JPH0652855B2 - Variable impedance circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Object of the Invention (Industrial field of application) The present invention relates to a circuit capable of changing impedance by electrical control, and is particularly excellent in linearity and suitable for a monolithic integrated circuit form. The present invention relates to a variable impedance circuit.

(Prior Art) A variable impedance circuit capable of electrically controlling impedance is used, for example, in a gain control stage of an automatic gain control (AGC) circuit. As a variable impedance circuit of this type, for example, there is a circuit disclosed in Japanese Patent Laid-Open No. 54-121044.

This circuit is basically a circuit as shown in FIG.

The input terminal 1 is connected to the base of the transistor Q11.
Transistors Q11 and Q12 form a differential transistor pair whose respective emitters pass through resistors R12 and R13,
It is connected to the second power supply (VEE) terminal 3 via the current source Ic1.

Transistors Q11, Q12, resistors R12, R13, current source Ic1
Form a differential voltage-to-current converter 11. The base of the transistor Q12 is AC-grounded.

The collector of the transistor Q11 is the emitter of the transistor Q17, and the collector of the transistor Q12 is the transistor Q11.
Connected to 18 emitters. The collectors of the transistors Q17 and Q18 are connected to the first power source (Vcc) terminal 2, and the common base is connected to the ground potential terminal via the diode-connected transistor Q19 and to the power source (Vcc) terminal via the resistor R11. It is connected.

The circuit 12 composed of the transistors Q17 and Q18 is
The non-linearity is a compensation circuit that cancels out the non-linearity of the transistors Q13 and Q14 forming the differential circuit 13 and compensates for the overall non-linear characteristic.

The bases of the differential transistors Q13 and Q14 are connected to the emitters of the transistors Q17 and Q18, respectively. The common emitters of the transistors Q13 and Q14 are connected to the second power source (VEE) terminal 3 via the current source Ic2.

The collector of the transistor Q13 is the collector of the transistor Q15, and the collector of the transistor Q14 is the transistor Q13.
It is connected to the collector of 16 and the bases of transistors Q16 and Q15. The transistors Q15 and Q16 form a current mirror circuit 14, and the emitter is the second power supply (Vc
c) Connected to end 2. Further, the collector of the transistor Q15 is connected to the input terminal 1 and provided with a feedback path.

Now, when the instantaneous voltage vin is supplied to the input terminal 1, the currents ic1 and ic2 flowing in the collectors of the transistors Q11 and Q12 are given.
Is represented as follows.

Re1 and re2 are transfer conductances of the transistors Q11 and Q12, and are expressed as follows.

re = VT / Ic (2) where VT is a thermal voltage (about 26 mV). Also, RE = R
12 = R13.

The transistors Q17 and Q18 are driven by the collector currents of the transistors Q11 and Q12, and apply the difference voltage between the base and the emitter to the bases of the transistors Q13 and Q14. The bases of the transistors Q17 and Q18 are biased by a diode-connected transistor Q19, and the reference voltage is kept at about 0.7V. Therefore, transistors Q13 and Q14
If the collector current components of are ic3 and ic4, then ic1 · ic4 = ic2 · ic3 (3) Next, the input current iin flowing from the input terminal 1 is the output current of the current mirror circuit 14, which is (ic3−ic
4) The above equation is approximated as follows, assuming that RE is sufficiently large compared to re11 and re12.

From this, the impedance Rin seen from the input terminal 1 is And the current ratio Ic1 / Ic of the two current sources Ic1 and Ic2
A variable impedance can be obtained by controlling c2.

Therefore, in the above circuit, the input terminal 1 is connected to, for example, the control terminal of the automatic gain control amplifier, and the gain control signal is applied to the current control units of the current sources Ic1 and Ic2. The gain of the amplifier can be controlled.

(Problems to be Solved by the Invention) When the above circuit is actually configured, since there are various factors (base current of transistor, Early effect, pairing error of transistor, etc.), input offset voltage (i
There is an input DC vin) required to make in = 0. However, if the resistance value RE is increased to such an extent that the approximate expressions shown in the equations (6) and (7) hold, the transfer conductance of the circuit (the conversion ratio 1 / RE of the input voltage and the output current) becomes too low and the input offset The voltage increases. That is, a large amount of offset voltage vin is required to set iin = 0.

