JPS5848514A - Differential amplifying circuit - Google Patents

Abstract

PURPOSE::To make a gain of a differential amplifying circuit variable with simple constitution, by changing a current transfer ratio of a current mirror circuit. CONSTITUTION:An emitter circuit of output transistors (TRs) Q5, Q6 of a current mirror circuit is provided with N-channel junction type FETs Q9, Q10. A specified voltage is applied to each gate of the FETs Q9, Q10 from an AGC detector 11. In this case, an output of the detector 11 is set to be an optimum bias for the FETs Q9, Q10. The variable resistance property of the FETs is used in the explanation above, and depletion MOSFETs can also be used, and in general, variable resistive elements can be used.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a differential amplifier circuit configured to take out an output current using a current mirror circuit, and an object thereof is to make it possible to control the gain of such a differential amplifier circuit.

FIG. 1 shows a conventional differential amplifier circuit that extracts an output current using a current mirror circuit. In the same figure, Q
1 and Q2 are transistors, and R1 is a resistor, which constitute a differential amplifier circuit section.

Io is the transistor Q1. The current flowing through the constant current source connected to the emitter of Q2, υin, is the input terminal 1 of the differential amplifier circuit. Q3 and 05, Q4 and Q6 are transistors (hereinafter referred to as current mirror circuits Q3Q6 and Q4Q6) that constitute a current mirror circuit that takes out the output current from the differential amplifier circuit.
Q7 and Q8 are current mirror circuit Q3
Q6. Q4Q6! These transistors (hereinafter referred to as current mirror circuits Q7 and Q8) constitute a current mirror circuit that takes out the obtained current. vCC is a DC bias power supply terminal, and Z'out is an output terminal of the differential amplifier.

The gain of the above differential amplifier circuit is calculated using the following equation: 31. □
- be recognized. Transistor Q1. The collector current flowing into Q2 is lcl ” ”IO+'mZ' i n
”' ”・””)Ic2 ” ” IO'mZ' I
n-...(2) Here, a is the base grounding and amplification factor of the transistors Ql and Q2 constituting the differential amplifier circuit, and - is the mutual conductance in the differential amplifier.

The collector current I of the transistor Q is IC6=5
Q5)! . , (MP is the current transfer ratio of the current mirror circuit Q3Q6, and this value is also the current transfer ratio in Q4Q6)
, from Equation 1(1) above, I06=MP(aJ0−v,n
) ・・・・・・・・・(3) The collector current "Q6" of the transistor Q is "Q6 = MplC2", and from the above formula (2), Ice = Mp"IO'mZ'in ) '
(4) The collector current of transistor Q7 is 1.7=MnI,6 (Mn is the current transfer ratio of current mirror circuit Q7Q8), and from the above equation (4), Ic7 =MnMp (αIO'mZ"in) Hit...
(5) Therefore, the output current −Iout in the differential amplifier circuit with is Iout” from the above equations (3) and (5).
"Ic5"c7 = Mpα(I Mn) I o +Mp (1+ M
n)'rni)in......(6) - Now, the DC current amplification factor hFE of transistors Q7 and Q8 configuring the current mirror circuit Q7Q8 is
>1, the current transfer ratio Mn in the current mirror rotation i is as follows. Therefore, the m-flow I in the above equation (6). U, ha, I
out ” 2Mp 'mZ'i n
It is simplified and expressed as ``'''・''' (8).

Therefore, the output voltage Z'ou of the differential amplifier circuit in FIG.
t is Z'out=R2out=2Mpfrf,v1nR2...Group (9), and thus the gain in the differential amplifier circuit is expressed as: 7-=2MpfmR2...1 group QQ.

Here, in the case of Figure 1, which shows the conventional example, Mp', 1, G=2tmR2......αη.

Therefore, conventionally, in order to make the gain G variable,
The mutual conductance frn or the output resistance R2 may be made variable, but - is determined by the constant current source of the differential amplifier circuit, and R2 is also fixed by the circuit design.

