RU1800585C - Active filter with switchable pass band - Google Patents

Abstract

The invention relates to radio engineering and can be used in audio processing paths. The purpose of the invention is to reduce non-linear distortion. An active filter with a switchable bandwidth contains the first and second L-shaped NS circuits 1 and 2, an amplifier 3, the first and second key transistors 4 and 5, the first, second, third and fourth resistors 6-9, diode 10, the capacitor 11 and key 12. The operation of an active filter with a switchable bandwidth is based on the introduction of a nonlinear OS in the control circuit of the key transistors, 1 ill.

Description

&

-0

The invention relates to radio engineering and sound engineering and can be used in amplification paths for processing audio frequency signals.

The purpose of the invention is to reduce non-linear distortion.

The drawing shows a schematic diagram of an active filter with switchable bandwidth,

The active filter contains two series-connected RC-links 1 and 2, an amplifier 3, two key transistors 4 and 5, the bases of which are connected through the resistors 6 and 7 to the control bus. The transverse arm outputs of the RC-links 1 and 2 are connected through the key transistors 4 and 5, respectively, with the output of amplifier 3 and with a common bus. A voltage divider is connected between the positive potential bus + E1 and the output of amplifier 3, consisting of a resistor 8, a diode 9, and a parallel RC circuit of a resistor 10 and a capacitor 11. The connection point of the resistor 8 and the anode of the diode 9, i.e. the output of the divider, is connected with control bus Control, which is connected via key 12 to the negative bus

 potential -E2.

Active filter works as follows. The input signal passes through two RC-links 1 and 2 and is fed to the input of amplifier 3, from the output of which the processed signal is taken. With the key 12 closed on the control bus negative bus voltage -E2 is present. In this case, the diode 9 is in the reverse connection, and currents flow through the resistors 6 and 7 to the bases of the key transistors 4 and 5, the Open key transistors 4 and 5 close the outputs of the transverse arms of the RC links 1 and 2, respectively, to the output of the amplifier 3 and to the common bus . In this case, the filter takes the form of an active filter of the second order with a controlled voltage source and limits the signals passing through it by frequency, i.e. it is in a state of a narrow passband. The large base currents of transistors 4 and 5 provide the filter with small

 nonlinear distortion in this mode of operation. With the switch 12 open, the voltage divider enters into operation and the current through it opens the diode 9.

On the control bus a constant component of the positive voltage is formed, which is equal to

. (Ei-UglRio,, |. M uy P- -Sja + Rio + U9 (1) where Ei is the voltage potential of the bus of positive potential

and

Uj is the voltage drop across the open diode 9;

Rs, Rio - resistance of resistors 8 and 10.

This voltage closes the junctions of the key transistors 4 and 5, and all the points at the inputs and outputs of the RC links 1 and 2 and the amplifier 3 become equipotential in a wide frequency band, which ensures that the active filter has a wide passband state. The capacitor 11, which is connected in parallel with the resistor 10, provides a floating bias voltage on the control bus at high frequencies of the signal, which allows linearizing feedback through the relatively low resistance resistors b and 7 to reduce the effect of non-linear capacitances of the key transistors 4 and 5 on distortions filter output signal. The capacitance of the capacitor 11 is selected according to the following relation

Cu 1/2 JTf0Ri2, (2) where fo is the filter cut-off frequency, which is 25 nafo-t / 2 -rVR c -.

Ria is the resistance of the resistor 12. The transmission function of the active filter is as follows:

- for narrow bandwidth 30

10

fifteen

twenty

Fi (p)

1 + (1 -K) Ri Ci + (Ri + R2) + Ri R2 Ci C2pz

(3)

35

for wide bandwidth

Fa (p)

1+ (1 - K) Ct + (Ri + R2) (Ci + С3) p + RiR2C «(Cs + СзУ

d)

where K is the gain of the amplifier 3; I J Ri, R2 resistance of resistors of RC-links 1 and 2;

Ci, Cs — capacitances of capacitors of RC units 1 and 2;

C, CE - capacitances of transitions of closed key transistors 4 and 5;

C3 is the input capacitance of amplifier 3. Expressions (3) and (4) are based on the assumption that the input resistance of amplifier 3 is much greater than the sum of the resistances Ri + Ra, and also that the capacitances Cm and Cg satisfy the following relations: Gi C Ci and Cs C C2.

The parameters of the RC links 1 and 2 and the amplifier 3 select the outcome from the desired frequency response of the filter. The resistances of resistors 8 and 10 of the voltage divider are selected so that the positive voltage Uynp. was greater than the amplitude of the signal at the input of the filter. The specific values of these resistances may be tens or hundreds of kilo-ohms. The resistances of the resistors 6 and 7 are usually selected close in resistance and as small as possible, which is determined by the maximum current of the control bus. and the load capacity of amplifier 3, since the resistor 6 with a narrow filter band turns out to be connected by alternating current to the output of amplifier 3. It is desirable to have the gain of the amplifier close to unity, but this is not necessary. A reduction in nonlinear distortion can also be obtained with a gain from 0.8 to 1.3, the decrease being at least two times,

Using the proposed structure of an active filter with a switchable bandwidth allows reducing nonlinear distortions of the processed signal by approximately an order of magnitude compared to the prototype, that is, for a filter with a cutoff frequency of 9 kHz, it is possible to reduce the frequency band from 40 to 16000 Hz at a signal level of 1 V nonlinear distortion from 0.04-0.09% to 0.001-0.004%. The reduction of nonlinear distortion can also be obtained in active filters, which use not only

bipolar key transistors, but also field-effect key transistors.

Claims (1)

Formula of the invention An active filter with a switchable bandwidth, containing two series-connected L-shaped RC links, in the longitudinal arm of which a resistor is connected, and in the transverse capacitor, the outputs of the transverse arm of the first and second L-shaped RC links are connected through the first and the second key transistors respectively with the amplifier output and with a common bus, and the bases of the first and second key transistors are connected to the first terminals of the first and second resistors, the second terminals of which are connected to each other and are a control input, and t is characterized in that. in order to reduce non-linear distortion, a key, a third resistor and a series circuit consisting of a series-connected diode and a parallel RC circuit are connected between the control input and the output of the amplifier, the input of which is connected to the output of the second L-shaped RC link, and the first the terminals of the third resistor and key are connected to the control input, and their second are inputs for connecting the corresponding bias voltage source.