Therefore, an object of the present invention is to provide a variable impedance circuit that eliminates the influence of the internal resistance of the transistor of the voltage-current converter, does not excessively reduce the transfer conductance of the circuit, and does not lose its linearity.

[Means for Solving Problems] (Means for Solving Problems) According to the present invention, the configuration of a voltage-current converter to which an input voltage is supplied is configured so that a base is connected to a differential input terminal. First and second transistors, a collector connected to the emitter of the first transistor, a third transistor whose base is connected to the emitter of the second transistor, and a base connected to the emitter of the first transistor A fourth transistor whose collector is connected to the emitter of the second transistor, and an impedance element interposed between the emitters of the third and fourth transistors,
Two equal-sized current sources connected to the emitters of the third and fourth transistors, respectively.

(Operation) In the voltage-current converter, so-called differential quartet operation is obtained by the above means, that is, the input voltage applied to the differential input terminals is composed of the first to fourth transistors and the impedance element. It is represented by the product of the path and the current flowing in this path. At this time, the sum of the voltage components generated in the internal resistance of each transistor is canceled, so that the input voltage is not affected by the internal resistance of the transistor. The circuit has excellent linearity.

Embodiment An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows an embodiment of the present invention, in which the same parts as those of the conventional circuit are designated by the same reference numerals. This circuit is voltage-
A differential quartet circuit (Precis
ion differential voltagquartet. R ・ Caprio, Elect, Le
tt., vo19, pp.147-148,1973).

That is, the input voltage vin is supplied to the input terminal 1 and guided to the base of the transistor Q11. Transistors Q11, Q
Reference numeral 12 is a differential transistor pair, and the base of the transistor Q12 is connected to the ground potential terminal. And transistor Q
The emitters of 11 and Q12 are connected to the emitters of transistors Q21 and Q22. The base of the transistor Q21 is connected to the emitter of the transistor Q12 and the transistor Q22.
Is connected to the emitter of the transistor Q11.

An impedance element R11 is connected between the emitters of the transistors Q21 and Q22, and each transistor Q21,
The emitter of Q22 is connected to the second power source (VEE) terminal 3 via current sources Ic11 and Ic12, respectively.

Therefore, in the voltage-current converter 100, assuming that the input voltage vin is the reference voltage (zero), equal currents flow through the respective transistors Q11, Q12, Q21, Q22, but the voltage at the input terminal 1 is balanced. The current ic1 flows when is increased or decreased with reference to zero.

The collectors of the differential transistors Q11 and Q12 are
In the same manner as in the conventional case, they are connected to the emitters of the transistors Q17 and Q18 that form the compensation circuit 12 and to the bases of the transistors Q13 and Q14 that form the differential circuit 13.

The collectors of transistors Q17 and Q18 are the first power supply (Vcc)
It is connected to the end 2 and the base is connected to the ground potential end via the diode-connected transistor Q19 and is also connected to the power supply (Vcc) end 2 via the resistor R11. The transistors Q17, Q18, Q19 and the resistor R11 set the base bias of the transistors Q13, Q14,
The operating point is set to compensate for the non-linearity.

The emitters of the transistors Q13 and Q14 are connected to the second power source (VEE) terminal 3 via the current source I2. The collector of the transistor Q13 is connected to the collector of the transistor Q15, and the collector of the transistor Q14 is connected to the collector and base of the transistor Q16 and the base of the transistor Q15. The transistors Q15 and Q16 form a current mirror circuit 14, the collector of the transistor Q16 is an input part, the collector of the transistor Q15 is an output part, and the emitters of the transistors Q15 and Q16 are connected to the first power supply (Vcc) terminal 2. ing.

Further, the output section of the current mirror circuit 14 and the input terminal 1 are connected to form a feedback path.

This embodiment is configured as described above, and the current sources Ic11, Ic1
The impedance seen from the input terminal 1 can be varied by varying the ratio of the current flowing through 2 (same value) and the current flowing through the current source Ic2. This function will be described below.

