RU2527185C1 - Class av two-step amplifier - Google Patents

Abstract

FIELD: radio engineering, communication.

SUBSTANCE: class AV two-step amplifier includes a bias voltage controller and a stabilisation module which includes two branch current meters and an adder, wherein the output of the stabilisation module is connected to the input of the bias voltage controller, and a detector of the minimum value of the sum of the branch currents, the output of which is the output of the stabilisation module.

EFFECT: reduced distortions in a wide signal current range while maintaining high temperature stability and stability during possible dispersion or drift of transistor parameters.

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Description

FIELD OF THE INVENTION

The present invention relates to electronics, and more specifically to linear amplifiers of audio signals, which can be used both in high-class equipment and high reliability, and due to its relative simplicity, efficiency and self-tuning property in most transistor sounding devices, as well as in monitoring and measurement devices , automation and specialized microcircuits.

State of the art

Class AB amplifiers are the most common among linear amplifiers, since, while remaining the simplest type of amplifiers, ideally they allow combining the high efficiency of Class B amplifiers with non-distorting amplification of Class A amplifiers. However, the formation of the bias voltage necessary for this voltage, supporting the push-pull cascade arms on the verge of opening by positive and negative signal half-waves, it was and remains the main problem of constructing such amplifiers, the problem of stabilizing their parameters s. This is explained by the instability of the control characteristics of transistors, their dependence on temperature, and on the signal level, and on the spread or drift of the parameters of the same transistors. As a result of this, an unambiguous method (rule, criterion, algorithm) for regulating the bias voltage has not yet been formulated, which would allow the amplifier to automatically set the optimal bias voltage for its transistors. The complexity of the problem is illustrated even by the title of the article by V. Groshev. “Methods for ensuring the certainty and stability of the initial mode of push-pull bipolar transistor amplifiers,” published in the scientific and theoretical journal Radiotechnika No. 2, 1989. It is not about stabilization, but only about ensuring the “certainty” of the mode.

The level of modern technology of class AB amplifiers can be estimated from the book: Sukhov N.E. Radio hobby. The best designs of ULF and do-it-yourself subwoofers. - St. Petersburg: Science and Technology, 2012. It contains the most interesting circuit designs of power amplifiers, made by Russian and foreign authors on various components, including imported ones.

Among the solutions protected by copyright certificates and patents, push-pull power amplifiers are known, consisting of a main powerful amplifier stage and an auxiliary channel that controls the main mode of operation with an adjustable bias circuit. Such are the two-stroke power amplifier (SU 652686, publ. 03/15/1979, IPC: H03F 3/20), in which the auxiliary cascade contains pulse-width converters, power amplifier (SU 1119163 A, publ. 10/15/1984, IPC: H03F 1 / 34), into which a parallel limiter is introduced from above, and a power amplifier (SU 1730714 A1, publ. 04/30/1992, IPC: H03F 3/20), containing limiters, adders, additional transistors and resistors. The main goal of these inventions - reducing distortions and increasing efficiency - is solved by different means, but in all cases these solutions are quite complex, not obvious, and the controls for the amplifier modes themselves need to be configured.

There are many technical solutions that solve the problem of stabilizing the quiescent current of amplifiers through the use of special feedbacks. Known, in particular, a transformer push-pull low-frequency amplifier (SU 307487 A, publ. 01.01.1971, IPC: H03F 3/26, H03F 1/34), covered by a negative voltage feedback circuit, which increases the coefficient of non-linear distortion due to non-linear non-inertial current feedback of the transistors of the terminal stage, introduced to stabilize the initial current of the amplifier.

The closest analogue, according to the author, is an amplifier, known from document SU 663073 A (publ. 05/15/1979, IPC: H03F 1/34). It describes a transformerless transistor push-pull amplifier of class AB, covered by negative voltage feedback, containing series-connected preliminary, as well as terminal and terminal stages made in series-parallel circuit, while the collector-series gap is connected in series to the transistor collector circuit of the last preliminary stage the emitter of the first additional transistor and a resistor, to the terminals of which the base of the transistors is preliminarily connected about the cascade, a parallel diode-resistive circuit is included in each collector circuit of the terminal stage transistors, the base-emitter junction of the second additional transistor is connected to the junction of the collectors with the diode-resistive chains of the terminal stage, the collector of which is connected to the RC circuit through the parallel RC circuit the corresponding power bus, and directly to the base of the first additional transistor.

