JP2621994B2 - Voltage controlled variable gain amplifier - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial application field) The present invention relates to a voltage-controlled variable gain amplifier incorporated in a semiconductor integrated circuit.

(Prior Art) FIG. 4 shows a conventional voltage-controlled variable gain amplifier incorporated in a semiconductor integrated circuit. An input signal from the signal source 11 is applied to the inverting input terminal of the operational amplifier 12, the collector of the transistor Q1, and the collector of the transistor Q3 via the resistor R1. The non-inverting input terminal of the operational amplifier 12 is grounded. The base of transistor Q3 is grounded and the transistor
The base of Q1 is connected to control voltage source 13. The emitter of the transistor Q1 is connected to the emitter of the transistor Q2. The base of the transistor Q2 is grounded, and the collector is connected to the collector of the transistor Q4. The emitter of the transistor Q4 is commonly connected to the emitter of the transistor Q3, and is connected to the output terminal of the operational amplifier 12 via the resistor R3. In addition, transistors Q3 and Q4
A current source circuit including resistors R4 and R5 and a transistor Q5 is connected between the common emitter of the transistor Q1 and the common emitter of the transistors Q1 and Q2, and a power supply voltage is supplied from the power supply Vcc via the resistor R2.

The output from the common collector of the transistors Q2 and Q4 is supplied to the inverting input terminal of the operational amplifier 14 whose non-inverting input terminal is grounded, and is led out to the output terminal 15 via the resistor R6.

Assuming that the input signal supplied to the resistor R1 has a positive half cycle, the transistors Q3 and Q4 operate. The operational amplifier 12 works so that its inverting input terminal is equivalent to the ground potential, and flows a current through the resistor R3. Assuming that a current of (Vin / R1) flows through the resistor R1, a current equal to this flows through the transistor Q3. Assuming that the control voltage VC is zero, the current flowing through the transistor Q3 and the transistor
The current flowing in Q4 is equal. The current flowing through the transistor Q4 also flows through the resistor R6.

Here, when the control voltage VC is changed, the amount of current flowing through the transistor Q4 is also changed, and as a result, the output Vout also changes. The current of the transistor Q4 can be changed according to the following equation according to the control voltage VC.

IQ4 = IQ3 × exp (VC / V _T ) (1) V _T = KT / q KT: Boltzmann constant, q: electric charge, IQ4 and IQ3 are currents of transistors Q3 and Q4, respectively.

On the other hand, in the negative half cycle of the input signal, transistor Q1
And Q2 operate, and the current can be expressed by the following equation.

IQ2 = IQ1 × exp (VC / V _T ) (2) That is, the current flowing through the resistor R6 can be changed by the control voltage VC.

(Problems to be Solved by the Invention) The above-described conventional voltage controlled variable gain amplifier has a configuration in which the PNP transistors (Q1, Q2) and the NPN transistors (Q3, Q4) operate alternately every half cycle of the input signal. is there. For this reason, it is required that the characteristics of the PNP transistor and the NPN transistor be strictly matched. However, in general, a lateral (lateral) transistor must be used as a PNP transistor in a bipolar integrated circuit. In the case of a lateral PNP transistor, the base-emitter voltage VB
The relationship between E and the collector current is not a perfect exponential function characteristic, that is, a log characteristic. Equations (1) and (2) above assume that the collector current and VBE have perfect log characteristics.

As a result, in the conventional amplifier, when the control voltage VC is set to a value other than zero volt, output distortion is potentially present.

Accordingly, it is an object of the present invention to provide a voltage controlled variable gain amplifier that can improve output distortion even when using a Rural PNP transistor.

[Means for Solving the Problems] The present invention provides an operational amplifier in which an input signal is supplied to an inverting input terminal, and first and second opposite polarities in which the input signal is supplied to a common collector. , Third and fourth transistors of opposite polarity whose output signal is derived from a common collector, control voltage supply means for supplying a control voltage to the bases of the first and fourth transistors, A fifth transistor and a third transistor, which have opposite polarities to the third transistor and whose emitters are connected to the emitters of these transistors, respectively, and whose base is connected to the output terminal of the operational amplifier via a bias circuit; And the fourth transistor has a polarity opposite to that of the fourth transistor, and has an emitter connected to the emitter of each of the transistors and a base connected to the output terminal of the operational amplifier. Those having a transistor of the eighth.

