JPH0220109A - Variable gain amplifier - Google Patents

Abstract

PURPOSE:To prevent the change of a direct current part from generating at an output even when the gain is controlled by taking out the output of a differential amplifier through the transistor of a base ground, providing a dummy circuit and a detecting circuit and feeding back the detecting output. CONSTITUTION:An intermediate frequency signal supplied from front and rear S1 is amplified and taken out from a differential amplifier 1 through the signal line of transistors Q11 and Q12 of the emitter ground to the corrector. The gain of the amplifier 1 is changed, the circuit functions as a variable gain amplifier and AGC is executed. When the corrector current of a transistor Q26 is reduced, the reduction of the corrector current of a transistor Q26 due to an AGC voltage Ea is cancelled. Reversely, even when the corrector current of the transistor Q26 is increased by the AGC voltage Ea, it is cancelled in the same way. Thus, the corrector current of the transistor Q26 is kept at a constant value regardless of the AGC voltage Ea.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to a variable gain amplifier.

[Summary of the invention]

The present invention provides a variable gain amplifier that performs gain control by controlling a constant current source of a differential amplifier.
In addition to extracting the output of the differential amplifier through a base-grounded transistor, a dummy circuit and a detection circuit are also provided, and the detection output is fed back to prevent DC changes in the output even when the gain is controlled. It is something.

[Conventional technology]

There are various types of AGC circuits provided in radio receivers, one of which uses a variable gain amplifier.

In addition, when the main part of a radio receiver is made into a single chip IC, in order to reduce the number of external terminal pins on the IC, the volume may be adjusted using an electronic volume, and in this case, a variable gain amplifier is also used. There is something to do.

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

Alternatively, as shown in FIG. 3, the transistor Qd,
Some have added a differential amplifier based on Qe.

[Problem to be solved by the invention]

However, in the variable gain amplifier shown in FIG. 2, when the collector current of transistor Qc changes due to the control signal, the collector currents (DC component) of transistors Qa and Qb also change.

It is not possible to directly connect subsequent circuits to Qb.

Therefore, in this variable gain amplifier, the subsequent circuit is connected through the DC cuff capacitor, but in this case, when the signal subject to gain control has a low frequency such as an audio signal, the capacitor Because of the large capacity, it is not possible to convert it into an IC.

In this respect, in the variable gain amplifier shown in FIG. 3, the collector currents of the transistors Qd and Qe are determined by the collector current of the constant current source transistor Q[, and are not changed by the gain control signal. Therefore, it is possible to directly connect subsequent circuits and also to integrate it into an IC.

However, in this variable gain amplifier, the collector currents of transistors Qa and Qb are
, the base-emitter voltage of Qe (J, 7V) and the collector-emitter voltage of transistor Qf (!=i
0.2V), 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 1.9V. lV
More than that is required.

However, if the power source is one AA battery and its final voltage is 0.9V (this is 60V with a rated voltage of 1.5
%) This amplifier will not work.

This invention attempts to solve the above problems.

[Means to solve the problem]

Therefore, in the present invention, in a variable gain amplifier whose gain is controlled by controlling a constant current source of the differential amplifier, the output of the differential amplifier is taken out through a transistor whose base is grounded, and a dummy circuit and A detection circuit is provided and its detection output is negatively fed back.

[Effect]

If the DC component of the output changes, this is canceled by the feedbank.

〔Example〕

In FIG. 1, the emitters of transistors Ql and Q2 are connected to each other, and a constant current source transistor Q3 is connected between these emitters and ground.

The collector and emitter of Q4 are connected in parallel to form a differential amplifier (1). And transistor Q,
, Q2 are connected to the power supply terminal T, through resistors R3, R,2, and the collectors of transistor Q1
For example, if the base of
The base of transistor Q2 is connected to terminal T1. Further, the AGC voltage Ea is supplied to the base of the transistor Q3°Q from the terminal T3.

In addition, the collectors of the transistors Q, Q2 are connected to the emitters of the transistors Qs, Qs, resistors R5, R6 are connected between the collectors and ground, and the bases of the transistors Qs, Qs are connected to the elements described later. The transistors Q s are connected to R,,,Q,,
, Q a are assumed to be base grounded.

Furthermore, the collectors of the transistors Qs, Qg are connected to the bases of the transistors q, Q, whose collectors are grounded, and between their emitters and the terminal T, the transistors Qs, Q+. This, the lid.
The emitters are connected, and transistors Q7゜Q8 ('
), , Il-mink is a transistor (e41+,
Q! It is connected to the base of 2, and its dummy is grounded across the resistor R11, R, and the 77" force (connected to the subsequent circuit (not shown) is connected to t7. , transistor Q3. Q, . is connected to terminal '1'2;
-7:' The base bias voltage bb of t+lT is supplied. Q and Q8 are considered to be Emi9's Noorowa.

