RU2183380C2 - Multistage transistor amplifier - Google Patents

Abstract

FIELD: electronics. SUBSTANCE: multistage transistor amplifier can be used to design amplifiers based on transistors which do not distort amplified signal. It has stages built on p-n-p and n-p-n transistors connected with emitters through resistor R. Resistive loads are connected to collectors of transistors which bases form common controlling electrode coupled to power supply source via one common element of feedback of stages. Bases of other transistors of each next stage starting with second transistor are connected to load of previous stage. Collector loads of stages are comprised of 2N semiconductor diodes connected in series and of resistor of NR value, where N is integral number. EFFECT: improved compensation for nonlinear distortions. 12 dwg

Description

 The invention relates to the field of electronic technology and can be used to build amplifiers on transistors that do not distort the amplified signal. According to established terminology, such amplifiers made on vacuum tubes and transformers are called High-end (hi-end: "so good that there is nowhere else"). The invention can simplify, reduce the cost, miniaturize High-end equipment.

 A well-known amplifier (cascade of a multistage amplifier), which is a combination of transistors of pnp and npn types of conductivities, is a book by Low A. et al. "Fundamentals of semiconductor electronics", trans. from English edited by Halperin E.I., M .: Soviet Radio, 1956, p. 258. This device is called Lowe "amplifier with additional symmetry." According to modern terminology, this is an “amplifier with complementary transistors” (K-cascade, K-amplifier). These cascades on complementary transistors are available in the claimed object.

 A diagram of a K-cascade adapted to our needs is shown in FIG. 1. Here, using sources 3, 5, 9 of resistors 4, 7, transistors 2, 6 are set to direct currents, voltages. Input signal 1 (sinusoidal voltage) applied to the base of transistor 2 increases the currents of transistors 2 and 6 if the half-cycle of the signal is positive. A negative half-cycle reduces the currents of the transistors. Alternating current flows through the emitter resistance 4 and the collector load, consisting of a resistor 7, a diode 8.

Figure 1 shows the condition for the compensation of nonlinear distortions: if the resistance R and the desired voltage gain N = 1, 2, 3 ... are selected, then the collector load should be constructed according to the formula Z _K = NR + 2ND, i.e. 2N diodes should be connected in series with the resistor in the collector. Then the nonlinear distortion of the signal at the outputs of K6, E2 are compensated. See the proof of distortion compensation below.

 The technical result of the claimed invention cannot be achieved using K-stages, since a multi-stage amplifier built of n pieces of K-stages, in the usual way, has a significant drawback: a significant number of auxiliary elements (RC circuits, etc.) are required to connect the cascades. . This is bad - each circuit introduces non-linear distortion during transients.

 The most suitable technical solution for the functional purpose and technical essence is an n-cascade amplifier, in which "cascades with additional symmetry" are rationally connected, the operation of the cascades for alternating current is provided by one capacitor (Fig. 2). A multistage transistor amplifier whose cascades are built of pnp and npn transistors connected by emitters, resistive loads are included in the collectors of one of the cascade transistors, the base of one of the transistors of each subsequent stage, starting from the second, is connected to the load of the previous cascade, and the bases of all others transistors are connected together, forming a common cascade control electrode connected to a power source through one common cascade feedback element. This amplifier is available in the claimed object.

 The scheme was proposed by the authors of the application in 1972: Prishchepov G.F., Prishchepova T. M. Multistage amplifier. Auth. testimonial. 327568 (USSR). Publ. in the bull. "Discoveries. Inventions. Industrial Designs. Trademarks", 1972, 5. A scientific publication on this subject is given in the collection "Semiconductor Electronics in Communication Technology". Vol. 19. Ed. I.F. Nikolaevsky. Moscow, Communications, 1978, p. 35-45: article "Transistor structure for balanced amplifiers." This transistor structure is accepted by us as a prototype.

 The technical result of the claimed invention cannot be achieved in the prototype due to its shortcomings. The disadvantages of the amplifier of FIG. 2 are that the non-linear distortions are the same as for a conventional transistor amplifier. In other words, there is no compensation for non-linear distortions.