This amplifier has all the elements except the distinctive that are used in the claims of the present invention (see drawing SU 663073 A). So, the bias voltage regulator is implemented by a transistor 9 and a resistor 10, two shoulder current meters are implemented by resistors 13 and 14, the adder is formed by a series connection of these resistors, from which the total voltage is removed. Elements 11 ÷ 18, which do not directly participate in signal amplification and serve only to stabilize the amplifier mode, can be allocated to the stabilization module.

The disadvantage of this amplifier is that it does not reduce distortion in a wide range of signal currents. This is explained by the fact that with sufficiently strong feedback signals, a bias voltage is set at which the initial current of the amplifier, i.e. the amplifier current when the amplified signal passes through zero becomes less than the quiescent current of this amplifier or even equal to zero. In this case, distortions of the “step” type arise.

The claimed invention is aimed at eliminating the disadvantages of the known solutions.

Disclosure of invention

The technical result of the invention is the formation of such a bias voltage that supports the terminal transistors on the verge of opening or in a state of weak controlled opening, that is, ensures a constant initial current of the amplifier, i.e. current when the amplified signal passes through zero at any changing signal level. Ensuring the constancy of the initial current of the amplifier at the level of its quiescent current provides a reduction in distortion in a wide range of signal currents while maintaining high temperature stability and stability with a possible spread or drift of transistor parameters.

The specified result is achieved in the proposed push-pull amplifier of class AB, which contains a bias voltage regulator and a stabilization module with two shoulder current meters included in it and a summing device, where the output of the stabilization module is connected to the bias voltage regulator input, while ensuring a constant initial current with varying At the signal level, a detector of the minimum value of the sum of the currents of the arms, the output of which is the output of the stabilization module, is introduced into the amplifier.

It is known that to stabilize something, in this case, the initial current of the amplifier, is possible only by measuring it, comparing it with a reference value and automatically controlling it. In order to identify the possibility of measuring the initial current of the amplifier against a background of a randomly changing signal current, the inventor proposes to consider the changes in the arm currents of the push-pull cascade and their sum under the assumption that the signal current in the load changes according to the simplest sinusoidal law:

i _n = I _m sin α,

Where

i _n - instantaneous value of the load current,

I _m is its amplitude,

α = Ωt is the phase angle,

Ω is the operating frequency,

t is time.

The nature of the change in currents in the shoulders of the push-pull cascade is shown in Fig. 1a, and the sum of the absolute values of the currents is shown in Fig. 1b.

The load current in the push-pull cascade is determined by the difference of the shoulder currents or the sum of the absolute values of the increments of the shoulder currents:

i _n = [Δi ₁ ] + [Δi ₂ ]

At low signal currents, both arms of the amplifier operate in Class A linear mode. The increments of the arm currents in absolute value are equal to half the load current

$$\left[\Delta i_{\mathrm{one}}\right]=\left[\Delta i_2\right]=0.5i_n=\frac{I_m}{2}\sin \alpha$$

and expressions for the shoulder currents will look like:

при 0≤α≤α_кр $$\left\{\begin{array}{c}{i}_{\mathrm{one}}=I_{n\mathrm{but}h}+\frac{I_m}{2}\sin \alpha \\ i_2=I_{n\mathrm{but}h}-\frac{I_m}{2}\sin \alpha \end{array}\right\}$$

at 0≤α≤α _cr

Hereinafter, α _cr designates the critical phase angle, above which the amplifier switches from class A mode to class B.

