JPH0230604B2 - RITOKUKAHENKAIRO - Google Patents

Description

[Detailed description of the invention] The present invention relates to a variable gain circuit.

As shown in Figure 1, the conventional variable gain circuit puts the differential pair Q ₁ and Q ₂ in an unbalanced state, changes the shunt ratio of the currents I ₁ and I ₂ , and changes the signal obtained through I ₁ [Q ₃ or
Since the gain of the alternating current signal applied to the base of the _Q4 was variable, the temperature characteristics were extremely poor, as shown in Figure 2. In Figure 2, the horizontal axis shows the temperature (°C) and the vertical axis shows the gain, and as the splitting ratio deviates from 1:1, it changes to A, B, C,
The disadvantage is that the temperature drift increases as shown in (D), (E), and (F), and it is difficult to compensate for this.

In addition, in the conventional circuit, the control sensitivity by the volume (VR) is determined by the ratio of resistors R ₁ and R ₂ , so
In order to increase the control sensitivity, the ratio between resistors R ₁ and R ₂ must be selected to be large, but in ICs, the larger the resistance ratio, the more the circuit characteristics will vary, which is not desirable.

The present invention proposes a novel and effective variable gain circuit devised to eliminate such drawbacks.

The embodiments will be described in detail below according to the embodiments shown in the drawings.

In the present invention _, as _{shown in FIG} . , fourth transistor T ₅₂ ,
Each emitter of a transistor pair consisting of T ₅₃ and T ₅₁ ,
A relatively high (for example, about 2.2KΩ) resistors R ₆₄ and R ₆₅ are connected between the emitters of T ₅₄ , and an alternating current input to at least one of the first and second transistors T ₅₁ and T ₅₄ . The signal gain can be adjusted by adjusting the volume (VR) of the first and second resistors.
By changing the potential at the connection point of R ₆₈ and R ₇₀ , that is, the base potential of the third and fourth transistors T ₅₂ and T ₅₃ , and thereby changing the emitter currents of the third and fourth transistors T ₅₂ and T ₅₃ , the result is The gain is varied by changing the DC current flowing between the collectors and emitters of the first and second transistors T ₅₁ and T ₅₄ , and the AC signal with the variable gain is transmitted to the second transistor, for example.
It is designed to be taken out of the T ₅₄ collector.
E ₁ and E ₂ represent constant bias power supplies, and E ₂ can also be grounded.

In the above embodiment, the resistors R ₆₄ and R ₆₅ may be shared by one resistor. In addition, in the illustration, the collectors of the third and fourth transistors T ₅₂ and T ₅₃ are
Although they are coupled to the collectors of the first and second transistors T ₅₁ and T ₅₄ , respectively, they may be directly connected to the power supply line without such coupling.

FIG. 5 shows an example in which the variable gain circuit of the present invention is used as the automatic variable gain circuit 2 in a color amplifier circuit of a color television receiver, and 21 shows a composite color signal (carrier color signal + color burst). This is a band bus circuit that allows the

The manual gain variable circuit 2 is the transistor described above.
T ₅₁ , T ₅₂ , T ₅₃ , T ₅₄ , T ₅₅ , resistance R ₆₃ , R ₆₄ , R ₆₅ ,
R ₆₆ , R ₆₇ , R ₆₈ , R ₇₀ , and a volume (VR) part, and the automatic gain variable circuit 3 includes transistors T ₄₆ , T ₅₆ , T ₅₇ , T ₅₈ , T ₅₉ , T ₆₂ , and resistor R ₆₉ ，
The transistor is formed by _R72
T ₄₈ , T ₄₉ , T ₅₀ , diode D ₈ , D ₉ , resistor R ₅₄ ,
_R55 , _R56 , _R57 , _R58 , _R59 , _R60 , and _R61 form a constant voltage circuit. _T65 is a transistor constituting the second color amplifier 4, and its output is outputted to point K via an emitter follower transistor _T66 .

The composite color signal applied through the bandpass circuit 21 is supplied to the base of one transistor _T51 forming the first differential pair via a resistor _R62 , and is supplied from the output point a of the automatic variable gain circuit 2 to the automatic variable gain circuit. 3 to the output point d of the second color amplifier 4.
is input.

