JPS6027210B2 - 3-stage direct-coupled amplifier circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a three-stage direct-coupled amplifier circuit, which has a single input terminal, the polarity of the first and final stage transistors is different, and the direct current resistance of the feedback path is reduced from the output terminal. The present invention provides an amplifier circuit characterized in that current changes in each stage due to changes in power supply voltage are extremely small by performing DC feedback through the power supply voltage.

In recent years, with the spread of operational amplifiers, CR type active filters using operational amplifiers have been used in various fields.

As is well known, operational amplifiers have a wide range of applications such as comparators, voltage amplifiers, and current amplifiers, but because a differential amplifier circuit is used in the first stage for versatility and improved characteristics, positive-phase It has an input terminal with a phase opposite to the input mode. For this reason, active filters using operational amplifiers have a feedback path constructed of a capacitor and a resistor between an output terminal and a positive-phase or negative-phase input terminal. At this time, input terminals to which no feedback path is connected need to be grounded or connected to a reference voltage source. When the operational amplifier is mounted in a membrane integrated circuit to configure a CR type active filter, the presence of the input terminal often becomes a factor that complicates the circuit configuration of the active filter. For this reason, an amplifier having a single input terminal is often effective in constructing an active filter.

Conventionally, a three-stage directly coupled amplifier circuit shown in FIG. 1 has been considered as an amplifier circuit having a single input terminal. Figure 1 shows transistors Q, , Q2, Q and resistor R. , R2, and R3, and the resistor RF is a negative feedback resistor for stabilizing the DC operating point. The DC operating point and current flowing through each stage of the three-stage direct-coupled amplifier circuit shown in FIG. 1 are given by equations '1', '2' and '3', respectively. 、． =vC
C-VBE2 '1}R,2=VCC
-V883 [2'R2 et al: VCC-V
BE・ [31R3 Formulas {1}, '2} and {3}1 where symbols 1, 12 and 13 indicate the current flowing through the first stage, second stage and third stage, respectively, and the symbol VB
E, , VBE2 and VB83 represent the base-emitter voltages of transistors Q, , Q2 and Q3, respectively.

In addition, the base currents of transistors Q, , Q2 and Q are sufficiently small compared to currents 1, , 12 and W, and the equation '1
}, {21 and [3'' are omitted. Formula {1)~
From (2) and (2), it can be seen that the current flowing through each stage increases or decreases approximately in proportion to the power supply voltage Vcc. The gain of each stage of the circuit shown in Figure 1 is determined by the ratio of the load resistance of each stage to the small signal emitter resistance of the transistor of each stage, when the load of each stage is sufficiently low compared to the input impedance of the next stage. 4. Since the signal emitter resistance is inversely proportional to the current flowing to the emitter of the transistor, the gain of the amplifier circuit increases or decreases in proportion to the increase or decrease in the power supply voltage.

Normally, an amplifier circuit is used with deep negative feedback applied to the feedback path, so the gain fluctuation is compressed by the amount of negative feedback.

For this reason, as a negative feedback amplifier, gain fluctuations in the low frequency range are often not a problem. However, in amplifiers using this active filter, it is necessary to perform phase compensation for stability in the high frequency range. Gain fluctuations are related to stability in the bridge frequency region, and if the stability is impaired, the amplifier may oscillate in the high frequency region.As mentioned above, the amplifier circuit shown in Figure 1 has a drawback in the power supply voltage fluctuation characteristics. be.

The present invention provides an amplifier circuit that improves the above-mentioned drawbacks.

The present invention will be explained in detail below using the figures. FIG. 2 shows a circuit connection diagram of the first embodiment of the present invention.

The first stage is composed of transistors 11 and 12 of different conductivity types whose collectors are connected to each other.
The second stage is composed of a transistor 13 of the same conductivity type as the transistor 11 and a resistor 15.

The third stage is transistor 4, which has the opposite polarity to transistor 11.
, and a resistor 16.

A resistor 17 represents a DC resistance of a feedback path connected between the input terminal 1 and the output terminal 2. The capacitor 8 is for phase compensation of the amplifier circuit. Terminal 3 is connected to power supply Vcc, and terminal 4 is grounded. Terminal 5 is a reference voltage terminal that connects a circuit that always has a constant voltage difference with respect to the power supply Vcc connected to terminal 3. The circuit operation of the first embodiment (FIG. 2) will be explained below. Note that the current amplification factor of each transistor is sufficiently large, and the explanation will be omitted with reference to the base current. transistor 2
The emitter current of is the potential difference between its base and emitter V88
, which is equal to the potential difference between the voltage Vcc at terminal 3 and the voltage Vr at terminal 6, and can be obtained from formulas (4} and [5}. Vcc-Vr=VB8,
'4}18,=1. exp(qVBE,/kT)
(5} Here, 1.

