JPS61189710A - Oscillation preventing system of amplifier - Google Patents

Abstract

PURPOSE:To prevent the oscillations of an amplifier without giving attenuation to the load output for the frequencies outside the pass band of a filter connected to an amplifier, by providing an additional circuit serving as an impedance within the pass band to the output part of the amplifier. CONSTITUTION:An additional circuit 5 consists of a series resistance 6 and a capacity 7 and an inductance 8 connected in parallel to each other. The impedance of the circuit 5 is extremely large between the pass bands 9 and 10 of a filter 4 and approximates to the value R2 of the resistance 6 as the impedance gets further away from those pass bands. The value of the resistance 6 is set approximately equal to an impedance RO shown by the pass band of the filter 4. The load impedance of an amplifier 3 is reduced down to RO within the pass band of a BPF and also to R2 by the resistance 6 even at a position out of the pass band when the load side is viewed from the output part of the amplifier 3. Thus the oscillations are prevented in side and outside the pass bands of the filter 4. This ensured the stable amplification.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an oscillation prevention method for an amplifier, which is applied to a bandpass filter, for example.

After an electrical signal is amplified by an amplifier, a filter is sometimes connected to extract only the signal of a desired frequency, but it is desired that the characteristics of the amplifier are not disturbed by the connection of such a load.

[Conventional technology]

Conventionally, when a capacitive or inductive load is connected to an amplifier, oscillation occurs, so oscillation is prevented by adding a capacitor or inductance to the amplifier's feedback circuit to advance or delay the phase. ing.

Also, when a filter is connected, for example a bandpass filter, the phase characteristics are complex and can be capacitive or inductive, so simply advancing or delaying the phase will not stabilize the amplifier. I can't get it to work. Therefore, a method has been adopted in which a resistance attenuator is provided to suppress the influence of the load.

[Problem that the invention seeks to solve]

Although the above conventional methods can accommodate simple capacitive or inductive loads, they are not sufficient for filters with complex phase characteristics.

For example, in a bandpass filter, the input/output impedance characteristics have a large real part and a small imaginary part within the passband, resulting in a value close to that of a pure resistance, and impedance matching is performed.However, outside the passband, the imaginary part is large and impedance matching is performed. I can't get rid of the tingling.

An amplifier connected to the output section of such a bandpass filter is affected by the imaginary part impedance, especially outside the passband, and its closed-loop gain phase characteristics vary with frequency, resulting in a value that differs from the pure resistance, which is the nominal termination impedance. Therefore, the oscillation condition is satisfied, and the amplification effect becomes unstable.

Conventional systems that use resistive attenuators to stabilize the amplification effect have the problem of increased loss and are not particularly effective for high output amplifiers.

[Means for solving problems]

The above conventional problem is solved by the oscillation of the amplifier according to the present invention, in which an additional circuit is provided at the output section of the amplifier to have an impedance close to the impedance within the passband of the filter at frequencies outside the passband of the filter connected to the amplifier. Solved by prevention method.

[Function] According to the present invention, if the additional circuit provided at the output section of the amplifier is a band-pass filter, the additional circuit can be Even if the imaginary part of the input impedance of the filter increases outside the passband of the bandpass filter, it can be made to take a value close to the same value as the real part resistance in the passband of the filter, so the amplifier gain The risk of oscillation can be prevented without causing any change in the current.

〔Example〕

The gist of the present invention will be explained in detail below with reference to the drawings.

Fig. 1 shows an embodiment of the present invention; Fig. 2 is a block circuit diagram of an oscillation prevention system for an amplifier connected to a filter; Fig. 2 shows frequency impedance characteristics of an additional circuit; and Fig. 3 shows an example of an oscillation prevention method for an amplifier connected to a filter. Shows the load impedance characteristics of the amplifier.

In FIG. 1, ■ is the input part of the amplifier 3, 2 is the output part of the filter 4, 5 is an additional circuit provided between the amplifier 3 and the filter 4, and 678 is the resistance, capacitance, and inductance in the additional circuit.

The signal to be amplified is applied from an input section 1 to an amplifier 3, the output of which is input to a filter 4 via an additional circuit 5,
A signal in a desired band is extracted from the output section 2 of the filter 4.

In the illustrated embodiment, the additional circuit 5 comprises a series resistor 6, a capacitor 7 connected in parallel, and an inductance 8, the impedance characteristics of which are shown in FIG.

In FIG. 2, the frequency f1 is plotted on the horizontal axis, and the impedance value Z is plotted on the vertical axis, and the impedance of the additional circuit 5 has an extremely large value within the passband 9-10 of the filter 4. However, as we move out of the passband, the value R of the series resistor 5 becomes
Approaching 2.

In the additional circuit of the invention, the value of the series resistor 6 is set to be approximately equal to the impedance RO exhibited by the filter 4 in its passband.

FIG. 3 shows a load impedance characteristic curve when looking at the load side from the output section of the amplifier 3, with the horizontal axis representing the frequency f and the vertical axis representing the load impedance Z.

The load impedance of the amplifier 3 is suppressed to the value of R2 by the resistor 6 of the additional circuit 5 even if it is within the passband of the bandpass filter and outside the passband of RO1, and there is no risk of oscillation inside or outside the passband of the filter 4, making it stable. It performs an amplification effect.

〔Effect of the invention〕

As described above, the oscillation prevention method for an amplifier connected to a filter according to the present invention can prevent the danger of amplifier oscillation by simply installing a simple additional circuit without attenuating the load output. , and its effects are extremely large.

[BRIEF DESCRIPTION OF THE DRAWINGS] Fig. 1 shows an embodiment of the present invention, a block circuit diagram of an oscillation prevention method for an amplifier connected to a filter, Fig. 2 shows a frequency impedance characteristic curve of an additional circuit, and Fig. 3 shows a block diagram of an oscillation prevention method for an amplifier connected to a filter. 1 shows a load impedance characteristic curve of an amplifier with an additional circuit according to the invention; In the figure, 1 is the input section of the amplifier, 2 is the output section of the filter, 3 is the amplifier, 4 is the filter, 5 is the additional circuit, and 678 is the resistance, capacitance, and inductance in the additional circuit. Mine 1t! r ) Ding ησI! r cap kiseai town rakuraken 2 rfi □ to 7 ski buy J call book - main nail S minbu 3 times

Claims (1)

[Claims] An amplifier connected to a filter, characterized in that an additional circuit having an impedance outside the passband of the filter close to the impedance within the passband of the filter is provided at the output section of the amplifier in parallel with the filter. oscillation prevention method.