UMZCH with a symmetrical input without a common environmental protection. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers

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The amplifier is distinguished by the use of a compensation type local feedback loop, which reduces the distortion of the output stage. The use of a highly linear input stage eliminated the need to introduce a common feedback, and its symmetry in a wide frequency band practically eliminates the influence of external interference on the amplifier.

The advantages of UMZCH with a general environmental protection are well known and have been considered more than once in the specialized literature [1] and on the pages of the Radio magazine. However, despite the high technical characteristics, their real sound reproduction quality is often far from ideal, while relatively simple UMZCHs without a general OOS (or with OOS up to 20 dB) are more natural sounding than UMZCHs with deep OOS.

The developers came to the conclusion that the main culprit is the dynamic distortion associated with the poor choice and implementation of the frequency response and phase response of the amplifier stages covered by a deep OOS. In the audio industry, even a separate direction has arisen - these are amplifiers with a low-stage signal path without a common OOS, and sometimes with compensation for nonlinear distortions [2]. UMZCH of this type is performed on specially selected lamps or transistors operating in class A or AB with a high quiescent current, and are characterized by high cost. The developers of such UMZCH use only high-quality components, the input stages are built according to symmetrical (balanced) circuits, and to achieve a low output resistance, a large number of powerful transistors with selected parameters are used, which, in fact, ensures the repeatability of the declared characteristics of the UMZCH.

In the proposed UMZCH without a common OOS, a symmetrical input stage based on a current follower is used [3]. The UMZCH circuit is functionally simple and includes a voltage amplifier and a current amplifier. Such a structure corresponds to one of the principles of High End audio - a minimum of "electrical length", i.e. a minimum of amplification stages and components in the signal path. The amplifier uses local feedback to reduce output stage distortion.

When developing the UMZCH, the main attention was paid to reducing the number of amplifying stages and increasing the initial linearity of the voltage amplifier.

A feature of the UMZCH is the absence of amplifying stages made according to the scheme with a common emitter (OE) or with a common source (OI). It is known that a differential cascade usually consists of a pair of transistors connected according to a circuit with an OE or RO [1] and introduces noticeable nonlinear distortions [4]. Using switching circuits with a common base (OB), a common collector (OC) and a common drain (OS) together with a short length of the amplifying path, it was possible to create an UMZCH without a common OOS with parameters that are not inferior to those of industrial products. High amplifier parameters are achieved through purely circuit solutions and, unlike the exotic and material science approaches characteristic of the High End, do not require the use of expensive components.

UMZCH has a low-resistance balanced input (1200 ohms) and is designed to work with signal sources that have a symmetrical adjustable output. To fully realize the capabilities of the UMZCH, the signal source must have an "open" output (without coupling capacitors). Note that most modern high-quality signal sources are capable of transmitting a signal without distortion to a relatively low-resistance load (up to hundreds of ohms). In studio or professional equipment, the balanced output impedance of the signal source is already designed for a load of 600 ohms and this is the industry standard. Therefore, in such cases, it seems redundant to achieve a high input impedance in a high-quality UMZCH.

On fig. 1 shows a general block diagram, where the input stage consists of a symmetrical voltage amplifier based on transistors VT1, VT2, connected according to the OB circuit. This stage is loaded on the current mirror (transistors VT3, VT4), the VT5 tracking transistor and the R6CK circuit. The transistor in the switching circuit with OB has a more linear transfer characteristic and better frequency properties [5, 6]. The signal in the form of a differential input voltage (relative to the +U1 bus) is fed to two resistors R1, R2 of equal resistance and is converted into the input current of the emitters of transistors VT1, VT2. The terminal stage A1 is a voltage follower.

A similar voltage amplifier circuit with an additional input differential stage on field-effect transistors was used in [7]. Separate elements of this scheme were cited by I. Dostal in his monograph [8]. The principle of operation of such a voltage amplifier is described in detail in the literature [7, 8].

The final stage A1 can be made on bipolar or field-effect transistors. The output of the voltage amplifier (at point C) is fairly low impedance. This makes it possible to use a single-stage complementary voltage follower as A1, although the possibility of using a two- or three-stage structure with a high current gain in the final stage is not ruled out [1].

Such an UMZCH introduces less distortion into the output signal compared to an amplifier of a classical structure, and the real gain is 10 ... 12 dB. This is true, as a rule, always, if the signal source has a low output impedance and can drive a load of 600 ohms without increasing the non-linear distortion. In such a circuit, the signal source is connected to the +U1 power rail. The UMZCH uses two bipolar power supplies with a transformer T1: one for the voltage amplification stage (winding II, diode bridge VD4 and smoothing capacitors of the power filter C1, C2), and the second for powering the final stage (winding III, diode bridge VD5 and capacitors C3, C4). On fig. 1 common wire of power supplies and further indicated by a rectangle.

