JP2881771B2 - Variable gain amplifier - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a variable gain amplifier.

[Summary of the Invention]

According to the present invention, in a variable gain amplifier configured to perform gain control by controlling a constant current source of a differential amplifier, an output of the differential amplifier is taken out through a base-grounded transistor, and a dummy circuit and a detection circuit are provided. By feeding back the detected output, a change in the DC component does not occur in the output even when the gain is controlled.

[Conventional technology]

There are various types of AGC circuits provided in a radio receiver, and one of them uses a variable gain amplifier.

In addition, when the main part of the radio receiver is integrated into a one-chip IC, there is a type in which the volume is adjusted by an electronic volume type in order to reduce the number of external terminal pins of the IC.
Some use a variable gain amplifier.

As such a variable gain amplifier, as shown in FIG. 2, a differential amplifier is formed by transistors Qa and Qb, and the collector current of the constant current source transistor Qc is controlled by a control signal to control the gain. There is something to control.

Alternatively, as shown in FIG. 3, transistors Qd, Qe
Some have added differential amplifiers.

[Problems to be solved by the invention]

However, in the variable gain amplifier shown in FIG. 2, when the collector current of the transistor Qc changes according to the control signal, the collector currents (DC components) of the transistors Qa and Qb also change, so that the subsequent circuit is directly connected to the transistors Qa and Qb. I can't.

Therefore, in this variable gain amplifier, a subsequent circuit is connected through a DC cut capacitor, but when the signal to be gain controlled has a low frequency like an audio signal, the capacitance of the capacitor is reduced. Becomes too large to be integrated.

In this regard, in the variable gain amplifier of FIG. 3, the collector currents of the transistors Qd and Qe are determined by the collector current of the transistor Qf for the constant current source, and do not change by the gain control signal. Therefore, the circuit at the subsequent stage can be directly connected, and the IC can be formed.

However, in this variable gain amplifier, the collector potentials of the transistors Qa and Qb are equal to the base-emitter voltage (≒ 0.7 V) of the transistors Qd and Qe and the collector-emitter voltage (≒ 0.2 V) of the transistor Qf. Is the sum voltage
0.9V, and the load resistors Ra and Rb also require a terminal voltage of about 0.2V, so in this amplifier the power supply voltage is 1.
1V or more is required.

However, if the power supply is a single AA battery and the final voltage is 0.9V (that is, 60% of the rated voltage of 1.5V), this amplifier will not operate.

The present invention is to solve the above problems.

[Means for solving the problem]

For this reason, according to the present invention, in a variable gain amplifier in which gain is controlled by controlling a constant current source of a differential amplifier, an output of the differential amplifier is taken out through a base-grounded transistor, and a dummy circuit and A detection circuit is provided, and the detection output is negatively fed back.

[Action]

When the DC component of the output changes, this is canceled by feedback.

〔Example〕

In FIG. ₁ , the emitters of transistors Q ₁ and Q ₂ are connected to each other, and between the emitters and the ground, the collectors and emitters of transistors Q ₃ and Q ₄ for a constant current source are connected in parallel. Thus, a differential amplifier (1) is configured. The collectors of the transistors Q ₁ and Q ₂ are connected to the resistor R
_1, is connected via a R ₂ to the power supply terminal T _1, the base of the transistor Q ₁ is, for example, is connected to the previous stage S ₁ of the intermediate frequency amplifier, the base of the transistor Q ₂ is connected to the terminal T _1. Further, the base of the transistors Q ₃ and Q ₄ has a terminal T ₃
Supplies the AGC voltage Ea.

The collector of the transistor Q _1, Q ₂ is connected to the emitter of the transistor Q _5, Q _6, together with the resistor R _5, R ₆ is connected between ground and a collector, the transistor Q _5, Q transistors Q _5, Q ₆ is connected to the element R _42, Q ₄₂ whose bases below ₆ is a base-grounded.

Further, the collector of the transistor Q _5, Q ₆ is connected to the base of the transistor Q _7, Q _8, with its collector being grounded, between its emitter and the terminal T _1, the transistor Q _9, Q ₁₀ collector-emitter is connected, the emitter of the transistor Q _7, Q ₈ is connected to the base of the transistor Q _11, Q _12, and its emitter is grounded via a resistor R _11, R _12. Then, the transistors Q ₇ , Q ₉ , Q ₁₁ and the resistor R
₁₁ and the transistor Q _8, Q _10, Q ₁₂ respectively with resistors R _12, constitutes a signal extraction circuit, the transistors of the signal extraction circuit of the respective Q
_11, the collector of Q ₁₂ is connected to a subsequent circuit (not shown). Then, a predetermined base bias voltage Vbb is supplied to the transistors Q ₉ and Q ₁₀ through the terminal T ₂ ,
_9, Q _10, together with the constant current source, thereby the transistors Q _7, Q ₈ is an emitter follower.

