JPH01291512A - Active filter circuit - Google Patents

Abstract

PURPOSE:To effectively suppress the dispersion of a tuning voltage due to the temperature drifting of the current amplification factor of a transistor by keeping the collector current of the transistor at an output stage constant. CONSTITUTION:In an active filter circuit, a DC tuning voltage VTU can be obtained by smoothing a pulse signal VP. At this time, a constant current circuit 2 consisting of, for example, the transistor Q3 and resistors (R7 to R9) is connected to the emitter of the transistor Q2 at the output stage. Thereby, the ratio of the current amplification factor DELTAhFE of the transistor Q2/the base current DELTAIb of the transistor Q2 can be kept constant. Therefore, the collector current Ic can be kept constant by the constant current circuit 2. And the correction of drifting can be performed easily, and an accurate tuning voltage can be obtained. Also, to keep the collector current Ic constant, it is possible to obtain a proximate characteristic even by connecting a high resistor instead of the circuit 2 in which the transistor is used.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an active filter circuit for smoothing a pulse signal to obtain a DC tuning voltage.

[Conventional technology]

Microprocessors are used in the tuning mechanism of receivers such as television receivers and radio receivers, in which pulse signals are used as tuning signals. Therefore, the tuning element of the tuning circuit (varactor diode, etc.)
An active filter is used to obtain the DC voltage that drives the.

FIG. 2 shows a circuit of such an active filter.

Pulse signal vP for tuning applied to input terminal
is amplified by transistor Q1, smoothed by a filter circuit consisting of resistors R4 and R5, capacitors C2 and C3, and transistor Q2, and a direct current tuning voltage V? is output from the emitter of transistor Q2. Output as U. This tuning voltage V, , is approximately 1 depending on the channel being handled.
It varies from v to 20v.

[Problem to be solved by the invention]

However, in the circuit shown in FIG. 2, a difference in temperature drift occurs in an extremely wide variation range of the chain gummy pressure VTtI, so it is difficult to correct the drift.

Now, power supply voltage B is 32V, and hFE is transistor Q.
Current amplification factor of 2, tuning voltage ■A. The base current of transistor Q2 when is 1V is 11.If the base current of transistor Q2 is 11.20V, then
, >>■, □, there is a difference in the amount of change between ■ and ■1□ due to the temperature drift of hFE.

That is, Δl, >>Δlb□. Since the base current flows into resistors R5 and R4, the voltage difference is approximately Δ1b+ (R4+R5) >> Δ■1□ (R4+R5)
This appears as a drift difference in the tuning voltage VTtl, resulting in a tuning error.

The present invention has been made in view of the above points,
The purpose is to make the tuning voltage insensitive to temperature drift.

[Means to solve the problem]

For this purpose, the present invention is configured to keep the collector current of the output stage transistor constant in an active filter circuit that smoothes a pulse signal to obtain a DC tuning voltage.

〔Example〕

Examples of the present invention will be described below. FIG. 1 is a diagram showing an active filter circuit of one embodiment. Components that are the same as those in FIG. 2 are given the same reference numerals. In this embodiment, a constant current circuit 2 consisting of a transistor Q3 and resistors 7 to R9 is connected in place of the emitter resistor R6 of the transistor Q2 shown in FIG.

As mentioned above, Ibl>>IbZ, and there is a difference in the amount of change due to the temperature drift of hFE, and the tuning voltage V
? This appears as an error of t+.

Therefore, in order to keep the amount of change constant, Δh
vt/ΔIb=constant. In other words, it is sufficient to keep the collector current 1c flowing through the transistor Q2 constant.

Therefore, the collector current 1c of the transistor Q2 is kept constant by the constant current circuit 2, and variations in drift due to the magnitude of the tuning voltage are suppressed, so correction of the drift (not shown in the figure, but can be realized by level shifting, etc.) is easy. This makes it possible to obtain accurate tuning voltage.

Note that, in order to make the collector current Ic constant, approximate characteristics can also be obtained by connecting a high resistance instead of the circuit 2 using a transistor.

〔Effect of the invention〕

As described above, according to the present invention, there is an advantage that variations in tuning voltage due to temperature drift of current amplification factors of transistors can be effectively suppressed.

[Brief explanation of the drawing]

FIG. 1 shows an active filter circuit of the present invention, and FIG. 2 shows a conventional active filter circuit. 2...Constant current circuit agent Patent attorney Tsune Akira Nagao

Claims (3)

[Claims] (1) An active filter circuit that obtains a DC tuning voltage by smoothing a pulse signal, characterized in that the collector current of a transistor in an output stage is made constant. (2) The active filter circuit according to claim 1, characterized in that a constant current circuit is connected to the emitter circuit of the transistor. (3) The active filter circuit according to claim 1, characterized in that a high resistance is connected to the emitter circuit of the transistor to achieve a constant current.