The amplifier in fig. 1 is characterized by a fundamentally linear input characteristic that sets the initial linearity of the entire UMZCH. In addition, the UMZCH gain is determined only by the ratio of resistors R6 / R2 (or R6 / R1) and does not depend on the parameters of the transistors used. It can be set with a high degree of accuracy and varied over a wide range. Measurements show that without resistors R5, R6, the gain of the cascade is quite high and is more than 400 at a frequency of 500000 Hz.

The disadvantages of UMZCH include some restrictions on the parameters of the signal source. It must be symmetrical and preferably with an open DC output. In addition, a circuit with a current follower at the input degrades the signal-to-noise ratio [3].

Now consider the UMZCH schematic diagram shown in Fig. 2. The amplifier features high performance and no feedback circuits. The input amplifier is made on transistors VT3, VT4, which are loaded on a cascode-type current mirror VT5, VT6.1, VT6.2, VD5, R8, R13, in which a pair of matched transistors K159NT1V (VT6) is used to improve accuracy.

(click to enlarge)

The main load of the voltage amplifier is resistor R17. Active current sources VT1, VT2 (with elements VD6, VD7, R7, R15) in the emitter circuits of the input transistors increase the linearity of the voltage amplifier in the large signal mode. As a result, the harmonic coefficient of the voltage amplification stage is reduced by almost an order of magnitude and amounts to, for example, 0,007% at a frequency of 2 kHz with an output voltage of 31 V (rms).

The composite voltage follower on the elements VT9, VT10, VT12-VT14, VD13, R18, R19, R22 provides effective decoupling of the voltage amplifier from the final stage. This solution almost completely eliminated the influence of the non-linear capacitance of the gate-drain transistor VT9 on the parameters of the voltage amplifier. In this follower, the input capacitance VT9 practically does not change, since the voltages between the terminals of this transistor are fixed.

Incomplete use of the supply voltage in the repeater on the positive half-wave of the signal required its increase, so the bipolar supply voltage is asymmetrical with respect to the common wire of the power supply and is +57 V and -52 V.

The final stage of the UMZCH has no features and is a push-pull follower on powerful transistors VT15 - VT20, operating in class AB with a quiescent current of 300 mA. A stable current source of 220 mA (VT7, VT8, R11, R14, VD9-VD12) is also built according to the cascode OB-OB circuit. Transistors VT7, VT8, VT10, as well as powerful transistors, are located on heat sinks. The quiescent current of the final stage stabilizes the temperature sensor on the VT11 transistor, which has thermal contact with the transistors of the final stage.

The integrator based on precision op amp K140UD17 (DA1) and elements R1-R4, R17, C1-C4, VD1-VD4 maintains the minimum DC voltage at the UMZCH output, independent of temperature and supply voltage asymmetry.

To decouple the stages, increase the linearity of the UMZCH and increase the efficiency of the final stage, the voltage amplifier is powered by a stabilized voltage of +57 V and 52 V, and the final stage is supplied with an unstabilized voltage of ±44 V.

The differential gain of the UMZCH is determined by the ratio 2 (R17 / R6) and is about 45. Connecting the amplifier output to point A through the R5C5 circuit leads to partial compensation of the nonlinear distortions of the final stage and reduces the output impedance of the UMZCH at a frequency of 1 kHz from 0,2 to 0,035 ohms (measurements were made without the L1R28 output circuit). The output impedance of the UMZCH varies slightly in the frequency range up to 10 kHz and is 0,05 ohms at a frequency of 20 kHz. The measurements showed that the output resistance of the UMZCH does not depend on the change in the quiescent current of the terminal stage over a wide range (in the range of 50 ... 3000 mA), which indicates the effectiveness of the applied environmental protection.

To measure the harmonic coefficient (Kg) of the UMZCH, an automatic non-linear distortion meter S6-8, a spectrum analyzer S4-74, and a GZ-118 signal generator together with a balancing device were used. Three 20 Ohm PEV-50 resistors connected in parallel (resistance 7 Ohm) are used as a load equivalent, and five such resistors are used for a 4 Ohm equivalent. The output voltage was measured using a VZ-39 voltmeter. The lower limit of measuring Kg with such a device is almost -90 dB.

The total Kg of the UMZCH without distortion compensation (the R5C5 circuit is disabled) with an output power of 105 W and a load of 7 ohms at a frequency of 1 kHz was 0,099%, and at 20 kHz - 0,096%. The signal spectrum contains mainly the second and third harmonics of comparable amplitude, as well as higher harmonics of smaller amplitude (the consequences of the operation of the final stage in the AB mode).