The above is the main signal line, and a dummy circuit ('2) equivalent to this signal line in terms of paleo-DC is provided. ,t
'That is, between the terminal T and ground, there is a resistor R22&,
) Ran Sufuta Q22. Q24C); 1, dummy 7, and 7 are connected in series, and the transistor/difter Q2
The base of □ is connected to the lower terminal 2, and the transistors 0,
The base of 24 is connected to terminal 'I'3.

Similarly, the damifter of the transistor [U2□] is connected to the collector of the transistor Q22, and the resistor R26 is connected between the collector and the ground.
Its base is connected to the base of the transistor Q6, and the base of the transistor Q7 is grounded. Also, the transistors Q 2 R, Q 3 , ),
C, L 12 and resistor R12 are transistor Q,
+, Q, ;- and the resistor Ri 2 are connected to the terminal 'r, and the 5-junction jtI2B1, and the cross of the transistor Q28 is connected to the collector of the transformer and the 5-star Q26. .

Furthermore, a DC potential detection circuit (3) is installed, ・-1, that is, a transistor for the base 2f! current is connected between the node of the transistor Q3□ and the end ``■゛. Q4
1 is connected between the bottom and the bottom, the base is connected to the terminal T2, and the transistors Q,..., I
The 11° lid of the transistor Q42 is connected to the (-) terminal, its terminal is grounded, and the resistor R42 is connected between its collector and the terminal -F'I''. The collectors of these are transistors Q5, Q6, Q,...
t17) Connected to the base.

Note that the corresponding transistors and resistors have the same characteristics and values as ζl-. , that is, Q=Q2=Q
22. Q3=Q4=02-, Qs=Qa=Qz
T.

Q7=Qa=Q, e, Qs=Q+o=Q3o Q
x-CJ-iQ3□, R1=R2=Rzz, R3-・R
6=R7,+,RR,2-2) Ze...! : To be done. .

According to the configuration 1, a current i s which is the difference between the collector current of the difter Q,1 and the collector current of the transistor Q32 is supplied to the base of the transistor Q42, and this transistor A collector current of a magnitude corresponding to the difference current IS flows through the collector of Q42, and its collector becomes a DC potential corresponding to the difference current 1s.This DC potential causes transistor 9 to
．． The base of QL Ql& is biased and Torah”-
School Q11. 5, Q26 operates as a base ground. When an intermediate frequency signal is supplied from the front and rear S1, this intermediate frequency signal is transmitted to the previous stage S, → differential amplifier (1). →Base grounded transformer difter Qs, Qs
→ Emitter lower transistor Q, QIl → Emitter grounded transistor/jifter Q, Q, It is amplified and taken out to its top and lid through the signal line of 2.

In this case, when the AGC voltage Ea of the terminal 'r changes, the transistor 0. .

Since the collector current of Q changes, the gain of amplifier (1) changes, so this circuit works as a variable gain amplifier and AGC is performed, and the GC voltage Ea to
Transistor QI.

If the collector current of Q4 changes, the transistor Q,
, Q2 also changes, and the DC potential of its collector also changes, but elements R12, Q22 . Q24
is the element R2, Q on one side of the differential amplifier (1) 2. Q
Since the connection relationship is equal to + and the operating conditions are the same, the DC potential of the nifter of the transistor Q22 also changes according to the AGC voltage Ea, following the DC potential of the collector of the transistor Q2.

Now, if the collector current of transformer...lid Q22 (and Q,...Q2) increases due to AGC voltage Ea, then the DC potential of the collector decreases - the base-emitter voltage of transistor Q26 decreases.・The collector current should decrease.

However, when the collector current of transistor Q26 decreases, the DC potential of its collector decreases ~ → Since the transistor 28 is an emitter follower, the DC potential of its emitter decreases → the collector current of transistor Q32 decreases.
By the process of increase in the difference current Is → increase in the collector current of transistor Q, 2 - decrease in the DC potential of its collector → increase in the collector current of transistor Q26, AG
The decrease in the collector current of transistor Q25 due to C voltage Ea is canceled.

Also, on the contrary, AGC? Transistor Q due to J pressure Ea
Even if the collector current of 26 tries to increase, it is canceled in the same way. Thus, rs+.

In this state, transistors Q2,4 Q2. −＋Q
32→Q4. The feedback loop of is stabilized, and the collector current of transistor Q26 is kept at a constant value regardless of the GC voltage Ea.

