JPH0241931Y2 - - Google Patents

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to an ALC circuit that compensates for input offsets that occur when ALC is turned on and off, and is suitable for IC implementation.

[Technical background of the invention]

Generally, an ALC circuit is configured as shown in FIG. In other words, a signal from an input signal source 1 such as a microphone or a tuner is inputted via an input resistor 2 to the base of a transistor Q 1 whose PNP collector is grounded and whose base is grounded via a bias resistor 4 of a transistor Q ₁ . are doing. The amplifier 3 includes, in addition to the transistor Q ₁ , an NPN transistor Q 2 whose emitter is connected to the emitter of the transistor Q ₁ and whose collector is connected to the power supply Vcc via the constant current load 5 , and the collector of the transistor Q ₂ and the collector of the transistor Q ₂ . An NPN transistor Q 3 has a base connected to a common connection point with the constant current load 5, a collector connected to the power supply Vcc, and an emitter connected to the base of the transistor Q ₂ via the resistor 6, and the base of _{the transistor Q 2 .} It consists of a series circuit of a resistor 7 and a capacitor 8 between the ground, and a constant current load 9 inserted between the emitter of the transistor _Q3 and the ground. The output of the amplifier 3 derived from the emitter of the transistor Q ₃ is connected to the output terminal 10. Furthermore, the output of this amplifier 3 is the rectifier smoothing circuit 1
The collector of this transistor Q ₄ is connected to the input of _the amplifier 3 at its emitter.

Here the transistor Q ₁ which is the input of amplifier 3
When a signal from the signal source 1 is applied to the base of the transistor Q1 through the resistor 2, the transistor _Q1 operates as an emitter floor type, the transistor _Q2 operates as a common base type, and the load is the active current source of the constant current source 5. The impedance as a load and the impedance seen from the base side of the transistor _Q3 are parallel, and the gain is determined by this load and the emitter resistance re of the transistors _Q1 and _Q2 (re is 26Ω at an emitter current of 1 mA).
Also, resistors 6 and 7 are connected from the emitter of transistor _Q3 .
It is fed back to the base of transistor Q ₂ by capacitor 8, and determines the closed circuit gain. Therefore, the closed circuit gain of the amplifier 3 is approximately determined by the resistor 6/resistor 7.

The base of transistor _Q1 has some potential (usually several
mV to several tens of mV), but can operate at a level that is almost close to ground potential compared to ordinary differential amplifiers.

Therefore, the output of the amplifier 3 passes through the rectifying and smoothing circuit 11 and is connected to the base of the ALC transistor _Q4 .
The ALC effect is obtained by changing the internal impedance between the collector and emitter, which is biased to zero DC.

[Problems with background technology]

However, in the above ALC circuit, the base potential of the transistor _Q1 is determined by its base current and the bias resistor 4, and normally has a potential of several mV to several tens of mV, but once the ALC control transistor _Q4 operates, its collector-emitter The internal impedance between them is several hundred ohms to several tens of ohms, and the transistor
The base potential of Q ₁ becomes even lower. At this time, when a rectifying and smoothing circuit 11 with a fast attack time is used, fluctuations in the DC at the input will fluctuate the DC at the output, and as shown in FIG. It has stable ALC characteristics.

[Purpose of invention]

The present invention provides an ALC circuit that minimizes input offset and provides stable ALC characteristics.

[Summary of the idea]

In the ALC circuit of the present invention, a bias resistor and an ALC control transistor are inserted between the base of a PNP transistor having base, emitter, and collector electrodes and ground, and an ALC control current is passed through the base of the ALC control transistor. In order to minimize the offset caused by the on-resistance between the emitter and collector of the ALC control transistor at this time, current is caused to flow into the bias resistor of the PNP transistor.

[Example of idea]

Hereinafter, one embodiment of the present invention will be described in detail with reference to FIG. 1, but the same parts as in FIG. 3 are given the same numbers and the explanation thereof will be omitted.

The rectifying and smoothing circuit 20 in Figure 1 converts its output into an ALC
In addition to flowing into the base of the control transistor _Q4 , the control current is also configured to flow into the input of the amplifier 3, that is, the base of the transistor _Q1 .

As a result, the base potential of the transistor _Q1 is determined by the internal impedance between the collector and emitter of the transistor _Q4 , the parallel impedance of the bias resistor 4, and the base current of the transistor _Q1 . Here, when the transistor Q ₄ is turned on by the ALC control current of the rectifier and smoothing circuit 20 and is at a potential lowered due to the internal impedance between the collector and emitter, the transistor Q ₁
ALC control current also flows into the base of. Therefore, the ALC control current is not output from the rectifying and smoothing circuit 20, that is, the base potential of the transistor _Q1 is set to be the same as the base potential of the transistor Q1 when the ALC is off, thereby preventing input offset from occurring.

[Effect of idea]

As described above, according to the present invention, by inputting the ALC control current to the ALC control transistor and also flowing into the input of the amplifier, the input potential of the amplifier is compensated for the offset in the same way as when the ALC is off. are doing. Therefore, for the input signal shown in FIG. 2a, a stable ALC characteristic as shown in FIG. 2c is obtained at the output of the amplifier.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram showing one embodiment of the present invention;
Figure a is an input signal waveform diagram that is the subject of the present invention.
Figure b is a conventional output waveform diagram, Figure 2c is an output waveform diagram of the present invention, and Figure 3 is a conventional circuit diagram. Q ₁ , Q ₂ , Q ₃ ... transistor, 3 ... amplifier,
20... Rectifying and smoothing circuit, Q ₄ ... ALC control transistor.

Claims (1)

[Scope of utility model registration request] a first transistor whose base is directly connected to an input signal source and whose collector is grounded; and a second transistor whose emitter is connected to the emitter of the first transistor and whose collector is connected to a DC power supply via impedance means. and a third transistor, the base of which is connected to the common connection point of the second transistor and the impedance means, the collector of which is supplied to the DC power supply, and the emitter of which is connected to the base of the second transistor via a resistor. A direct-coupled input amplifier consisting of a transistor, a circuit for rectifying and smoothing the output of the amplifier, a base connected to the output terminal of the circuit, an emitter grounded, and a collector connected to the base of the first transistor for ALC control. In an ALC circuit that includes a transistor and drives the first transistor with a ground bias, the on-resistance of the ALC control transistor is increased by flowing a control current also to the input of the amplifier. An ALC circuit characterized by reducing the offset that occurs when