JPH0241932Y2 - - Google Patents

Description

[Detailed explanation of the invention] (a) Industrial application field The present invention relates to an AGC circuit of an amplifier, and is particularly applicable to a video tape recorder (VTR) or an audio tape recorder (ATR), from a playback state to a recording state. This invention relates to a circuit for improving the response speed of AGC when a large input is applied to the amplifier when switching to the amplifier.

(b) Prior art In general, in video tape recorders (VTR) or audio tape recorders (ATR),
During recording (recording or recording), the audio circuit is AGC or
A feedback circuit called an ALC circuit is provided to suppress the recording level within a predetermined level. On the other hand, when reproducing an audio signal recorded on a magnetic tape, the output of the detection circuit in the AGC (ALC) circuit is set to a non-operating state.

An example of this is an example of an ICLA7040 application circuit published in "Sanyo Semiconductor News" No. 1139A published by Tokyo Sanyo Electric Co., Ltd.'s Semiconductor Division, which is shown in FIG. In FIG. 2, 1 and 2 are input terminals and output terminals, 3 and 4 are coupling capacitors, 5 is an amplifier, 6 is an AGC detection circuit consisting of capacitors 7 and 8 and diodes 9 and 10, and 11
is the AGC amplification transistor, 12 is the diode 1
3 and a current mirror circuit consisting of a transistor 14; 15 is an AGC drive transistor; 16 is a switching transistor that sets the control terminal 17 to high level during playback and turns on and off during recording; 18 is a DC power supply terminal; 19 is the above-mentioned This figure shows an IC in which the above-mentioned predetermined elements such as the amplifier 5 are built-in.

When currently set to playback mode, terminal 17
becomes high level, the switching transistor 16 is turned on, and the output side of the AGC detection circuit 6 is grounded.
AGC amplification transistor 11 is cut off,
The feedback system is inactive, that is, the AGC loop is not working, and the charge on the capacitor 8 of the AGC detection circuit 6 is completely discharged and is zero. At this time, the audio signal applied to input terminal 1 is transmitted to amplifier 5.
The signal is amplified at a predetermined gain and an output signal is obtained from the output terminal 2.

When switching from playback mode to recording mode (recording or recording mode) and setting terminal 17 to low level, switching transistor 16 is turned off and the output of amplifier 5 is applied to AGC detection circuit 6, and capacitors 7 and 8 and diodes 9, 10
After being voltage-doubled and rectified by the AGC amplification transistor 11, the signal is DC amplified by the AGC amplification transistor 11, and negatively fed back to the input side of the amplifier 5 by the AGC drive transistor 15.

However, if a large input signal is applied to input terminal 1 immediately after switching from the playback mode to the recording mode described above (for example, when a signal level that is 20 dB higher than the level at which AGC starts working is applied to the input of amplifier 5), the amplifier The output of the amplifier 5 has a waveform as shown in FIG. 3A, and depending on the level of this input signal, the amplifier 5 becomes saturated.

(c) Problems to be solved by the invention As mentioned above, the AGC circuit's response becomes slow due to the application of a large input signal immediately after switching from playback to recording mode.
The AGC detection circuit was designed in advance to solve the problem that the function could not follow the amplification characteristics of the amplifier and a large oscillator signal would be output, or the amplifier would become saturated and a distorted signal would be output. This is to improve the response of the AGC circuit by applying a predetermined voltage.

(d) Means for solving the problem In this invention, the
A shot key diode for applying a predetermined voltage to the AGC detection circuit through the switching transistor in advance in the regeneration mode is connected to the switching transistor, and the switching transistor is switched from an on state to an off state. Solve the problem at the time.

(e) Effect: Connected to the output end of the amplifier in this invention
In the AGC detection circuit, the collector of the switching transistor that is turned on in the playback mode
This configuration uses a switching transistor connected to the emitter path to apply a voltage smaller than the base-emitter rise voltage of the transistor, for example, 0.4 V or less, so that the switching transistor is turned off, that is, in the recording mode. , even if a large input signal is applied to the input terminal of the amplifier, the AGC circuit operates immediately.

(F) Embodiments The present invention will be explained with reference to the drawings. In FIG. 1, the same elements as in FIG. 2 are given the same reference numbers, and 20 indicates a Schottky diode.