The challenge facing the inventor was that it was necessary to compensate (destroy) the nonlinear distortion of the cascades of the multistage transistor amplifier of FIG. 2. The technical result is that:
- there is a condition for the compensation of nonlinear distortions for the amplifier stage on the transistor (figures 1 and 2);
- change the collector and emitter circuit of the cascade amplifiers of figure 2 (prototype), etc. the transition is made to figure 3 - the claimed object.

 The technical result is achieved by the fact that in series with the emitters of the transistors of each stage, resistors of the value R equal to the resistor of the emitter load of the last stage are connected, and the collector loads of the previous stages are composed of 2N pieces of series-connected semiconductor diodes and a resistor of the value NR, where N is an integer (1, 2, 3 ...).

 As a result, new significant features of the claimed object appear in comparison with the prototype: the improved transistor structure of Fig. 3 contains sets of transistors of pnp and npn types of conductivity, sets of diodes, sets of resistors, only one capacitor.

New significant features of the claimed object give rise to the following new properties:
- non-linear distortions of amplified signals are compensated. Transistor High-end equipment achieves the quality of "tube-transformer High-end";
- High-end amplifiers can be miniaturized, can be made in the form of a cheap microcircuit with low energy consumption.

 These properties, previously unknown for the prototype, suggest that the claimed technical solution meets the criteria of "novelty", inventive step and "industrial applicability".

 Theoretical evidence of the possibility of achieving the goal is given below in the description of the statics and dynamics of the amplifier.

 The present invention is clarified by the drawings.

 In FIG. 1 shows a “cascade with additional symmetry” (according to Low A.) or a complementary amplifier, K-amplifier.

 Figure 2 presents a method of connecting K-stages to a multi-stage amplifier (transistor structure). The structure can be extended, consist of n-pieces of cascades, where n is an even number. The last cascade is performed by an emitter follower.

Figure 3 shows the inventive object is a multi-stage transistor amplifier. It is made up of complementary transistors. A resistor of nominal R is introduced into the emitters of each pair of transistors, collector loads are constructed according to the formula Z _K = NR + 2ND. N = 1, 2, 3 ... The expression 2D arises here due to the series connection of two pn junctions of the BE transistors, since it is necessary to compensate for the nonlinearity of these two junctions. Conventionally, only one diode is shown in collector loads (instead of 2N pieces of diodes).

 In FIG. 4 shows a single stage without auxiliary circuits with a transistor and a load diode. The cascade is used to prove the non-linear distortion compensation condition.

Figure 5 shows a graphical solution to the problem of amplifying the test signal u ₁ (linearly changing voltage) by a cascade in which a semiconductor diode is used as a collector load. The output signal is u ₂ . It is obtained by the projection method.

Figure 6 shows the experimental dependence of the cut-off frequency of the frequency response in the low frequency region for a parallel-symmetric transistor structure with variations in capacitance C _OB .

 7 is an illustration of a method for measuring instantaneous nonlinear distortion of a signal transmitted through an amplifier.

The list of circuit elements:
In Fig.1: 1 - signal source; 2, 6 - transistors of the amplifier;
3, 5, 9 - power sources that determine direct currents and voltages of transistors 2, 6;
4 - resistor in the emitters of the cascade, the resistor value is R Ohm;
7, 8 - collector load of the amplifier, - serial connection of diodes (2N units) and a resistor of 2NR value. Here N is an integer.

Figure 2: transistors are numbered according to the cascades: 1, 1 ₀ ; 2, 2 ₀ . . . The index "o" indicates the connection of the base of this transistor to the common OB electrode.

4, 5 - power sources;
6 - a resistor depicting the resistance of the signal source;
7, 8, 9 - collector load resistors of cascades 1, 2, 3;
10 - load resistance of the output emitter follower;
11 - capacitor of the common electrode of transistors C _AB ;
n, is the nth, even cascade repeater of the extended structure.