(Operation) By the above means, in the positive half cycle of the input signal,
The set of the second, seventh, eighth, and fourth transistors operates, and in the negative half cycle, the set of the first, fifth, sixth, and third transistors operate. Each set includes the same number of PNP and NPN transistors, and in each set, the VBE of the PNP and NPN transistors are in series, so that a symmetric operation of the characteristics can be obtained in positive and negative half cycles, and the output Signal distortion is reduced.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 shows an embodiment of the present invention. The signal from the input signal source 21 is supplied to the resistor R11. The input signal derived from the resistor R11 is supplied to the inverting input terminal of the operational amplifier 22, and the first and second transistors Q1
1, Supplied to the common collector of Q12.

The set of the first and second transistors Q11 and Q12 is paired with the set of the third and fourth transistors Q13 and Q14 and is in a parallel relationship. An output signal is derived from a common collector of the third and fourth transistors Q13 and Q14. The common collector is supplied to an inverting input terminal of the operational amplifier 24 and one end of the resistor R12. Output terminal of operational amplifier 24 and resistor R1
The other end of 2 is connected to output terminal 25.

The emitter of the fifth transistor Q15 is connected to the emitter of the first transistor Q11.
The collector of 15 is connected to the positive power supply Vcc, and the base is connected to the output of the constant current source I11. Similarly, the emitter of the third transistor Q13 is also connected to the emitter of the sixth transistor Q16. The collector of the transistor Q16 is connected to the positive power supply Vcc, and the base is connected to the output of the constant current source I11. . The input side of the constant current source I11 is connected to the positive power supply Vcc.

The emitter of the second transistor Q12 is connected to the emitter of the seventh transistor Q17, and the collector of the transistor Q17 is connected to the negative power supply VEE.
The emitter of a similar fourth transistor Q14 is connected to the emitter of an eighth transistor Q18, the collector of which is connected to the negative power supply VEE.

Next, between the positive power supply Vcc and the output of the operational amplifier 22, a current bias circuit including a current interruption source I11, resistors R13 and R14, and a transistor Q19 is connected. The operational amplifier 22
And 24 non-inverting input terminals are grounded. In addition, the bases of transistors Q12 and Q13 are grounded,
A control voltage VC is applied to the bases of Q11 and Q14.

In the above circuit, when the input signal has a positive half cycle, the transistors Q12 and Q17, Q14 and Q18 operate.
Here, the operational amplifier 22 works to maintain its inverting input terminal (common collector of the transistors Q11 and Q12) at the same potential as the ground potential. This is obtained by supplying the base current of the transistor Q17 by supplying it to the operational amplifier 22 side.

Conversely, when the input signal has a negative half cycle, the transistors Q11, Q15, Q16, and Q13 operate. At this time, the operational amplifier 22 also works to maintain its inverting input terminal at the same potential as the ground potential. This is obtained by the fact that the base current of the transistor Q15 affects the current on the output side of the operational amplifier 22.

During the positive half cycle of the input signal, a current IR1 = Vin / R11 flows through the resistor R11. Then, a current equal to the current IR1 flows through the transistors Q12 and Q17.

Here, assuming that the control voltage VC applied to the base of the transistor Q14 is zero (ground potential), the same current flows through the transistors Q17 and Q18. Accordingly, the relationship between the current of each of the transistors Q12, Q14, Q17, and Q18 and the base-emitter potential VBE can be expressed as follows.

_{VBEQ12 = V T × ln (IQ12} / Isn) VBEQ17 = V T × ln (IQ17 / Isp) VBEQ14 = V T × ln (IQ14 / Isn) VBEQ18 = V T × ln (IQ17 / Isp) However Isn is of the NPN transistor The reverse saturation current Isp is the reverse saturation current IQ12 = IQ17 and IQ14 = IQ18 of the PNP transistor.

Further, since the bases of the transistors Q17 and Q18 are commonly connected, the following equation is established.

VBEQ12 + VBEQ17 = VBEQ14 + VBEQ18−VC From the above equations, the following equations can be summarized.