Now, the following relational expression is established between the input voltage vin and the current ic1 flowing in the differential quartet circuit.

vin = -re12.ic1-re21.ic1 + R21.ic1 + re22.ic1 + re11.ic1 = ic1 (re22-re12 + re11-re21 + R21) (8) where the current amplification factor β of each transistor is sufficiently high. Since the collector currents of Q12 and Q22 are equal, and the collector currents of transistors Q11 and Q21 are also equal, Is established. Therefore, equation (8) is Can be expressed as As can be understood from this equation, it is understood that vin is converted into a current without being affected by the internal resistance re of the transistor. As a result, the input current iin flowing from the input terminal 1 is Input impedance R seen from input terminal 1
in is It is represented by.

Therefore, according to the above circuit, the linear resistance of the voltage-current converter is further improved as compared with the conventional one without being affected by the internal resistance re of the transistor, and the resistance R21 is arbitrarily set without considering the magnitude of the internal resistance re. You can choose.

In the above circuit, when the difference between the reference voltage (zero potential at the midpoint) and the power supply voltage is large, due to the Early effect of the transistors Q13 and Q14 (effective reduction of the baseband),
The input offset voltage increases and may be accompanied by non-linear characteristics.

In such a case, as shown in FIG. 2, it can be improved by providing a current mirror circuit 14A and a voltage shift circuit 15. Various other examples of the current mirror circuit 14A can be considered. In this example, the emitter of the transistor Q16 is the collector of the transistor 26 so as to eliminate the DC potential difference between the input section and the output section of the current mirror circuit. , The emitter of the transistor Q15 is connected to the collector and base of the transistor Q26, and the transistor Q27.
Connected to the base of. And transistors Q26 and Q
The emitter of 27 is connected to the power supply (Vcc) terminal 2 through the emitter collector of the transistor Q25. Therefore, the current flowing into the emitters of the transistors Q15 and Q16 is kept at the same value by the current mirror circuit of the transistors Q26 and Q27.

On the other hand, the bases of the transistors Q17 and Q18 are connected to the first via the diode-connected transistors Q23 and Q24 and the resistor R11.
It is connected to the power supply (Vcc) terminal 2 and voltage-shifted.
Also, the base of transistor Q25 is biased by the voltage appearing at the collector of transistor Q24. Where now
If vin = 0, the collector of the transistor Q13 has a direct current potential of 0v with respect to the reference potential. According to this structure, it is possible to obtain a variable impedance circuit having good linearity in a wide power supply voltage range.

[Effects of the Invention] As described above, the present invention can provide a variable impedance circuit having good linearity without being affected by the internal resistance re of the transistor.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, FIG. 2 is a circuit diagram showing a second embodiment of the present invention, and FIG. 3 is a diagram showing a conventional variable impedance circuit. Q11-Q19, Q21-Q27 ... Transistors, Ic11-I
c12, Ic2 ... current source, R11, R12 ... resistance, 100 ...
... voltage-current converter, 12 ... compensation circuit, 13 ... differential circuit, 14, 14A ... current mirror circuit, 15 ...
Voltage shift circuit.

Claims (1)

[Claims] 1. A differential voltage-current converter having a differential input terminal pair and a differential output terminal pair, and forward biased by an output current from the differential output terminal pair of the voltage-current converter, And a pair of transistors whose bases are commonly connected and kept at a constant potential, and a second differential transistor pair to which a differential voltage between the base-emitter voltage of this transistor pair is applied to the differential input pair. A variable impedance circuit having a current mirror circuit connected to the second differential transistor pair and a feedback path for returning an output signal of the current mirror circuit to the input terminal, A first and a second transistor whose bases are respectively connected to the differential input terminal pair, and a collector connected to the emitter of the first transistor, A third transistor to which the emitter base of the second transistor is connected, a fourth transistor whose base is connected to the emitter of the first transistor, and a collector to the emitter of the second transistor; A variable characteristic comprising: an impedance element interposed between the emitters of the third and fourth transistors; and two current sources of equal magnitude connected to the emitters of the third and fourth transistors, respectively. Impedance circuit.