In FIG. 3: an R-value resistor is introduced into the emitters of each pair of transistors, collector loads are constructed according to the formula Z _K = NR + 2ND. N = 1, 2, 3 ... The expression 2D arises here due to the series connection of two pn junctions of the BE transistors, since it is necessary to compensate for the nonlinearity of these two junctions. Conventionally, only one diode is shown in collector loads (instead of 2N pieces of diodes).

 Transistors connected to the OB electrode (common base of the structure) are assigned numbers with indices "o". These pnp transistors have even numbers, and npn transistors have odd numbers.

The structure can be extended, consist of n cascades, where n is an even number. The last cascade is made by an emitter follower. The low resistance resistor R _c at the input represents the resistance of the signal source.

Two power sources (batteries) E1, E2 can be replaced by one source E = E1 + E2. In this embodiment, the signal is supplied to the base B1 through the capacitor C _B1 . Resistor R _B1 and source E determine the direct current base of the first transistor - see FIG. 3b.

1 and 1 ₀ ; 2 and 2 ₀ ; 3 and 3 ₀ ; n and n ₀ are complementary transistors of the first, second, third and nth stages of a multistage amplifier;
4 - resistors in emitters of complementary transistors (nominal R Ohm);
5 - collector load resistors (nominal NR Ohm);
6 - diodes compensators for nonlinear distortion 2N - pieces;
7 - capacitor in the circuit OB structure;
8, 9 - sources of EMF;
10 - resistance of the input signal source;
11 is an embodiment of connecting an input signal through a capacitor 11;
12 is a resistor that sets the base current for the first transistor.

Figure 4: u ₁ , u ₂ - voltage input and output signals.

 1 - transistor; 2 - diode-load of the collector circuit.

Figure 5: point 1 voltage u _{1 is} projected on the current-voltage characteristic of the transistor 2, then on the load line 3 and down. Here comes the "horizontal of time." At the intersection of these lines, point 4 is obtained, which belongs to the output voltage u ₂ .

5 - family of output current-voltage characteristics of the transistor;
ab - axis of symmetry of nonlinear graphs.

Figure 6 shows the experimental dependence of the cutoff frequency of the frequency response in the low frequency region for a parallel-symmetric transistor structure with variations in capacitance C _OB (N = 10, R = 100 Ohms, I = 1 mA, β = 50).

In FIG. 7a: G — test signal generator (ramp voltage); U - studied amplifier; A - attenuator (voltage divider); F - phase shifter; DO - two-channel oscilloscope;
u ₁ , u ₂ - input and output voltages reduced to the same level using an attenuator; Y, Δy; X, Δx - measurement results.

 An example of a specific implementation of the amplifier is shown in Fig.3. Using this figure, the statics and dynamics of the device are explained.

Statics, communication elements. Ensuring the normal operation of the structure by direct current with selected N, R, I, the same transistors, diodes, known voltages U _be , U _d , β transistors is reduced to providing EMF sources E1, E2: E ₁ = NRI + 12nu _d ; E = E ₁ + 2RI + 4U _BE. Here, N = 1, 2, 3 ... is the desired voltage gain for the cascade of the structure; I is the selected collector current. If one power source is used, then its EMF
E = E ₁ + E ₂ .

The current base of the first transistor in this embodiment is set using the resistance R _B1 , the signal is fed through a capacitor:
R _B1 = E ₁ / I _b1 , I _b1 = I / β.
As a result of the above conditions, the currents of the transistors 1, 1 ₀ , 2, 2 ₀ , etc. turn out to be the same and equal to the selected current I. The base-collector voltages of the transistors 1, 3, 5 ... are equal to E1. The voltage of the base-collector of transistors 1 ₀ , 2 ₀ ... is equal to IR + 2U _BE .

Typical temperature instability of the voltage of the pn junctions is - 2 mV / deg, therefore, to stabilize the transistor currents, it is necessary to ensure the voltage drift E1, E2, E (mV / deg):
- 4N for E1; - 4 (N + 4) for E2;
- (8N + 16) for E.

