.VD3.

At the output of the amplifier, a Bushe compensator R29, C6 is connected, with the help of which the load impedance becomes purely active.

To avoid the appearance of interface distortions, loudspeakers (ACs) must be connected to the amplifier with wires of the largest possible cross section.

The amplifier is made on a printed circuit board (Fig. 2). Posting details here. Coil L1 is wound on resistor R31 with PEV-2 0,69 wire and contains 14 turns. Transistors VT12, VT13 are mounted on ribbed heatsinks sized 20x15x10. Transistor VT5 can be replaced by diode D220 in direct connection.

High quality transistor UMZCH

(click to enlarge)

Adjusting the amplifier is reduced to setting the quiescent current of the output transistors and setting half the supply voltage at an ungrounded midpoint. In the case of using a stereo pair of amplifiers, each channel is powered by a separate rectifier.

The amplifier was tested together with a corrector amplifier [7] and showed good results. The operation of the amplifier favorably differs in high fidelity of reproduction, which is manifested in an increase in detail and transparency of sound.

References:

V. Ivanov "Transformerless ULF". "Radio" No. 2/70, pp. 29-30.
I. Akulinichev. "ULF with common-mode mode stabilizer". "Radio" No. 3/80, p.47.
I. Akulinichev "On the criticality of power amplifier supply" "Radio" No. 11/84, p.33-35.
Japanese Patent 60-23531, publ. 24-126-85, p.73.
AS USSR 1259472, publ. 01-133-87, p.6
N. Sukhov. "On the issue of assessing linear distortion UMZCH". "Radio" No. 5-7/89.
A. Petrov. "Modern amplifier-corrector" / "Radio amateur" No. 2/92, p.33-34.

Author: A.Petrov

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