JPH0514571Y2 - - Google Patents

Description

[Detailed description of the invention] [Industrial application field] The invention consists of a first-stage differential amplifier, a second-stage differential amplifier, and their respective loads, and feeds back the output to the first-stage differential amplifier. The present invention relates to a variable impedance circuit that controls output impedance by controlling the mutual conductance of an amplifier.

[Conventional technology]

Conventionally, this type of variable impedance circuit has been applied to an amplifier with automatic gain control characteristics.
This automatic gain control characteristic is a characteristic that maintains the output level at a constant level without distortion even if the input signal level exceeds a certain value, as shown in FIG.

FIG. 3 is a block diagram of a conventional example of an amplifier having automatic gain control characteristics. A signal is input from an AC signal source 3 via a capacitor 9 and a resistor 8, and the AC output of a fixed gain amplifier 5 whose gain is set by gain setting resistors 41 and 42 is converted to DC by a detection/rectifier circuit 6 and then controlled. The open gain of the full feedback amplifier 30 is controlled through the circuit 7. Since the output impedance of the full feedback amplifier 30 is the value obtained by dividing its characteristic output impedance by the open gain, the impedance at point B is controlled by controlling the open gain, and the impedance at point B is
By connecting the point A with the capacitor 29, the impedance of the point A can be controlled. Since the input signal level of the fixed gain amplifier 5 is obtained by dividing the signal level of the signal source 3 by the fixed resistor 8 and the output impedance of the full feedback amplifier 30, the output level can be kept constant. In other words, the full feedback amplifier 30 operates as a variable impedance circuit.

FIG. 4 is a circuit diagram specifically showing the full feedback amplifier 30 of FIG. 3. In FIG. In this conventional example, the full feedback amplifier 30 consists of a first-stage differential amplifier and a second-stage differential amplifier. The first stage differential amplifier has a transistor 20 whose base is connected to point B and whose collector is connected to ground.
, a transistor 21 whose base is connected to the reference power supply 2 and whose collector is connected to the ground, whose base is connected to the emitter of the transistor 20, whose collector is connected to the anode of the diode 23, and whose emitter is connected to the resistor 26.
The transistors 18 are connected to one end of the constant current source 27 through the
a transistor 1 whose collector is connected to the anode of a diode 22 and whose emitter is connected to one end of a constant current source 27 via a resistor 25;
9, a power supply connected to the other end of the constant current source 27,
The anodes of the cathodes of diodes 22 and 23 are connected to each other, and a diode 24 has a cathode connected to ground. Further, the next-stage differential amplifier includes a transistor 17 whose base is connected to the anode of the diode 22 and whose collector is connected to point B.
, a transistor 11 whose collector is connected to point B, an emitter connected to the positive side of the power source 1, a transistor 10 whose emitter is connected to the positive side of the power source 1 as a base, and a collector connected to the base of the transistor 11;
A transistor 16 whose collector is connected to the collector of the transistor 10 and whose base is connected to the anode of the diode 23, and one end is connected to the transistors 16 and 17.
, and a variable current source 28 controlled by a control circuit 7 whose other end is connected to the negative electrode side of the power source 1.

5 has a configuration in which the variable current source 28 in FIG. 4 is replaced with a constant current source 27, and the constant current source 27 is replaced with a variable current source 28.

[Problem that the invention attempts to solve]

Here, Vcc...potential of power supply 1, Vref...potential of reference power supply 2, _V10 , _V16 , _V17 ...transistors 10, 16,
Base-emitter voltage of 17, V ₂₂ , V ₂₃ , V ₂₄ ...diodes 22, 23, 2
4, the collector-emitter voltage of transistor 16 is given by Vcc - V ₁₀ - (V ₂₃ + V ₂₄ - V ₁₆ ) ≒ Vcc - 1.4 (V), whereas transistor 17 The collector-emitter voltage of is given by Vref−(V ₂₂ +V ₂₄ −V ₁₇ )≒Vref−0.7(V). That is, transistor 16,1
The voltage between the collector and emitter of 7 is different.
Therefore, as shown in FIG. 6, a difference occurs in the collector currents of transistors 16 and 17 due to the Early effect of the transistors, which causes transistor 1
A difference is generated between the collector current of transistor 9 and the collector current of transistor 18, and a difference is also generated between the base potentials of transistor 20 and transistor 21. Specific numerical examples are shown below. In the circuit of Fig. 4, the collector currents of transistors 16 and 17 are respectively
Assuming Ic ₁₆ and Ic ₁₇ , the difference ΔV _16-17 between the base potentials of the transistor 16 and the transistor 17 is expressed as ΔV _16-17 =kT/qln Ic ₁₆ /Ic ₁₇ . This ΔV _16-17 directly becomes the voltage difference between the anode of the diode 22 and the anode of the diode 23. Therefore, the collector current of transistors 18 and 19 is
Assuming Ic ₁₈ and Ic ₁₉ , Ic ₁₉ /Ic ₁₈ = Ic ₁₇ /Ic ₁₆ .

Here, Ic ₁₉ −Ic ₁₈ = 2×g _n ×ΔV _19-18 However, g _n ...transistors 18, 19, resistor 25,
Mutual conductance of the differential amplifier composed of 26 ΔV _19-18 ...Voltage difference between the base potential of transistor 19 and the base potential of transistor 18 Also, if the current value of constant current 27 is I ₀ , then Ic ₁₈ = I ₀ /(1+Ic ₁₇ /Ic ₁₆ ), Ic ₁₉ =I ₀ /(1+Ic ₁₆ /Ic ₁₇ ) Therefore, I ₀ (Ic ₁₇ /Ic ₁₆ −Ic ₁₆ /Ic ₁₇ )/2+Ic ₁₇ /Ic ₁₆
+Ic ₁₇ /Ic ₁₆ × 1/2 × g _n =ΔV _19-18 (1).