If all currents are normalized with respect to the maximum load current:

и $$\frac{I_m}{I_m}=1$$

, то $$$$

$$\left\{\begin{array}{c}{i}_1=I_{нн}+\frac{1}{2}\sin \alpha \\ i_2=I_{нн}-\frac{1}{2}\sin \alpha \end{array}\right\}$$

при 0≤α≤α_кр $$\frac{I_{n\mathrm{but}h}}{I_m}=I_{nn}$$

and $$\frac{I_m}{I_m}=\mathrm{one}$$

then $$$$

$$\left\{\begin{array}{c}{i}_{\mathrm{one}}=I_{nn}+\frac{\mathrm{one}}{2}\sin \alpha \\ i_2=I_{nn}-\frac{\mathrm{one}}{2}\sin \alpha \end{array}\right\}$$

at 0≤α≤α _cr

For α = α _{cr, the} current of the second arm decreases to zero, i.e.

Из этого определяем α_кр=arcsin2I_нн $$I_{nn}-\frac{\mathrm{one}}{2}\sin {\alpha }_{\mathrm{to}R}=0$$

From this we determine α _cr = arcsin2I _nn

In class B mode, the load current is determined by the current increments of only one arm:

при $${\alpha }_{кр}\le \alpha \le \frac{\pi }{2}$$

$$\left\{\begin{array}{c}{i}_{\mathrm{one}}=\sin \alpha \\ i_2=0\end{array}\right\}$$

at $${\alpha }_{\mathrm{to}R}\le \alpha \le \frac{\pi }{2}$$

характер изменения токов повторяется в обратном порядке, а при α>π меняется знак тока нагрузки и инициатива в его формировании переходит ко второму плечу.At $$\alpha \le \frac{\pi }{2}$$

the nature of the change in currents is repeated in the reverse order, and for α> π the sign of the load current changes and the initiative in its formation passes to the second arm.

The sum of the currents of the shoulders:

$$\{\begin{array}{c}{i}_{\mathrm{one}}+i_2=2I_{nn}\\ i_{\mathrm{one}}+i_2=\sin \alpha \end{array}\begin{array}{c}PR\mathrm{and}0\le \alpha \le {\alpha }_{\mathrm{to}R}\\ PR\mathrm{and}{\alpha }_{\mathrm{to}R}\le \alpha \le \frac{\pi }{2}\end{array}$$

has a constant minimum value, determined only by the initial current of the amplifier:

(i ₁ + i ₂ ) _min = 2I _nn

The above calculations allowed the author of the present invention to conclude that in order to stabilize the class AB amplifier mode at any desired initial current, it is necessary and sufficient to stabilize the minimum value of the sum of the currents of the arms, equal to twice the initial current of one arm. This principle has been used in the development of the present invention.

Thus, regardless of the specific design of the amplifier, whether it is a transformer or transformerless amplifier with parallel or series switching of the arms, we can name the elements necessary for stabilizing its mode. These elements are depicted in a generalized block diagram of the proposed class AB amplifier (see Figure 2).

Brief Description of the Drawings

Figure 1 shows the nature of the change in currents in an amplifier of class AB.

Figure 2 shows the structural diagram of a push-pull amplifier class AB.

The implementation of the invention

The proposed amplifier operates as follows.

Current meters 1 and 2 constantly measure the shoulder currents of the push-pull output stage. The summing device tracks the sum of the instantaneous values of the currents of the shoulders. The detector of minimum values of the sum of the currents of the arms determines the minimum value of this sum, which occurs in the idle mode of the amplifier or when the amplified signal passes through zero.

The output voltage of the detector is determined only by the initial current of the amplifier and nothing more. It regulates the bias voltage of the amplifier so that the initial current of the amplifier is kept constant for any destabilizing factors and for any signal amplitude. This ensures stability of the mode and minimization of distortion in any real conditions.

Claims (1)

A push-pull amplifier of class AB, containing a bias voltage regulator and a stabilization module with two shoulder current meters included in it and a summing device, the output of the stabilization module being connected to the bias voltage regulator input, characterized in that in order to maintain a constant initial current with a changing signal level in the amplifier introduced a detector of the minimum value of the sum of the currents of the arms, the output of which is the output of the stabilization module.

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