Or 5. Transistor Q2 due to ΔGC voltage Ea
Even if the DC potential of the emitter of transistor Q26 changes, the DC potential of the base of transistor Q26 follows and changes due to the feed bank loop, so that the voltage between the base and emitter of transistor Q26 is maintained at a constant value.

At this time, the transistor Q of the dummy circuit (2)
22. Q24, Q26 and resistor L2
．． The connection relationship of R211 and the main signal line transistor Q2゜(L, Qll (and Ql, Q3.
QS) and resistors R2, R11 (and R, , R3) are connected equally, and the transistor Q
Since the base bias voltage equal to that of the transistor 02B is supplied to the transistors s and Qs, the collector current of the transistor 05°Q6 is kept at a constant value equal to the collector current of the transistor Q28 regardless of the AGC voltage Ea. In addition, as a result, the subsequent transistors Q-Q8. The operating point of is also kept constant regardless of the AGC voltage Ea.

At this time, since the connection relationships between the transistors Q7 to QI2 and the connection relationship between the transistors Q30% and Q32 are the same, the collector currents of the transistors Q1□, Q, and 2 are the same as those of the transistors Q1. is equal to the collector current of
Therefore, transistor Q41, which is a reference constant current source,
approximately equal to the collector current of .

Thus, according to the present invention, gain control can be performed by controlling the constant current source transistors Q 3 and Q 4 of the differential amplifier (1), but in this case,
In particular, according to the present invention, the differential amplifier (1), which is the main signal line, has a common base transistor Qs,
In addition to connecting Q e, a dummy circuit (2) equivalent to these in terms of DC is provided, and the DC potential of the bases of transistors Q and Q6 is corrected through this dummy circuit (2). The DC component of the collector current of , can be maintained at a constant value regardless of the control voltage.

Therefore, since the front and rear circuits can be directly connected and a DC cut capacitor is not required, it is easy to integrate the circuit into an IC.

Also, when viewed from the power supply voltage Vcc of terminal T, terminal '
The voltage required across the elements connected in series between FI and ground does not exceed 0.9v, so
It can be operated with a single AA battery up to its final voltage of 0.9V.

In the above description, in order to simplify the explanation, the constant current source transistor of the amplifier (1) is replaced with the transistor Q3.
．． For example, by making the junction area between the base and emitter of transistor Q3 twice that of the original transistor Q, the voltage between the base and emitter becomes equal, and the transistor when The collector current of Q3 is 2 times the collector current of the original transistor Q3.
If it is doubled, the transistor Q can be omitted.

Also, for example, transistors Q1 to Q1. Q22
, Q The junction area between the base and emitter of 24 is Q,
(Q2) :Q22= 03(Ql) : Q24=
If N : l (N > 1), then the transistor Q
,,, the output current of Q,2 is the N of transistor Q41.
It can be doubled.

〔Effect of the invention〕

According to this invention, base-grounded transistors Q s and Q are connected to the differential amplifier (1) which is the main signal line.
6 is connected, and a dummy circuit (2) equivalent to these in terms of DC is provided, and the DC potential of the bases of the transistors Q s and Q e is corrected through this dummy circuit [2]. =,, The DC component of the collector current of Q 6 can be kept at a constant value regardless of the control voltage.

Therefore, it is possible to directly connect the front and rear circuits, and there is no need for a DC capacitor, making it easy to integrate into an IC1. , terminal T, and ground, the voltage required for the elements connected in series is +1. There is no J exceeding gv, so it can be operated with one AA battery up to its final voltage of 0.9V.

[Brief explanation of the drawing]

Figure 1 is a connection diagram of one example of this invention, Figures 2 and 3.
The figure is for explanation. 1゜(1) is a differential circuit, (2) is a dummy circuit, and (3) is a detection circuit. ey+Don5 Figure 2 Complete rotation Figure 4 Figure 3

Claims (1)

[Claims] A differential amplifier to which an input signal is supplied; a pair of base-grounded transistors from which differential outputs of the differential amplifier are taken out; and a DC connection between the differential amplifier and the pair of transistors. It has an equivalent dummy circuit and a detection circuit that detects the DC operating point of this dummy circuit, and controls the output current of the constant current source of the differential amplifier using a control signal to control the gain, and also controls the gain. A signal is supplied to the dummy circuit, a detection output of the detection circuit is fed back to the dummy circuit, and a detection output of the detection circuit is supplied to the bases of the pair of transistors to connect the collectors of the pair of transistors. The gain is controlled according to the control signal from
A variable gain amplifier that extracts an output signal having a constant DC component regardless of this gain control.