JPS62258509A - Gain variable type amplifier - Google Patents

Abstract

PURPOSE:To improve the S/N and to suppress the generation of distortion by inserting a diode having a forward resistance changed in response to the same forward current as to that of an equivalent emitter resistance of a differential transistor(TR) to the emitter of the TR. CONSTITUTION:Diodes D1, D2 having the forward resistance changed in response to the same forward current as the equivalent emitter resistor of the differential TRs Q4, Q5 are inserted respectively to the emitters of the TRs Q4, Q5. A noise voltage Vn is expressed in equation I. An output noise voltage Von2 at an output terminal 10 is expressed in equation II. Captions re4, re5 are equivalent emitter resistances of the TRs Q4, Q5, captions re6, re7 are forward resistors of the diodes D1, D2, and captions Vn1, Vn2 are noise voltages of the TRs Q4, Q5. Then the gain of the voltage Von2 with respect to the voltages Vn1, Vn2 is decreased by the resistors re6, re7. Thus, the S/N is improved and the generation of distortion is suppressed.

Description

TECHNICAL FIELD The present invention relates to a variable gain amplifier whose gain can be controlled by direct current voltage.

BACKGROUND ART A typical prior art equivalent circuit is shown in FIG. This variable gain amplifier is used to perform high frequency or intermediate frequency amplification in amplitude modulation receivers and the like. The signal voltage vi is applied from the input terminal 1 to the transistor Q3 and is converted into a signal current i. This signal current I
i is a differential transistor Q1.i controlled by a DC voltage applied to input terminal 2.3. A series resistor Re is connected to the transistor Q2, and the left side of the transistor Q2 serves as an output terminal 4. 4U41, reference numerals rel, re21, transistor Q1. is the equivalent emitter resistance of Q2, and Vnl
, vn2 equivalently represent the noise voltages of the transistors Q 1 and Q 2, respectively.

By controlling the voltage Vc applied to the terminal 2.3 and the bases of the transistors Q 1 and Q 2 , the current Ii is shunted, the output current Io flowing through the resistor Rc is diverted, and the gain is changed.

In such prior art, the transistor Ql.

There is a problem that the S/N ratio deteriorates due to the noise voltage vnl l vn2 of Q2. In other words, the noise voltage vn generated by the differential transistors Ql and Q2 as a whole is vn = I - 4 ears... (1)
The output noise voltage Von at this time is expressed by Equation 1a (%). This deterioration of the S/N ratio is due to the high gain with respect to the noise voltage V11.

Another prior art technique for solving this problem is shown in FIG. In this prior art, resistors R are inserted into the emitters of transistors Q1 and Q2, respectively, in order to lower the output noise voltage Von mentioned in IIq. In Figure 5, f
jS4 The same reference numerals are given to the parts corresponding to the prior art in FIG. According to this prior art, the output noise voltage V on 1 at the output terminal 4 is as shown in the second equation6, where Vn is as shown in the first equation.

As a result, the gain for the noise voltage vn can be lowered, the output noise voltage Vonl can be lowered, and the S/N ratio can be improved.

Problems to be Solved by the Invention In the prior art shown in FIG. 5, the resistor R is connected to the differential transistor Q1 . By inserting it into the emitter of Q2, a new problem arises: increased distortion. This distortion can be determined by considering how the output current ro flowing through the resistor Re with respect to the current 1 flowing through the transistor Q3, that is, how the offset current Ic due to the voltage Vc applied to the terminals 2 and 3 changes depending on the current 11.

Here, 1 voltage VcJ is more than the young power trannosta Ql.

Assuming that the current ratio flowing through Q2 is J, the current ratio J is expressed by the third equation.

1', l1l=J-1i...
(4) I i2 = (1-J)・■1
(5) Also, the current Ic is expressed by the sixth equation.

rel=K1/Ii+
・=(7) re2=ni2/ Ding i2
...value 8
)Ii1+l12=Ii...(
9) K 1 and K 2 are transistors Q 1 and Q 2
is a constant determined by the voltage, typically about 26-V at 25°C. Reference group Ii1. Ii2 is a current flowing through the differential transistor QllQ2. Therefore, the first
Equation 0 holds true.

Ic Vc Ii (2R+rel+re2)・Ii...(10
) According to this 10th equation, by inserting the resistor R, the current Ic is affected by the current Ii, and if you convert 3,
This will cause distortion.

An object of the present invention is to provide a variable gain amplifier that improves the S/N ratio and suppresses the occurrence of distortion.

Means for Solving the Problems The present invention provides a transistor for converting a signal voltage into a current;
In an amplifier in which the gain is varied by branching control of the signal current by a differential transistor controlled by a DC voltage, a forward current that is the same as the equivalent emitter resistance of the transistor is applied to each emitter of the differential transistor. This is a variable gain amplifier characterized by the insertion of a guiode with forward resistance.

According to the present invention, a giode is inserted in series with the emitter of each differential transistor, and this giode has a forward resistance that varies according to the forward current, which is the same as the equivalent emitter resistance of the differential transistor. . This makes it possible to reduce the gain for the output 'S sound voltage and improve the S/N ratio. Furthermore, it becomes possible to suppress the occurrence of distortion.