When the local R5C5 circuit was connected, Kg UMZCH at a frequency of 1 kHz decreased to 0,035%, and at a frequency of 20 kHz - up to 0,043% with the same output power. With a maximum output power of 125 W at 7 ohms at a frequency of 1 kHz (output signal at the limit threshold), distortion in the UMZCH still does not exceed 0,1%.

It should be noted that the terminal transistors are not specially selected, and in the case of their preliminary selection, it is possible to improve the characteristics of the UMZCH. It so happened that in this UMZCH layout, the actual spread of the gain in the emitter current of a complementary pair of equivalent transistors turned out to be small, about 10%. The generalized value of the current gain at lK \u1d 5 A and Uke \u864d 96 V for the upper side (three KT865A transistors connected in parallel) is 87, and for the lower one (three KT4A transistors) - 170. At high values ​​​​of the collector current, the current transfer coefficient of the base of the transistors final stage is reduced. The maximum output power of the UMZCH at a load of 1 ohms is 0,18 W (at the same time, at a frequency of 4 kHz, Kg = 2%). Using more powerful imported devices in the final stage, the output power of the UMZCH at a load of XNUMX ... XNUMX Ohms can be increased even without increasing the number of transistors.

Intermodulation distortion in UMZCH is less than -70 dB (0,03%) when a measuring signal with an amplitude just below the limiting level acts on a 7 ohm load, which is the sum of two sinusoidal signals of equal amplitude with frequencies of 20 and 21 kHz. Intermodulation distortions were evaluated using the S4-74 spectrum analyzer, which has a dynamic range of at least 70 dB. The difference frequency component of 1 kHz was estimated. The amplitude of this spectral component lies at the noise level of the spectrum analyzer and is distinguishable only at large integration times of the analyzer (bandwidth - 300 Hz, sweep - 5 s). It should be noted that this measurement mode was chosen as the most informative, and when real sound signals are amplified, such an extreme situation is unlikely.

Ниже приведены main technical specifications UMZCH layout (Fig. 2) when working on an active load equivalent (resistor).

• Rated input differential voltage, V......1,2
• Rated input impedance, Ohm......619
• Rated frequency range, Hz......5...200000
• Signal-to-noise ratio, dB, unweighted (in the band from 0,5 kHz to 1 MHz), not less than ...... 93
• Output power (long-term) at a load of 7 ohms, in the frequency range of 20 ... 20000 Hz, W, not less than ..... 105
• Output power (long-term) at a load of 4 ohms, in the frequency range of 20 ... 20000 Hz, W, not less than ..... 156
• Output impedance in the frequency range 20...20000 Hz with connected circuit L1R29, Ohm, not more than......0,06
• Output voltage slew rate, V/µs, not less than......60
• The level of intermodulation distortion at the maximum output voltage of the UMZCH at a load of 7 ohms (frequency 20 and 21 kHz, amplitude ratio 1:1),%, not more than ...... 0,03
• Total harmonic distortion at a power of 50 W / 75 W / 105 W at a load of 7 Ohms,%, at a frequency of 1 kHz ...... 0,017 / 0,026 / 0,035
• 10 kHz ..... 0,018 / 0,031 / 0,039
• 20 kHz.....0,026/0,033/0,043
• Total harmonic distortion at a power of 75 W / 100 W / 156 W at a load of 4 ohms,%, at a frequency of 1 kHz ...... 0,043 / 0,055 / 0,082
• 10 kHz.....0,045 / 0,056 / 0,097
• 20 kHz......0,049/0,06/0,131

In UMZCH, domestic and imported components can be used. Transistors KT9115A (VT3, VT4) are best selected in pairs with the same current gain (even better - use matched pairs of high-voltage pn-p transistors made on the same substrate). Instead of KT9115A, you can use KT632B or imported devices 2SA1184, 2N5415. Instead of 159NT1V, you can use any matched pair of transistors of the npn structure (the selection criterion is the largest possible h21E). In UMZCH, instead of KP902A, low-power MOS transistors of the KP305 series work well. Resistors R5-R8, R13 and R15-R17 - C2-29, with R6 and R16, R7 and R15 with the smallest possible tolerance (in the author's version, these resistors have a tolerance of 0,05%). The remaining resistors are MLT and C5-16MV. Coil L1 contains 9 turns of insulated wire with a diameter of 1,53 mm, wound with a pitch of 2,5 mm on a mandrel with a diameter of 10 mm. Capacitors - KM-6, K73-16, K73-17.

Due to the peculiarities of connecting the signal source to the UMZCH input, the principle of "grounding" of the amplifier housing must also be changed. The "+57 V" bus of the UMZCH stabilized power source should be connected to the metal case of the structure. The common wire of the signal source is connected to the same point of the common wire.

