RU2500065C2 - Modulator on mos-transistors - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: excitation frequency and modulated signal frequency are different: modulation is carried out at doubled frequency in respect to excitation frequency. The device uses a complementary pair of MOS-transistors (CMOS-pair), connected in accordance with the parallel circuit and excited by sign-variable rectangular pulses of current with duration π/2, at the same time at the outlet of the modulator there is a high-pass RC-filter.

EFFECT: reduced shift and drift of modulator zero.

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Description

The invention relates to the field of amplification technology and can be used to modulate weak direct and infra-low-frequency electric currents and voltages in amplifiers of the MDM type (modulator-demodulator).

When the input signal to the amplifier has a frequency below or near the lower limit of the frequency range of the amplifier, the solution is to convert the input signal to a modulated signal of a higher frequency, its amplification, and then demodulation to the original frequency (modulation-gain-demodulation). The characteristics of the modulator make a very significant and often decisive contribution to the final amplification results, especially at a low input signal level.

In MDM amplifiers, contact and transistor modulators are predominantly distributed. The latter are characterized by high reliability and durability [1-3].

Known transistor modulator containing two MOS transistors included in the signal circuit and excited (controlled) by alternating rectangular current pulses, and a matching device in the form of an RC chain [3, p. 54, Fig. 2.19].

This device is accepted by us as a prototype.

A diagram of a known device is shown in figure 1, a.

The diagram shows: 1 - two MOS transistors; 2 - exciting current pulses i ₁ (t) and i ₂ (t); 3 - matching device in the form of an RC chain; U ₀ is the input voltage; u _o (t) is the output modulated voltage.

Figure 1, b shows graphs explaining the operation of the known device. Shown here (from top to bottom): i ₁ (t) and i ₂ (t) are the exciting current pulses and u _output (t) are the output voltage pulses. The angle α is given by the initial settings of the excitation generator.

MOSFETs work like keys, switching input voltage. The best results are shown in the regime in which α → 0 and the pulse duration i ₁ (t) and i ₂ (t) is π (duty cycle is 2). MOS transistors operate out of phase, and the modulated current i ₀ (t) takes the form of a meander. In this case, the voltage of the fundamental frequency u _{output 1} (t) = U _m1 sin (ωt + φ ₁ ), where φ ₁ is the initial phase, which serves as the output signal of the modulator, is isolated from the spectrum of the output voltage containing the fundamental frequency and higher odd harmonics .

A disadvantage of the known device is that the output signal and the excitation current supplied to the modulator have the same frequency. Since part of the energy from the excitation circuit inevitably penetrates the signal circuit of the modulator, being an internal noise, this significantly limits the characteristics of the device, especially when amplifying very small currents and voltages. It is not possible to filter out this interference because it has an output signal frequency.

The aim of the invention is to reduce the offset and zero drift of the modulator (and in general MDM amplifier) compared with the prototype.

The specified disadvantage of the prototype is eliminated by the fact that in the present invention, the excitation frequency of the modulator and the frequency of the modulated signal are separated (different): the signal is modulated at twice the frequency relative to the excitation frequency, which makes it possible to effectively filter the useful signal from the interference of the fundamental frequency.

Double frequency modulation is achieved by the fact that the inventive device uses a complementary pair of MOS transistors (CMOS couple) connected in a parallel circuit and excited alternately by current pulses of π / 2 duration. According to dependent clause 2 of the claims, the element that cuts off the main frequency is an RC high-pass filter,

The diagram of the inventive device according to claim 2 is shown in figure 2, a.

The diagram shows: 1 - input low-pass RC filter; 2 - CMOS couple of transistors; 3 - i ₁ (t) and i ₂ (t) current pulses of duration π / 2; 4 - output filter RC-high frequencies; U ₀ and u _o (t) are the input and output voltages, respectively.

Figure 2, b shows graphs explaining the operation of the claimed device. Shown here (from top to bottom): i ₁ (t) and i ₂ (t) are exciting current pulses of duration π / 2 (α = π / 4, duty cycle is 4); u _o (t) is the output voltage of the modulator. The angle α = π / 4 is given by the initial conditions for tuning the excitation generator.

The inventive device according to claim 2 works as follows. Due to the use of a complementary pair of MOS transistors connected in parallel, opening alternately with a duration of π / 2, rectangular voltage pulses are formed in front of the output RC filter, which appear twice during the period of excitation current of the main frequency. The spectrum of this sequence contains the second and highest even harmonics. For a given pulse duration, the second harmonic has the greatest weight, the interference of the first harmonic is cut off by an RC high-pass filter.

This device is designed to work with a high-resistance source of input voltage. To reduce input noise and increase input impedance in the proposed modulator uses an RC low-pass filter.

To work with low-impedance sources of input voltage, a variety (option, modification) of the inventive device according to claim 3 is proposed. The purpose of this modification of the invention is to increase the sensitivity and signal-to-noise ratio at the output of the modulator and reduce its input resistance when working with low-impedance sources of input voltage.