Embodiment FIG. 1 is an electrical circuit diagram of an embodiment of the present invention. The input terminal 7 has 7.0 to be amplified. Flow No. 111 is given.

be scarce. This signal voltage at input terminal 7 is converted into a current by transistor Q6. A resistor RE is connected in series to the transistor Q6. The signal current Ii from the transistor Q6 is applied to the differential transistor Q4. Diversion control is performed by Q5. Differential transistor Q 41 Q
A resistor R is connected in series to one of the transistors Q5.
e is connected. The connection point between the transistor Q5 and the resistor Re is an output terminal 10. Differential transistor Q4. The characteristics of Q5 are close to each other. A diode D1tD2 is connected in series to the differential transistors Q4 and Q5.
are connected in the forward direction.

FIG. 2 is an equivalent circuit diagram of the embodiment shown in FIG.

Reference mark re4. res are reference symbols reG and re7, which indicate the equivalent emitter resistance of transistors Q4 and Q5, respectively.
denotes the forward resistance that varies depending on the forward current of the diodes D 1 , D 2 with the reference vnltvn21±
, respectively transistor Q4. The noise voltage of Q5 is shown.

Differential transistor Q4. The 2I sound voltage vn generated as a whole of Q5 is as shown in the first equation, and at this time, the output noise voltage Von2 at the output terminal 10 is as shown in the eleventh equation.

From this 11th equation, the gain of the output noise voltage Von2 with respect to the noise voltage vnl+vn2 is smaller due to reGtre7 than in the 1st equation described in connection with the prior art described above. In this way, the output noise voltage can be reduced and the S/N ratio can be improved.

Differential transistor Q4. Equivalent emitter resistance re4 of Q5
．． re5 is as follows.

re4 = K4 / Ii4
= i12) res = K 5 / I is
-(13)K4. 5 is a constant determined by the transistors Q 4 and Q 5, and is typically 26-■ at 25°C as described above.

Forward resistance re6. which changes depending on the forward current of the diodes Di and D2. re7 is expressed as follows.

re6 = K G / I i4
・=(14)re7=K7/Ii5
- (15) where K G , K
7 is a value determined by the diodes D 1 and D 2 and is the same value as in the case of a transistor.

The offset current Ic due to the voltage Vc applied to the input terminal 8.9 is as follows.

...(16) Here, transistor Q4. If the current ratio flowing through Q5 is set as in the third, fourth, and fifth equations described in the above-mentioned prior art, the current Ii1 to current Ic is as follows.

From this 17th equation, it can be seen that the current Ic is not affected by the current Ii and therefore does not cause distortion. In this way, it is possible to suppress the occurrence of distortion.

A variable gain amplifier according to the invention can be used in the receiver in FIG. In this receiver, a high frequency signal from an antenna 12 is amplified in a high frequency amplification circuit 13 and provided to a mixing circuit 15 via a high frequency amplification automatic gain control circuit 14. A local oscillation signal from a local oscillation circuit 16 is applied to the mixing circuit 15 . The intermediate frequency No. 12 obtained in this way is amplified by nine times by an intermediate frequency amplification automatic gain control circuit 171, and then received in a superhetero gain method. Circuit 17 or C
) is detected by the detection circuit 18, amplified by the amplifier circuit 19, and made into sound by the speaker 20. The output from the detection circuit 18 is passed through the low-pass filter 2
1, the acoustic signal is cut off, and a DC voltage corresponding to the received electric field strength is led out to line 22. The voltage from line 2-2 is amplified by automatic gain control a1 amplifier circuit 23 and applied via line 24 to circuit 14.17.

The embodiment shown in FIG. 1 and FIG.
．． 17, and input terminals 8 and 9 are connected to line 2.
A DC voltage from 4 is applied, thereby achieving automatic gain control.

The invention is not only used for automatic gain control of receivers, but can also be implemented in a wide range of other applications.

Effects As described above, according to the present invention, a diode having a forward resistance that changes in accordance with the forward current, which is the same as the equivalent emitter resistance of the differential transistor, is inserted in the emitter of the differential transistor, so that the S/N ratio can be improved. It is possible to improve this and suppress the occurrence of distortion.

[Brief explanation of drawings]

Figure 1 is an electrical circuit diagram of one embodiment of the present invention, and Figure 2 is the electrical circuit diagram of one embodiment of the present invention.
FIG. 3 is a block diagram of a reception system implemented in conjunction with the variable gain amplifier according to the present invention, FIG. 4 is an equivalent circuit diagram of the prior art, and FIG. 5 is an equivalent circuit diagram of the embodiment shown in the figure. is an equivalent circuit diagram of another prior art. ? , 8.9...Input terminal, 10...Output terminal, Q4゜Q5...ffl! ) Lannostar, Q6...
Transistor, D1, D2...diode, R
E, Re... Resistance agent Patent attorney Shikyo Keiichi Figure 1■ Figure 2■ Figure 4■

Claims (1)

[Claims] In an amplifier that includes a transistor that converts a signal voltage into a current, and a differential transistor that controls the signal current by a DC voltage to control the shunt flow to vary the gain, each emitter of the differential transistor is connected to the equivalent emitter of that transistor. A variable gain amplifier characterized by inserting a diode with a forward resistance that changes according to the same forward current as the resistance.