The common wire of the power circuits and power filter capacitors must be isolated from the amplifier case. You also need to isolate the output terminals of the UMZCH. If the UMZCH uses two separate and completely independent power supplies for each of the channels, then their "+57 V" power buses should be connected at one point with the UMZCH case. The middle points of the power supplies do not need to be interconnected.

In the case of a "double mono" architecture, two UMZCH channels are connected to each other (and to the structure case) only through the +57 V power bus, which, in the absence of common high-current circuits, favorably affects the decoupling between the channels.

This version of the UMZCH was designed to work with a professional mixing console that does not have isolation capacitors at the output (DC output). With this method of "power supply" through the input resistors, the UMZCH always consumes a small direct current from the signal source (about 2 mA for each input). In other cases, for normal operation of the UMZCH, an audio signal source with a symmetrical low-impedance output and the ability to adjust the signal level will also be required.

In the absence of a signal source that has a balanced output, you can use any unbalanced signal source, supplementing it with a device that converts the unbalanced signal to balanced. Today, there are quite a few options for devices that implement this function: from the simplest ones based on a balancing transformer to specialized microcircuits, for example, SSM2142.

For the same purposes, the author sometimes used a device known as the "Di-Box" (Active Direct Inject Box), model Dl 100 from Behringer. Such devices are popular with musicians working with "live sound", and consist of a high-quality balun transformer and a voltage follower. The nonlinear distortions introduced by them are quite small (usually less than 0,005%).

On fig. Figure 3 shows a "simmetrator" circuit made with a cross-symmetrical OS on dual op amps DA1 (in one package) and precision resistors R1-R8.

The degree of symmetry of the output signal depends on the individual spread of paired resistors and will actually require additional adjustment (the resistance of these resistors can be units or tens of kilo-ohms). A more complex circuit with the possibility of adjusting the symmetry is shown in Fig. 4 (resistors R1-R14 have a tolerance of 0,05%). All measurements of the UMZCH parameters were performed using this device.

The proposed balancing devices can be used as a buffer element of the output stage of the signal source, although the best solution should be the use of a specialized SSM2142 microcircuit, which, at a cost of about $4, already contains all the necessary op-amps and resistors (30 kOhm) and is specially designed to operate on a load of 600 Ohm. The non-linear distortion of the node on the SSM2142 is less than 0,006% with an output signal of 10 V at a load of 600 Ohms in the frequency range of 20.. .20000 Hz.

A properly assembled amplifier almost does not need to be adjusted. Before turning on the engine of the tuning resistor R20 must be in the upper position according to the diagram. Before the first switching on and subsequent adjustments without load, two powerful protective resistors with a resistance of 10 ... 20 Ohms must be connected to the power supply circuits of the final stage. These resistors will protect the final stage transistors, for example, in case of wiring errors.

If self-excitation occurs, it is necessary to increase the capacitance of the neutralization and correction capacitors (C5, C6).

Next, check the constant voltage at the output of the UMZCH. It should not be more than 1...2 mV. Then, according to the voltage drop across one of the protective resistors, by adjusting the resistance R20, the quiescent current of the final stage is set. After warming up the amplifier for 1 - 2 hours, its value should be 300 ... 350 mA.

At this, the adjustment of the UMZCH should be completed and the protective resistors should be excluded from the power circuits of the final stage.

In a balancing device, the op amps should work well for a load of 600 ohms. Here you can use OPA604 (OPA2604), OPA134 (0PA2134, 0PA4134), LT1468, LT1469, LM6171, LM6172. LM837, AD841 are also suitable.

Literature

1. Shkritek P. Reference guide to sound circuitry: Per. with him. - M.: Mir, 1991, 446 p.
2. Shibasaki Tsutomu. Super-linear amplifiers without negative feedback. - Musen to jikken = Stereo Technic, 1989,76, No. 5, p. 96-103.
3. Polonnikov D. E. Operational amplifiers: Principles of construction, theory, circuitry. - M.: Energoatomizdat, 1983, 216s.
4. Op'Teynde Frank Wambaco Piet, Sansen Willy. On the relationship between the CMRR or PSRR and the second harmonic distortion of differential input amplifiers. - IEEE Journal Solid-State Circuits, 1989, 24, no. 6, p. 1740-1744.
5. Utochkin GV Integral and multi-transistor cascades of selective amplifiers. - M.: Energy, 1978, 80 p.
6. Voishvillo GV Modern technology of signal amplification. - M.: Soviet radio, 1978, 104 p.
7. Orlov A. UMZCH with single-stage voltage amplification. - Radio, 1997, No. 12, p. 14-16.
8. Got I. Operational amplifiers: TRANS. from English. - M.: Mir, 1982, 512 p. Author: A. Orlov, Irkutsk

Author: A. Orlov, Irkutsk

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