This goal is achieved by the fact that in the variant of the claimed device, a complementary pair of MOS transistors (CMOS couplers) are used, connected in a parallel circuit and excited in turn by current pulses of duration π / 2. According to dependent claim 3 of the invention, the filter element is a transformer, two identical primary windings of which are connected to transistors, and the secondary winding is tuned to resonance at double frequency, and an LC low-pass filter is installed at the input of the modulator.

A diagram of a variation of the claimed device according to claim 3 is shown in figure 3, a.

The diagram shows: 1 - input low-pass LC filter; 2 - CMOS couple of transistors; 3 - i ₁ (t) and i ₂ (t) current pulses of duration π / 2; 4 - a transformer, the secondary winding of which is tuned in resonance at a double frequency; U ₀ and u _out.2 (t) = U _m2 sin (2ωt + φ ₂ ) are the input and output voltages of the modulator, respectively.

Figure 3, b shows graphs explaining the operation of a variety of the claimed device. Shown here (from top to bottom): i ₁ (t) and i ₂ (t) are exciting current pulses of duration π / 2; u ₀ (t) - pulses of modulated voltage in the case when the secondary winding is not tuned to resonance; u _{output 2} (t) = U _m2 sin (2ωt + φ ₂ ) is the output voltage of the modulator when tuning the secondary winding of the transformer to resonance at double frequency (shown by a dotted line).

A variation of the claimed device operates as follows. The CMOS transistors operating alternately form two parallel input signal switching lines connected to the corresponding primary windings of the matching transformer. As a result, rectangular voltage pulses are induced in the secondary winding of the transformer (when it is not tuned to a double frequency resonance). The area of these pulses is significantly larger than the area of the output voltage pulses shown in figure 2, b, thereby increasing the sensitivity of the inventive variety of the modulator. Rectangular voltage pulses appear twice during the period of the excitation current of the fundamental frequency. The spectrum of this sequence contains the second and highest even harmonics. For a given pulse duration, the second harmonic has the greatest weight, which is released when the secondary winding of the transformer is tuned to resonance, which also increases the sensitivity of the modulator.

The inventive device according to the scheme of figure 2, and can be used in electrometric devices. The inventive device according to the scheme of figure 3, a - in magnetometric devices in which the source of the input voltage is an induction coil. In both cases, the effect of the implementation of the modulation at double frequency will be very significant.

Let us evaluate this effect quantitatively.

As noted above, the offset and zero drift of known modulators are determined by the level of interference generated by the excitation generator. The greatest contribution is made by the interference with the fundamental frequency of the generator, i.e. first harmonic of the excitation current. In the inventive device, a modulator with an output at a double frequency, the interference with the fundamental frequency is easily filtered out, due to which the effect of reducing the bias and zero drift is achieved.

However, due to the imperfection of real excitation generators, the doubled frequency component is usually present in the excitation current. Let us estimate its level at 1-2% of the current level of the fundamental frequency (in fact, in push-pull balanced oscillators this level is much lower). This component (with a frequency of 2ω) cannot be filtered out and will inevitably penetrate the output signal and cause a shift and zero drift of the modulator, just as the fundamental frequency component penetrates the output signal in known devices. Then, ceteris paribus, the offset and zero drift of the proposed device will be at least 50-100 times less than in the known modulators. In other words, we get a gain in reducing the displacement and zero drift not at times, but by 1-2 orders of magnitude.

Therefore, the introduction of the proposed device allows us to state the real possibility of developing not only the microvoltaic, but also the nano- and picovoltaic ranges of amplified currents and voltages.

Literature

1. Kalinchuk B.A., Pichugin O.A. Modulators of small signals. - L .: Energy. Leningrad branch, 1980 .-- 200 p.

2. Londarenko O.P., Silchev A.Yu., Tsybulenko N.I. Highly sensitive operational amplifier type modulator-demodulator. - Semiconductor electronics in communication technology, 1982, issue 22, p. 75-78.

3. Polonnikov D.E. Operational amplifiers: construction principle, theory, circuitry. - M .: Energoatomizdat, 1983 .-- 216 p.

Claims (3)

1. The modulator containing two MOS transistors included in the input signal circuit and excited (controlled) by alternating rectangular pulses of current from the excitation generator, characterized in that the input signal is modulated at twice the frequency relative to the frequency of the excitation generator, which is achieved using a complementary pair of MOSFETs transistors (CMOS pairs) connected in parallel and alternately excited by current pulses of duration π / 2, and by cutting off the main frequency of the device at the output of the modulator. 2. The modulator according to claim 1, characterized in that the RC high-pass filter serves as a cutoff device for the fundamental harmonic, and an RC low-pass filter is located at the input of the modulator. 3. The modulator according to claim 1, characterized in that the transformer cuts off the main harmonic, the two identical primary windings of which are connected by a signal circuit with the corresponding transistors, the secondary winding is tuned to resonance at twice the frequency, and an LC lower filter is located at the input of the modulator frequencies.

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