Of the differential pairs constituting the automatic variable gain circuit 3
The differential pair 23 consisting of T ₅₈ and T ₅₉ only works to keep the DC output level output to point d constant. On the other hand, among the transistors T ₅₆ and T ₅₇ that constitute the differential pair 22 that participate in signal amplification, T ₅₇ is off, but when the ACC signal goes up, the transistor T ₄₆ is turned off, and the base potential of the transistor T ₅₇ is Depends on ACC voltage.

When the transistor T ₅₇ is turned on by the ACC voltage, the DC current flowing through T ₅₇ is controlled by the ACC voltage, and therefore the DC current of the transistor T ₅₆ forming a differential pair with it also changes, and is output at point d. The gain of the resulting composite color signal will be automatically controlled by the ACC voltage.

T ₆₀ and T ₆₁ are transistors operated by color killer signals, and these transistors
T ₆₀ and T ₆₁ are connected as shown in the figure, and are turned off when the output of the color killer circuit given to point j is low level, and the transistors T ₅₁ , T ₅₂ , T ₅₃ , Color killer circuit 1 that is not involved in the function of T ₅₄ but is applied to point j
When the output of T51 is high level, it becomes conductive and raises the emitter potential of the transistors T51 _{and T51} .
_T54 is turned off, and the signal applied to the base of transistor _T51 is not output to the collector of transistor _T54 , and therefore to point a. Color killer is performed in this way.

According to the variable gain circuit of the present invention, the differential pair T ₅₁ ,
Since T ₅₄ and the transistor pair T ₅₂ and T ₅₃ always maintain an equilibrium state, the temperature characteristics are extremely good. Therefore, as shown in FIG.
The temperature characteristics are approximately constant even when the DC current is varied by the volume VR, and slight changes can be easily compensated for by making adjustments to the constant current source, for example. However, if the characteristics are as shown in FIG. 4, it can be put to practical use without such measures. Also, the third and fourth transistors
Changes in the base potentials of T ₅₂ and T ₅₃ are reduced by the resistors R ₆₄ and R ₆₅ at the emitters of the first and second transistors T ₅₁ and T ₅₄ , so the gain seen from the control terminal side (volume VR side) is Since it is limited by the resistances R ₆₄ and R ₆₅ , it is low, and there is also the advantage that there is no need to take a large ratio between the resistances R ₆₈ and R ₇₀ , making it suitable for IC implementation.

It is also conceivable to directly connect the emitters of the third and fourth transistors T ₅₂ and T ₅₃ to the emitters of the first and second transistors T ₅₁ and T ₅₄ without using the resistors R ₆₄ and R ₆₅ , but this In this case, the base-emitter capacitance of the third and fourth transistors T ₅₂ and T ₅₃ acts as a load (performs a detection function) for the first and second transistors T ₅₁ and T ₅₄ , which is not preferable, but the present invention Then, the base-emitter capacitance of the third and fourth transistors is isolated from the emitters of the first and second transistors T ₅₁ and T ₅₄ by the resistors _R 64 and R ₆₅ . No detection effect occurs.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing a conventional variable gain circuit, and FIG. 2 is an explanatory diagram thereof. FIG. 3 is a circuit diagram showing a variable gain circuit embodying the present invention, FIG. 4 is an explanatory diagram thereof, and FIG. 5 is a circuit diagram showing an example of its use. T ₅₁ , T ₅₄ ... Transistors forming the first differential pair, T ₅₂ , T ₅₃ ... Transistors forming the second differential pair, R ₆₄ , R ₆₅ ... Resistors, R ₆₈ ... First resistors , R ₇₀ ...Second resistance.

Claims (1)

[Claims] 1. A differential pair in which an input signal is applied to the base of at least one transistor and output is obtained from the collector of at least one transistor, and both emitters are connected to the emitter connection point of the differential pair. connected via a resistor, both collectors being connected to both collectors of the differential pair or the power supply line, respectively;
Furthermore, a pair of transistors having both bases connected in common, a first resistor having one end connected to a constant potential point, a second resistor having one end connected to the other end of the first resistor, and the other end of the second resistor. a volume having a variable terminal connected to its end; a connection point between the first and second resistors is commonly connected to both bases of the transistor pair; and a volume is connected to the transistor pair to vary the gain of the differential pair. A variable gain circuit consisting of variable means that commonly varies the flowing emitter direct current.