Operational amplifier signal compressor. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Color and music settings
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As is known, due to the small operating interval of the voltage of incandescent lamps in the CMU and SDU, it is necessary to artificially compress the dynamic range of the audio signal. This is achieved by using an AF signal compressor.
One of the options for a compressor that effectively compresses the dynamic range of the AF signal is described below. It is simple in design, contains a small number of parts, does not require adjustment. In addition, the amplitude of the output signal of the compressor does not depend much on the supply voltage, changing by no more than 1% when the supply voltage changes within 9 ... 30 V.
The compressor is made according to the scheme of an inverting amplifier on an op-amp with a thermistor in a negative feedback circuit (named after the scheme in Fig. 1). With an increase in the current flowing through the thermistor RK1, it heats up, its resistance decreases, which leads to a corresponding decrease in the compressor transfer coefficient.
The compressor transfer coefficient can be determined by the approximate formula: K=R4(1+Rt/R5)/R1, where Rt is the resistance of the thermistor RK1. The "cold" resistance of the thermistor T8E is 2 ... 3 kOhm. During the operation of the compressor, the resistance of the thermistor can change by 15 ... 20 times, which provides the compression effect. An analysis of the expression in brackets shows that with a decrease in the resistance of the resistor R5, the depth of compression increases due to an increase in the relative change in the transfer coefficient K, corresponding to the full interval of change in the resistance of the thermistor. However, it is necessary to increase the capacitance of the capacitor C3 to maintain the linearity of the frequency response of the compressor at low frequencies.
The amplitude characteristic of the compressor, taken at a frequency of 1000 Hz, is shown in fig. 2. Signal attenuation at the cutoff frequencies of the operating band (20 Hz and 20 kHz) does not exceed 3 dB. The distortion introduced by the compressor at input signal levels up to 300 mV is practically imperceptible in the operating frequency band.
The sensitivity of the compressor can be changed over a wide range by changing the R4/R1 ratio. If it is necessary to limit the operating frequency band from above, between the output and the inverting input of the op-amp, a capacitor with a capacitance of 270 ... 1000 pF is included (selected experimentally). This also makes it possible to reduce the compressor's sensitivity to impulse noise.
An unstabilized voltage source of 9...30 V was used to power the compressor. The current consumption does not exceed 10 mA.
Thermistor T8E can be replaced by any of the T8, T9, TV2-250, TK2-50 series. In addition to K140UD8B, the compressor can also use op-amps K140UD8A, K140UD8V, K140UD6, K544UD1 and others (preferably high-speed ones) with appropriate correction circuits.
The described compressor can also be used in sinusoidal voltage generators for amplitude stabilization, in speech signal companding devices in sound recording and radio transmitting equipment. When using the high-speed OA K574UD1, the compressor can be used in the RF path of the receiver as an amplifier with AGC.
Author: A. Belousov; Publication: cxem.net
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