Dynamics. According to alternating current, the cascades of the structure are autonomous, independent, if the resistance of the capacitance C _about = 0. Amplification stages are formed by transistors 1 and 1 ₀ , 2 and 2 ₀ , 3 and 3 ₀ , etc. However, at low frequencies through the system C is formed _{on the} "distributed" negative feedback stages. This communication system determines the lower cutoff frequency f _n of the passband at the level of -3 dB. The experimental graph f _n = f (C _about ), shown in Fig.6, obtained at I = 1 mA, N = 10, R = 100 Ohms, β = 50. The graph is well described by the formula
2πf _n = 1 / t; t = r _about C _about ; r _about = 40 Ohms.

Here r _about - the differential resistance of terminals ON - "earth".

 The theoretical justification for the possibility of implementing the device, the possibility of compensating for non-linear distortions is presented by the sections "simplified analysis" and "generalization".

Пусть вольтамперные характеристики (ВАХ) перехода БЭ и диода-нагрузки одинаковы: J= a exp(bu)-а. Здесь а - ток насыщения перехода, b - показатель степени экспоненты. Связь между напряжением диода и его током описывается функцией логарифма
u=1/b ln[(i+а)/а].Simplified analysis. We use the functional minimum of the amplifier stage: a bipolar transistor with a diode-load in the collector (Fig. 4). The cascade diagram is shown conditionally, by alternating current, without a power source, without auxiliary elements. Denote the input signal u _in = ΔU _BE = u ₁ ; the voltage increment of the diode load ΔU _D = u ₂ . The increment of the currents of the emitter, collector, diode:

Let the current-voltage characteristics (CVC) of the BE transition and the load diode be the same: J = a exp (bu) -a. Here a is the transition saturation current, b is the exponent. The relationship between the voltage of the diode and its current is described by the logarithm function
u = 1 / b ln [(i + a) / a].

If the current i is caused by u ₁ , and then the voltage u _{2 of the} diode occurs, then
u ₂ = (1 / b ₂ ) ln [(a exp (b ₁ u ₁ ) -a + a) / a] = u ₁ if b ₁ = b ₂ .

As a result, u ₂ , repeats u ₁ , - non-linear distortions are compensated. The non-linear distortion compensation is confirmed graphically using FIG. 5. It is indicated here:
1 - test input signal u ₁ - "sawtooth"voltage;
2 - current-voltage characteristic (CVC) of the BE transition;
3 - CVC of the collector load diode, built from point E to the left;
4 - output voltage u ₂ obtained by the projection method;
5 - family of output I-V characteristics of transistors.

 The segment OE is the supply voltage of the cascade. The lines with arrows reflect the sequence of projections. Graph 3 is a mirror copy of graph 2 relative to the ab axis.

 Note that the nonlinear distortion is compensated in the large signal mode, throughout the CVC of the nonlinear element.

Generalization 1. If the exponent exponents for the BE and diode transitions are different, if b ₁ / b ₂ = N, then distortion compensation is accompanied by a voltage increase of N times. A similar effect is obtained when N pieces of diodes are included in the collector whose I – V characteristics are identical to the I – V characteristics of the BE transition.

 Generalization 2. If the I – V characteristic of the load is known, then to compensate for the nonlinear distortion of the amplifier, one should choose an amplifier element with a current – voltage characteristic similar to the current – voltage characteristic of a load. For example, the load of a silicon transistor can be a germanium diode: both I – V characteristics are exponentials.

 In high-end amplifiers with electronic tubes, the primary winding of the transformer serves as the anode load. The current-voltage characteristic of the load corresponds to the magnetization curve of electrical steel.

 The graphs of the I – V characteristics of electron tubes depend on the variation of the grid-helix pitch along the length of the cathode rod. A classic example on this subject is presented to us by the 6K4 pentode varium, made for radios with automatic gain control (book: Vlasov V.F. Electronic and ionic devices. M., 1960. - p.232).