The above is the main signal line, but a dummy circuit (2) which is DC-equivalent to this signal line is provided. That is, between the ground and the terminal T _1, a resistor R _22, and between the collector and the emitter of the transistor Q _22, Q ₂₄ are connected in series, the base of the transistor Q ₂₂ is connected to the terminal T _1, transistor Q ₂₄ based is connected to the terminal T _3.

Then, similarly, the collector of the transistor Q _22, is connected the emitter of the transistor Q ₂₆ is, together with the resistor R ₂₆ is connected between ground and its collector, a base connected to the base of the transistor Q ₆ transistor Q ₂₆ Te is the base ground. Also, transistors Q ₂₈ and Q
_30, Q ₃₂ and resistors R ₃₂ is connected to the terminal T ₁ and the ground in the same relationship as the transistors Q _8, Q _10, Q ₁₂ and resistors R _12, the base of the transistor Q ₂₈ is the collector of the transistor Q ₂₆ Connected.

Further, a DC potential detection circuit (3) is provided. That is, between the collector and the terminal T ₁ of the transistor Q _32, the collector-emitter of the transistor Q ₄₁ for the reference current is connected, its base connected to the terminal T _2, the collector of the transistor Q _41, Q ₃₂ but is connected to the base of the transistor Q _42, the emitter grounded, with resistor R ₄₂ is connected between the collector and the terminal T _1, the collector of the transistor Q _5, Q _6, Q ₂₆ Connected to base.

Note that the corresponding transistor and resistor have the same characteristics and values. That is, Q ₁ = Q ₂ = Q ₂₂ , Q _{3
= Q 4 = Q 24, Q} 5 = Q 6 = Q 26, Q 7 = Q 8 = Q 28, Q 9 = Q 10 =
_{Q 30, Q 11 = Q 12} = Q 32, R 1 = R 2 = R 22, R 5 = R 6 = R 26, R 11
= R ₁₂ = R ₃₂

According to this structure, the collector current of the transistor Q _41, current Is of the difference between the collector current of the transistor Q ₃₂ is supplied to the base of the transistor Q _42, a difference current Is to the collector of the transistor Q ₄₂ , And the collector has a DC potential corresponding to the difference current Is. Then, the transistor Q ₅ base is biased transistors Q _5, Q _6, Q ₂₆ by this DC potential, Q _6, Q ₂₆ operates as a common base.

Therefore, when the intermediate frequency signal from the previous stage S ₁ is supplied, the intermediate frequency signal, front S ₁ → differential amplifier (1) →
Common-base transistors Q ₅ and Q ₆ → emitter follower transistors Q ₇ and Q ₈ → common-emitter transistors Q ₁₁ and Q
The signal is amplified by the collector through ₁₂ signal lines and extracted.

In this case, when the AGC voltage Ea of the terminal T ₃ changes, the collector current of the transistor Q _3, Q ₄ corresponds to change to the gain of the amplifier (1) is changed, therefore, this circuit AGC is performed acting as a variable gain amplifier.

Also, if the collector currents of the transistors Q ₃ and Q ₄ change due to the AGC voltage Ea, the collector currents of the transistors Q ₁ and Q ₂ also change and the current potentials of the collectors also change.
R _22, Q _22, Q ₂₄ on one side of the element R ₂ of the differential amplifier _{(1), Q 2, Q} 4
Together we are equal connection relation when, since it is equal to the operating conditions, the DC potential of the collector of the transistor Q ₂₂ is also changed following the DC potential of the collector of the transistor Q ₂ by the AGC voltage Ea.

And now, the transistor Q ₂₂ (and the AGC voltage Ea)
Assuming that the collector current of Q ₁ , Q ₂ ) increases, the DC potential of the collector should decrease, the base-emitter voltage of the transistor Q ₂₆ should decrease, and the collector current should decrease.

However, the collector current of the transistor Q ₂₆ is reduced, reduction in the DC potential of its collector → the transistor Q ₂₈
Since emitter followers, reduction → differential current I of the collector current of reduction → transistor Q ₃₂ of the direct current potential of the emitter
s increased → increase of the collector current of the transistor Q ₄₂ → increase in the collector current of reduction → transistor Q ₂₆ of the direct current potential of the collector, by a process of reduction of the collector current of the transistor Q ₂₆ is canceled by the AGC voltage Ea .

Conversely, even if an attempt is increased collector current of the transistor Q ₂₆ is the AGC voltage Ea, is canceled in the same manner. Thus, when Is ≒ 0, the transistor
The feedback loop of Q ₂₆ → Q ₂₈ → Q ₃₂ → Q ₄₂ stabilizes,
The collector current of the transistor Q ₂₆ is maintained at a constant value irrespective of the AGC voltage Ea.

Alternatively, the base of the direct current potential of the transistor Q ₂₆ by even a feedback loop DC potential of the emitter is changed transistor Q ₂₆ by the AGC voltage Ea is changed to follow, the base-emitter voltage of the transistor Q ₂₆ is a constant value Is kept.