V _T × In (IQ12 / Isn) + V _T × In (IQ12 / Isp) = V _T × In (IQ14 / Isn) + V _T × In (IQ14 / Isp) -VC × Isn × Isp) / (IQ14 × IQ14 × Isn
× Isp)｝ = (− VC / V _T ). When further arranged, (IQ12 / IQ14) = exp (−VC / 2V _T ) IQ14 = IQ12 × exp (VC / 2V _T ). Since the collector current of the transistor Q14 flows through the resistor R12, a voltage of R12 × IQ14 is generated at the output terminal. When these are put together, VOUT = Vin × exp (VC / 2V _T ) can be obtained. That is, the output voltage can be controlled by the control voltage VC.

The above description has described the case of the positive half cycle,
Similarly, in the case of the negative half cycle, the following equation is obtained.

_{VBEQ15 = V T × ln (IQ15} / Isn) VBEQ16 = the _{V T × ln (IQ16 / Isn} ) VBEQ11 = V T × ln (IQ11 / Isp) VBEQ13 = V T × ln (IQ13 / Isp) VBEQ15 + VBEQ11 + VC = VBEQ16 + VBEQ13 resistor R11 The current of IR1 = Vin / R11 flows, and the same current flows through the transistor Q11. The output current is
Appears at the collector of transistor Q13. Therefore, the output voltage V
out is Vout = R12 × IQ13 VOUT = Vin × exp (VC / 2V _T ).

As described above, according to this embodiment, in the positive half cycle of the input signal, the transistors Q12 (NPN) and Q17 (PN
P), Q14 (NPN) and Q18 (PNP) work, and in the negative half cycle, Q15 (NPN), Q11 (PNP), Q16 (NPN), Q13 (PNP)
Works. In other words, since the PNP and NPN transistors are involved in both cycles of the input signal, positive and negative symmetric operations can be obtained, and the output can be obtained without distortion.

FIG. 2 is a circuit showing the operational amplifier 22 and the bias circuit of the circuit of FIG. 1 in detail.

Therefore, the same parts as those in FIG. 1 are denoted by the same reference numerals.
The operational amplifier 22 includes a differential pair of transistors Q21 and Q22, and a common emitter of the transistors Q21 and Q22 is connected to a negative power supply via a constant current source I22. The collector of the transistor Q21 is connected to the collector and base of the transistor Q23 forming the current mirror circuit and the base of the transistor Q24. The collector of transistor Q24 is connected to the base of transistor Q22. The collectors of the transistors Q23 and Q24 are connected to the positive power supply Vcc. The collector of the transistor Q22 is connected to the base of the transistor Q25. The transistor Q25 forms a bias circuit, and has a collector connected to the positive power supply Vcc, and an emitter connected to the emitter of the transistor Q19 via a diode-connected transistor Q26.
The collector of the transistor Q19 is connected to the negative power supply VEE via the constant current source I23.

FIG. 3 shows another embodiment of the present invention, in which the collectors of transistors Q17 and Q18 are connected to a constant current source I23 together with a base. Also, the transistors supplying the control voltage VC are changed to Q12 and Q13. Even with this configuration, the same operation as in the previous embodiment is obtained. The other parts are the same as in the previous embodiment, and are therefore given the same reference numerals.

[Effects of the Invention] As described above, according to the present invention, lateral PNP
A voltage-controlled variable gain amplifier that does not cause distortion in output even when transistors are used can be obtained.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing one embodiment of the present invention, FIG. 2 is a circuit diagram showing a part of the circuit of FIG. 1 more specifically, and FIG. 3 is a circuit showing another embodiment of the present invention. FIG. 4 is a circuit diagram showing a conventional voltage controlled variable gain amplifier. R11 to R14: resistor, Q11 to Q19, Q21 to Q25: transistor, 22, 24 ... operational amplifier.

Claims (1)

(57) [Claims] 1. An operational amplifier having an input signal supplied to an inverting input terminal; first and second transistors having opposite polarities, wherein the input signal is supplied to a common collector; and an output signal derived from the common collector. Third of opposite polarity,
A fourth transistor, control voltage supply means for supplying a control voltage to the bases of the first and fourth transistors, and an emitter connected to the emitters of the first and third transistors, the emitters being opposite in polarity to the first and third transistors. And
Fifth and sixth transistors whose bases are connected to the output terminal of the operational amplifier via a bias circuit, and whose emitters are connected to the emitters of the second and fourth transistors, respectively,
A seventh having a base connected to the output terminal of the operational amplifier;
A voltage controlled variable gain amplifier comprising an eighth transistor.