 The developers of High-end amplifiers randomly or in a know-how manner use the law of compensation of non-linear distortions (generalization 2). They select the type of electronic lamp so that its I – V characteristic corresponds to the I – V characteristic of the primary winding of the anode transformer. Generalization 2 they call sound equilibrium. But distortion compensation can only be performed for the initial portion of the magnetization curve. Therefore, you have to make "a significant margin of gain in power, but not to use the entire power of the amplifier."

 Generalization 3. To compensate for the nonlinear distortion of the amplifier stage with resistor R in the emitter, N pieces (R + D) are required - collector load elements. If D are diodes with the I – V characteristics identical to the I – V characteristics of the BE transition, then the voltage gain of the compensated stage is N.

 Generalization 3 is best checked with the help of PC circuit analysis programs, when it is possible to set and check any I – V characteristics of nonlinear elements.

Nonlinear distortions arising during transients in RC circuits located between cascades or in a feedback loop covering a multistage amplifier were discussed in the literature. Conclusions are drawn about the benefits of feedbacks "distributed over cascades," about the minimum of RC circuits in an amplifier. The listed requirements are satisfied by the parallel-symmetric transistor structure of Fig. 2, if a resistor R is introduced into the emitters of each pair of complementary transistors, and collector loads are constructed according to the formula Z _к = NR + 2ND (Fig. 3). Coefficient 2 arises here due to the series connection of the two pn junctions of the BE cells of complementary transistors. In FIG. 3 conventionally shows one diode in the loads of cascades (instead of ZN units). The emitter structure follower (nth cascade of FIGS. 3, 2) is characterized in that an alternating voltage u _n is applied to the load R through the transition of the BE of the npn transistor, and is removed from the circuit (R + BE) of the nnp transistor. Wherein
u _O = u -u _{ben bn} + iR + u _beno, u = u _{ben beno,}
i.e., the non-linear distortion of the repeater is compensated. To increase the current amplification to the emitters of n, n ₀ transistors, you can connect another follower on complementary transistors according to Darlington's scheme, etc.

 Indication of nonlinear distortion. When setting up High-end amplifiers, a useful method for comparing the forms of the input and output signals at a given instant of time according to the scheme of Fig. 7. Here G is the test signal generator (ramp voltage); U - studied amplifier; A - attenuator (voltage divider); F - phase shifter; DO - two-channel oscilloscope; Y, Δy; X, Δx - measurement results. Measurements can be made at the mid-frequency and cut-off frequencies of the frequency response, at any amplitude of the input signal.

 The stated features of the amplifier of Fig. 3 allow us to recommend it for constructing a transistor High-end equipment that is not inferior to a tube high-quality audio equipment, but also superior to it in terms of economy, manufacturability, reliability, with less weight and volume.

 Transistor High-end equipment (based on figure 3) can be miniaturized using diode, transistor assemblies or custom integrated circuits based on base matrix crystals - see the book Digital Integrated Circuits: Reference / Maltsev P.P. et al. - M.: Radio and Communications, 1994. - p. 179. Chips of this type can be widely used in sound amplifiers of televisions, radios, etc.

 When comparing the technical and economic indicators of the amplifier of Fig. 3 and the prototype (Fig. 2), we note that the scope of the prototype has been expanded, that a situation arises when the prototype becomes a consumer product.

Claims (1)

 A multistage transistor amplifier, the cascades of which are built of pnp and npn transistors connected by emitters, the base of one of the transistors of each subsequent stage, starting from the second, is connected to the load of the previous stage, and the bases of all other transistors are connected together, forming a common control electrode of the cascades, connected to the power source through one common cascade feedback element, characterized in that the collector loads of transistors, the bases of which form a common cascade control electrode, of 2N pieces of series-connected semiconductor diodes and a resistor of magnitude NR, resistors of magnitude R equal to the resistor of the emitter load of the last stage are included in series with the emitters of the transistors of each stage, where N = 1, 2, 3.. . - amplification of the cascade voltage.

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