Next, to explain the operation of the present invention, first, during reproduction, a high level voltage is applied to the base of the switching transistor 16 to turn it on.
The forward voltage V _sf of the Schottky diode 20 is applied to the capacitor 8 of the AGC detection circuit 6. At this time, since the collector-emitter voltage V _CE of the switching transistor 16 can be considered to be approximately zero, the voltage V _c across the capacitor 8 is maintained at V _sf .

In this state, an input signal is applied to the amplifier 5 from the terminal 1 via the coupling capacitor 3, and an output signal is obtained from the output terminal 2.

Forward voltage of the Schottky diode 20
V _sf is lower than the base-emitter rising voltage V _BE of the transistor, that is, V _sf < V _BE (generally V _BE
= 0.7V, for example, V _sf = 0.4V) The AGC amplification transistor 11 is turned off, and accordingly the diode 13 and transistors 14 and 15 are turned off, so the AGC loop is kept inactive and the amplifier 5 performs amplification at a predetermined amplification degree.

Next, when switching from playback mode to record mode and applying a low level voltage to terminal 17, switching transistor 16 is cut off and AGC is turned off.
The output voltage of the detection circuit 6 is applied to the AGC amplification transistor 11.

Therefore, when a large input signal, for example 20 dB higher than the level at which the AGC loop operates, is applied to the input terminal 1 to the amplifier 5, the capacitor 8 of the AGC detection circuit 6 is charged with the voltage V _sf , so that
The AGC amplification transistor 11 has its rising voltage V _BE
This shortens the time it takes to reach this point, improving response speed.

This pattern is shown in the measured waveform as shown in Figure 3 (b).
When a large input signal that is 20 dB higher than the level at which the AGC loop starts operating is applied as an input signal to the amplifier 5, the AGC enters the operating state from the beginning of the large input signal, compared to the conventional example shown in Fig. 3A. Therefore, the output waveform of the amplifier 5 only needs to be slightly larger.

The output signal of the amplifier 5 is applied to the base of the AGC amplification transistor 11 via the AGC detection circuit 6, and the AGC voltage appearing on the collector side of the AGC amplification transistor 11 is applied to the base of the AGC drive transistor 15 via the diode 13 and the transistor 14. join. Due to the feedback by the AGC loop, amplifier 5 outputs a constant amplitude output to terminal 2.
occurs in

In the above embodiment, a case was explained in which a Schottky diode was provided at the emitter of the NPN type switching transistor 16, but the collector and emitter of the switching transistor 16 were reversely connected and a connection was made between the collector and the ground. A Schottky diode may be connected, and in this case as well, the control terminal 17 is set to high level in playback mode and low level in record mode, and the switching transistors are turned on and off, respectively, and the result is equivalent to the above example. perform the following actions.

(g) Effects of the invention According to the circuit for improving the response speed during mode switching of the AGC circuit of the present invention, the failure that conventionally occurs when a large input signal is applied to the amplifier during mode switching is removed, and the response speed is improved immediately. The AGC operation starts (the so-called attack time is shortened), and the circuit of the present invention is extremely useful for both VTRs and ATRs that record and play audio signals.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing an embodiment of a circuit for improving response speed during mode switching of the AGC circuit of the present invention, Fig. 2 is a circuit diagram showing a conventional circuit, and Fig. 3 is a circuit diagram showing a circuit diagram of the same circuit according to the present invention. An explanatory waveform diagram is shown. Explanation of main drawing numbers, 1...Input terminal, 2...Output terminal, 5...Amplifier, 6...AGC detection circuit, 1
1...AGC amplification transistor, 15...AGC drive transistor, 16...Switching transistor, 17...Control terminal.

Claims (1)

[Scope of utility model registration request] Connect the AGC detection circuit to the output terminal provided at the output end of the amplifier, and connect the AGC detection circuit to the output terminal of the AGC detection circuit.
In an AGC circuit that connects the input end of an amplification transistor and returns the output side of the AGC amplification transistor to the input side of the amplifier, a collector-emitter path of a transistor for regeneration setting is provided between the input end of the AGC amplification transistor and ground. and a shot key diode are connected in series, and a shot key voltage from the shot key diode is applied to the smoothing capacitor of the AGC detection circuit during playback.
The AGC is characterized in that the AGC amplification transistor is immediately activated when a large input signal is applied to the amplifier as the reproduction setting transistor is turned off when switching to a recording state.
A circuit that improves response speed when switching circuit modes.