; Reverse saturation current k; Borckmann constant T; Absolute temperature [KO] q; Electron charge The emitter current of the transistor 2 flows to the collector of the transistor 11 and is grounded through the emitter.

When the current flows through the emitter of the transistor 131, the base. An emitter voltage V882 is generated between terminal 1 and terminal 4 according to the equation. The voltage between the terminals and the terminals is applied to the terminal 2 via the resistor 17.

Letting the resistance value of the resistor 16 be R2, the current IE3 flowing through the resistor 16 can be found by formula {6. Ear 83=VBB2/R2
The current IE3 flowing through the '61 resistor 16 becomes the collector current of the transistor 14 having the opposite polarity to that of the transistor 11.

The collector current is the emitter current 1 of the transistor 14.
It becomes 83. The base of the transistor 14 is expressed as follows. An emitter voltage VBE3 is generated. The voltage between both terminals of the resistor 15 is the base-emitter voltage VBB3.

Therefore, if the resistance value of the resistor 15 is R, the current IE2 flowing through the resistor 15 can be found from the formula {7}. IE2=VB
The current 1 and 2 flowing through the resistor 15 of E3/R becomes the collector current of the transistor 13.

As described above, the currents flowing through each stage of the amplifier circuit of the first embodiment (FIG. 2) are 18, IE2, and IE3, which are determined from equations (5-, 2, and 2).

How the currents IE, , IE2 and IB3 in each stage change as the power supply voltage Vcc changes will be shown below. Now, the power supply voltage Vcc connected to terminal 3 in FIG.
Assume that .

As mentioned above, the voltage at the terminal 5 increases according to the relational expression {4--. Therefore, the voltage difference between terminal 5 and terminal 3 is the power supply voltage Vcc
increases to Vcc, the voltage at terminal 5 increases to Vcc.
r becomes V'r. In other words, the ceremony is established. Vcc
-Vr=VBE,=V'cc-V'r ■(8'
From equation V88, IE can be found from equation {5).

In short, any means for satisfying the formula '8} may be used as long as the emitter current of the transistor 12, and therefore the collector current of the transistor 11, are always constant regardless of fluctuations in the power supply voltage. That is, the transistor 12 constitutes part of a constant current circuit. Therefore, the means for substantially satisfying formula (2) is not limited to one. FIG. 3 is a circuit connection diagram showing the most desirable example of such a reference voltage generating circuit.

20, 25 and 26 are resistors, 21 and 22 are NPN
transistors 23 and 24 are PNP transistors;
3 is the power supply terminal, 5' is the output terminal, and 3 and 5' are the second
It can be connected to 3 and 5 in the figure.

It has an NPN transistor 21 and a PNP transistor 24 forming a common emitter amplifier circuit with a resistive load, and a transistor 22 forming a common emitter amplifier circuit with a diode-connected PNP transistor 23 as a load,
A negative feedback loop is formed by cascading these three amplifier circuits so as to make one circuit. Due to this negative feedback effect, the output voltage appearing between the output terminal 5' and the power supply terminal 3 is extremely stable. Therefore, formula [8} holds true. Although the capacitor 27 is used to prevent oscillation of the negative feedback loop, it may not always be necessary. As mentioned above, even if the power supply voltage Vcc connected to terminal 3 changes, VB8 does not change, so
18.No change.

This means that IE, which can be found from equation {5' and equation {6}
3 does not change, and IE2 obtained from the above-mentioned IE3 and equation (5' and equation 7) also does not change.In this way, the amplifier circuit of the first embodiment (Fig. 2) does not change due to power supply voltage fluctuations. It has excellent characteristics.

As described with reference to FIG. 1, the excellent power supply voltage fluctuation characteristics indicate an amplifier circuit with small open gain change. Furthermore, the above-mentioned excellent characteristics indicate that the feedback amplifier has excellent stability in the high frequency range. FIG. 4 shows a circuit diagram of a second embodiment of the present invention.

In FIG. 4, the same numbers are used for the same parts as in FIG. 2. The difference between FIG. 2 and FIG. 4 lies in the phase compensation method of the amplifier circuit using the capacitor 18. In the first embodiment,
A capacitor 18 is inserted between the collector and base terminals of the transistor 13 for phase compensation. In the second embodiment, a resistor 19 is inserted into the emitter terminal of the transistor 14, and a capacitor 18 is inserted into the connection point between the base terminal of the transistor 13, the resistor 19, and the emitter terminal of the transistor 4 to perform phase compensation. ing.
Since the resistor 19 is inserted, the emitter current flowing through the transistor 13 in the second embodiment differs from that in the first embodiment. As mentioned above, in the second embodiment, the transistor 1
The current IE flowing through 1 is determined from equations {4) and '5}.