As described above, in conventional circuits, it is difficult to change the gain of the differential amplifier circuit.

The present invention eliminates the above-mentioned conventional drawbacks, and uses current mirror circuits Q3Q6 and Q4Q in the above formula aQ.
The gain G of the differential amplifier is made variable by changing the current transfer ratio Mp of 6.

That is, in the typical current mirror circuit shown in FIG. 2, the relationship between the current transmission ground and R1 is as shown in FIG. 3. The figure is a depiction of the same.

According to the figure, by controlling the resistance value R1 from 200Ω to 6Ω, it is possible to control “2/I” within a range of 20.dB.

The present invention has been made in consideration of the various points mentioned above, and is intended to make the gain of a differential amplifier circuit variable.

A differential amplifier circuit according to an embodiment of the present invention will be explained below with reference to FIG.

FIG. 4 shows a differential amplifier circuit according to an embodiment of the present invention, and the same symbols are used for the same parts as in FIG. 1 showing the conventional example. The feature of this circuit is current mirror circuit Q3Q5
．． An n-channel junction field effect transistor (n-channel JFET) Q9 is installed as a load element in the emitter circuit of each of the output side transistors Q6 and Q6 of Q4Q6.
1Q1°, 7, - the n-channel lFETs Q9 and Ql. In this case, the output of the AGC detector 11 is an n-channel I F E T Os , Ql.

Set the bias state so that it is optimal for the In other words, Figure 6 shows the relationship between the drain-source voltage vDs and the drain-source current of an n-channel IFET (s characteristic from vDs-), and as seen in the same characteristics, the potential between the drain, source, and gate Relationship is the fourth
Acting as a suitable current limiting element in the circuit configuration shown, the output current l. It is necessary to take care so that ut is optimally controlled and a predetermined gain that satisfies the relationship of the above formula 〇〇 is obtained.

Note that in this circuit, the n-channel JFE
In order to prevent the drain-source voltage vDs of transistor Q6. The bases of Q6 are each configured to have a constant potential.

In addition, in the above embodiment, the variable resistance of the lFET.

Although the gain controllable current mirror differential amplifier circuit is shown, a depreciation/type MO5FET can be used in place of the above JFET, and in general, if a variable resistance element is used, the differential amplifier of the present invention can be realized. can be configured.

Further, a differential amplifier circuit having the same function as the above embodiment can be created by using the input differential circuit with a PNP configuration, the current mirror circuit with an NPN configuration, and a P-channel type JFETFi.

As explained above, the differential amplifier circuit of the present invention can control the gain with a simple configuration, and has high industrial utility value.

[Brief explanation of drawings]

Figure 1 is a circuit diagram of a conventional differential amplifier circuit, Figure 2 is a circuit diagram in which a resistor is inserted into one transistor of a current mirror circuit, and Figure 3 is a current transfer ratio of 12A in the circuit of Figure 2.
FIG. 4 is a circuit diagram of a differential amplifier circuit according to an embodiment of the present invention, and FIG. S is a diagram showing the current-voltage characteristics of an n-channel JFET used in the circuit. . ), Q3. Q4. Q6. Q6...transistor (pnp) transistor), QB+01
゜...Junction field effect transistor (n-channel IFET), R1, R2...Resistance. Name of agent: Patent attorney Nakao-Male and 1 other person 71

Claims (1)

[Claims] The control electrode of the first transistor is used as a signal input terminal, one main electrode of the first transistor is connected to a constant current source and also connected to one main electrode of the second transistor via a resistor, The one main electrode of the transistor is connected to a constant current source, and the control electrode thereof is connected to a constant voltage source, and the other main electrode of each of the first and second transistors is connected to an input side terminal of a current mirror circuit. and connecting the output terminal of each of the current mirror circuits to one main electrode of each of two transistors whose control electrodes are commonly connected and whose common connection point is connected to one main electrode of one of the transistors, A differential amplifier circuit characterized in that a variable resistance element is connected to at least one main electrode of the output side transistor of each of the above-mentioned main ≦ current mirror circuits.