At this time, the transistor of the dummy circuit (2)
The connection relationship between Q ₂₂ , Q ₂₄ , Q ₂₆ and the resistors R ₂₂ , R ₂₆ , the transistors Q ₂ , Q ₄ , Q ₆ (and Q ₁ , Q ₃ , Q ₅ ) and the resistor R of the main signal line ₂ and R ₆ (and R ₁ and R ₅ ) have the same connection relationship, and transistors Q ₅ and Q ₆ have transistors Q _{5
Since a} base bias voltage equal to ₂₆ is supplied,
Regardless of the AGC voltage Ea, the collector currents of the transistors Q ₅ and Q ₆ are kept at a constant value equal to the collector current of the transistor Q ₂₆ . This also allows the transistors Q _{7 to} Q
_{The twelve} operating points are also kept constant regardless of the AGC voltage Ea.

At this time, each connection of a transistor Q ₇ to Q _12, is equal to the connection of a transistor _{Q 30, Q 28, Q 32} , each collector current of the transistor Q _11, Q ₁₂ is of the transistor Q ₃₂ equal to the collector current, therefore, approximately equal to the collector current of the transistor Q ₄₁ is a constant current source for reference.

Thus, according to the present invention, gain control can be performed by controlling the transistors Q ₃ and Q ₄ for the constant current source of the differential amplifier (1). In this case, in particular, according to the present invention, Differential amplifier that is the main signal line (1)
Connected to the base-grounded transistors Q ₅ and Q ₆ , and a DC equivalent dummy circuit (2) is provided.
Since corrected based DC potential of the transistor Q _5, Q ₆ through the dummy circuit (2), the transistors Q _5, Q
The DC component of the collector current of No. ₆ can be kept constant regardless of the control voltage.

Therefore, the circuit at the preceding stage can be directly connected, and a DC cut capacitor is not required.

Also, when viewed from the power supply voltage Vcc of the terminal T _1, the voltage required for the series-connected elements between the ground and the terminal T ₁ is
It does not exceed 0.9V and can therefore be operated with a single AA battery up to its final voltage of 0.9V.

In the above description, the transistor for the constant current source of the amplifier (1) is a transistor
Q _3, and two and the Q ₄ is, for example, the junction area between the base and emitter of the transistor Q _3, by twice that of the original transistors Q _3, when the base-emitter voltage is equal the collector current of the transistor Q _3, if 2 times the collector current of the original transistors Q _3, can be omitted transistor Q _4.

Further, the base, for example a transistor _{Q 1 ~Q 4, Q 22,} Q 24
The junction area of the _{emitter, Q 1 (Q 2):} Q 22 = Q 3 (Q 4): Q
_{24 = N: 1 (N>} 1) and _{then, Q 5 (Q 6):} Q 26 = Q 12 (Q 11):
Q ₃₂ = N: 1 (N> 1), R ₂₆ : R ₅ (R ₆ ) = R ₃₂ : R ₁₁ (R
_{12) = R 22: R 1} (R 2) = N: If 1 (N> 1) and can be the output current of the transistor Q _11, Q ₁₂ to N times of the transistor Q _41.

〔The invention's effect〕

According to the present invention, the transistors Q ₅ and Q ₆ having a common base are connected to the differential amplifier (1) which is a main signal line, and a dummy circuit (2) which is DC-equivalent thereto.
And a transistor through the dummy circuit (2)
Since corrected based DC potential of Q _5, Q _6, it can be kept at a constant value irrespective of the DC component of the collector current of the transistor Q _5, Q ₆ to the control voltage.

Therefore, the circuit at the preceding stage can be directly connected, and a DC cut capacitor is not required.

Also, when viewed from the power supply voltage Vcc of the terminal T _1, the voltage required for the series-connected elements between the ground and the terminal T ₁ is
It does not exceed 0.9V and can therefore be operated with a single AA battery up to its final voltage of 0.9V.

[Brief description of the drawings]

FIG. 1 is a connection diagram of an example of the present invention, and FIGS. 2 and 3 are diagrams for explanation thereof. (1) is a differential amplifier, (2) is a dummy circuit, and (3) is a detection circuit.

Claims (1)

(57) [Claims] 1. A differential amplifier to which an input signal is supplied, a pair of grounded base transistors for extracting a differential output of the differential amplifier, and a collector connected to each of the pair of grounded base transistors. A dummy circuit which is DC-equivalent to each of the signal extraction circuits, the differential amplifier and the pair of grounded transistors, and which is DC-equivalent to the signal extraction circuit and detects a DC operating point of the dummy circuit. A control circuit for controlling the output current of the constant current source of the differential amplifier with a control signal to control the gain, supplying the control signal to the dummy circuit, and detecting the detection output of the detection circuit. The signal is fed back to the dummy circuit, and the detection output of the detection circuit is supplied to the bases of the pair of transistors whose bases are grounded. Variable gain amplifier with gain is controlled in accordance with control signals, and to take out the output signal having a constant DC component regardless of the gain control.