Active low pass filter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Knots of amateur radio equipment. Filters and matching devices
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On fig. Figure 1 shows a schematic of an active low-pass filter with a cutoff frequency of 3 kHz, which can be used in a microphone transmitter amplifier or direct conversion receiver. The filter contains two identical amplifying stages on transistors T1 and T2 and an emitter follower on transistor T3.
Rice. 1 (click to enlarge)
The frequency response of the first stage is formed by the R4C3C4 feedback circuit. The phase relationships in the circuit are such that at frequencies of 2-3 kHz there is some increase in gain, and at frequencies above 3 kHz, the gain drops sharply due to strong negative feedback. At low frequencies, the capacitance of the capacitors C3 and C4 is large and there is practically no feedback. The passive T-section R1R2C2 compensates for the gain boost and causes even more attenuation of frequencies above 3 kHz. Resistor R3 creates a bias and stabilizes the cascade mode. The second stage is assembled in a similar way. The emitter follower eliminates the influence of the load on the filter parameters. If the filter works on a high-impedance load (more than 5 kOhm), then the emitter follower can be excluded, and the output signal can be removed from the T2 collector.
The normalized frequency response of the device is shown in fig. 2. To avoid non-linear distortion, the input signal should not exceed 10 mV. The signal amplitude in this case reaches 2 V, that is, it is sufficient for direct supply, for example, to a semiconductor balanced modulator.
Fig. 2
The filter is relatively uncritical to the parameter of the resistors and capacitors included in it, so parts with a tolerance of + -10% can be used in it. Instead of those indicated in the diagram, you can use any low-frequency transistors with Vst \u50d 100-XNUMX. When properly installed, filter adjustment is not required.
With the ratings shown in the diagram, the most flat frequency response is obtained in the 300 Hz - 3 kHz band. A decrease or increase in the capacitance of capacitors C2 and C6 creates, respectively, an increase or decrease in the characteristics at frequencies of 2-3 kHz. If, in order to improve speech intelligibility, it is desirable to obtain a smooth decline in the characteristics in the low-frequency region, the capacitance of the transition capacitors C1 and C5 should be reduced. In this case, the signal amplification will decrease somewhat.
If the signal is fed to the filter input from the output of the transistor amplifying stage, the resistor R1 should be excluded, and the resistance of the resistor in the collector circuit of this transistor should be set to 2,2 kΩ.
Author: V. Polyakov (RA3AAE); Publication: N. Bolshakov, rf.atnn.ru
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