The emitter current 183 of the transistor 14 can be found from the above results and the equation (2). The base emitter voltage VB83 of the transistor 14 can be found from the emitter current IE8 and the above equation. Let the resistance value of the resistor 19 be R3. At this time, terminal 3
The voltage difference VB3 between the base terminal 3 of the transistor 14 and the base terminal 3 of the transistor 14 can be found from the equation. VB3: VBE30 IE3R3
'9} The second embodiment (
The emitter current 182 of the transistor 13 in FIG. 4) is determined. As described above, in the second embodiment, the currents 18, , IE2 and IE2 of each stage are calculated using the same formula as in the first embodiment.
E8 is determined.

This means that the second embodiment (FIG. 4) has the same excellent power supply voltage fluctuation characteristics as the first embodiment (FIG. 2). FIG. 5 shows a circuit diagram of a third embodiment of the present invention. The third embodiment has a resistor of 15 in the first embodiment.
This is an example in which the circuit is replaced with an active load circuit. The active load circuit shown in FIG. 5 is composed of a transistor 15 and a circuit that applies a constant voltage to the base terminal 6 of the transistor 5 from the power supply terminal 3. The third embodiment (Fig. 5) is the same as the first embodiment (Fig. 2) and the second embodiment (Fig. 4).
Compared to Figure 1), it is easy to increase the open gain.

In addition, in the first embodiment and the second embodiment, the transistor 11 and the transistor i3 are expressed by the above-mentioned formula '5},
As can be easily understood from [61,'7} and equation [9', it is necessary to have homogeneity from the viewpoint of power supply voltage fluctuation characteristics. In the third embodiment, transistor 11 and transistor 13 do not need to have the same polarity. The circuit in which the polarities of the transistors 13 and 15 in the third embodiment (FIG. 5) are swapped with each other and the voltage at the terminal 6 is made constant with the voltage at the terminal 4 is also equivalent to the third embodiment. It is clear that This clearly shows that the circuit current of the second stage constituted by the transistor 13 and the active load circuit does not depend on the voltage and current of the third stage. In addition, in the third embodiment, as the second embodiment (FIG. 4) can be considered from the first embodiment (FIG. 2), the difference between the emitter terminal of the transistor 14 and the terminal 3 in the third embodiment A resistor i9 is inserted between the emitter terminal and the base terminal of the transistor 13, and a capacitor 18 is connected between the emitter terminal and the base terminal of the transistor 13.
It is also possible to use a phase compensation method in which the

In the first embodiment, second embodiment, and third embodiment described above, the polarity of all transistors is reversed,
It is clear that an amplifier circuit in which the power supply voltage has the opposite polarity also has excellent power supply voltage fluctuation characteristics.

As described above, by implementing the present invention, there is almost no change in the open circuit gain of the amplifier due to changes in the power supply voltage, and it is possible to obtain a single-input three-stage direct-coupled amplifier circuit that has excellent stability in the high frequency region as a feedback amplifier. .

[Brief explanation of the drawing]

Fig. 1 is a wiring diagram of a conventional three-stage direct-coupled amplifier circuit, Fig. 2 is a circuit wiring diagram showing a first embodiment of the present invention, and Fig. 3 is a circuit wiring diagram showing an example of a reference voltage generation circuit. 4 and 5 are circuit wiring diagrams showing second and third embodiments of the present invention, respectively. 1...Input terminal, 2...Output terminal, 3...Power supply terminal, Tortoise...Ground terminal 5, 6...Reference voltage connection terminal, 5'...Reference voltage Output terminal, 11, 13,
21, 22, Q,, Q2, Q...・NPN transistor, 12, 14, turtle 5, 23, 24...--P
NP transistor, R,, R2, R3, 1 5, 1 6
, 20, 25, 26... Resistor, RF, 17... Feedback resistor, 19... Resistor for phase correction, i8, 27... Phase correction capacitor. Kaya J zuju 2 zu 3 zu kan 4 zu zu

Claims (1)

[Claims] 1. An input stage consisting of a first common-emitter transistor using a transistor active load that supplies a constant current in response to changes in power supply voltage, a second common-emitter amplifier stage connected in cascade to the input stage, and the input stage. A three-stage direct-coupled amplifier circuit, characterized in that a resistive load output stage consisting of a transistor with a polarity opposite to that of the transistor is connected in cascade, and a DC feedback path is provided from the resistive load output to the base of the input transistor.