The early voltage of transistors 16 and 17 is
20V, Vcc is 10V, and the voltage Vref of the reference power supply 2 is 5V, the voltage between the collector and emitter of transistor 16 is 8.4V, and the voltage between the collector and emitter of transistor 17 is 8.4V.
The emitter voltage is 4.3V, so transistor 1
The collector current ratio of 6 and 17 is Ic ₁₇ /Ic ₁₆ = 4.3+
20/8.4+20=24.3/28.4. I ₀ value 200μA,
If the value of the emitter resistors 25 and 26 is 750Ω, the mutual conductance g _n is 4.96×10 ^-4 S (Siemens)
Therefore, ΔV _19-18 is 12.2mV. As a result,
If the input point A of the fixed gain amplifier 5 and the output point B of the variable impedance circuit are directly connected, the above voltage difference will also be DC amplified, and the output operating point C of the fixed gain amplifier 5 will shift, causing a disturbance in the automatic gain control characteristics. , causing deterioration of the distortion rate of the output signal.

The above description has been made regarding the transistors 16 and 17, but the same can be said about the transistors 10 and 11. Therefore, in the conventional circuit example, the fixed gain amplifier 5 and the output point B of the variable impedance circuit must be coupled through the capacitor 29, which has the disadvantage of increasing the number of components and increasing the number of pins when integrated.

(Means for solving the problem) The present invention is a full feedback amplifier in which the output impedance at the output terminal is controlled by the control output from the control circuit, and is composed of a first stage differential amplifier and a second stage differential amplifier. , the first stage differential amplifier includes a first transistor having a base connected to a reference voltage source and a collector connected to a common potential, and a second transistor having a base connected to the output terminal and a collector connected to a common potential. a third transistor whose base is connected to the emitter of the first transistor;
a fourth transistor having a base connected to the emitter of the second transistor; and a first current source having one end connected to the emitters of the third transistor and the fourth transistor via a resistor and the other end connected to a power supply. , a first diode having an anode connected to the collector of the third transistor, a second diode having an anode connected to the collector of the fourth transistor, and cathodes of the first diode and the second diode connected to the anode. and a third diode whose cathode is connected to a common potential, and the next stage differential amplifier includes a fifth transistor whose base is connected to the collector of the fourth transistor, and whose base is connected to the collector of the third transistor. a sixth transistor whose collector is connected to the output end; a current inflow end connected to the power supply and a current outflow end connected to the collector and the output end of the fifth transistor; A first current mirror circuit serving as a load of the transistor, and a second current source having one end commonly connected to the emitters of the fifth transistor and the sixth transistor and the other end connected to a common potential, In the variable impedance circuit, one of the source and the second current source is a variable current source whose current is controlled by the control output of the control circuit, and the other current source is a constant current source, a second current mirror circuit inserted between the first current mirror circuit and the collector of the fifth transistor and the output terminal; an anode connected to the output terminal; and a collector of the sixth transistor of the next stage differential amplifier; a seventh transistor having a base connected to the reference voltage source, a collector connected to the second current mirror circuit, and an emitter connected to the collector of the fifth transistor. It is characterized by

Therefore, if the variable impedance circuit of the present invention is applied to an amplifier with automatic gain control characteristics, the DC voltage at the output end of the variable impedance circuit will be approximately equal to the DC bias voltage at the input end of the amplifier, so it can be directly connected without a capacitor. be able to.

〔Example〕

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

FIG. 1 is a circuit diagram showing an embodiment of an amplifier having automatic gain control characteristics and having a variable impedance circuit according to the present invention.

This embodiment is constructed by adding transistors 12, 13, 14 and a diode 15 to the conventional circuit shown in FIG. With the addition of transistor 14 and diode 15, the voltage between the collector and emitter of transistors 16 and 17 is approximately Vref.
-1.4 (V), and the influence of the Early effect of the transistor can be ignored. In addition, transistors 12,1
3, the voltage between the collector and emitter of transistors 10 and 11 is about 0.7V, and the influence of the Early effect can be ignored.

Here, as a specific example, Vcc=10V, Vref=5V
Then, the collectors of transistors 16 and 17 are
The emitter voltage is 3.6V in both cases, and the difference is less than 0.1V. At this time, Ic ₁₆ /Ic ₁₇ is approximately 23.6/23.7 at the maximum, and ΔV _19-18 obtained from equation (1) is 0.21 mV, which is an improvement of 1/57 compared to 12.2 mV in the conventional example.

As mentioned above, since the Early effect can be ignored, the difference between the voltage at point B and the voltage at reference power supply 2 can be reduced,
Point A and point B are directly connected.

[Effect of idea]

As explained above, the present invention provides a conventional variable impedance circuit that includes a Wilson current mirror circuit that serves as a load for the next-stage differential amplifier, a diode inserted between the output terminal and the collector of the next-stage differential amplifier, By having the emitter, collector, and base each having a collector different from the collector of the next-stage differential amplifier, the low impedance side of the current mirror circuit, and a transistor connected to a voltage source that is DC-equal potential to the output terminal, the transistor It is possible to reduce the voltage difference between point B and the reference voltage due to the Early effect, and it is now possible to directly connect a variable impedance circuit and a fixed gain amplifier, which was previously impossible. This has the effect of enabling an automatic gain control device.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing an embodiment of an amplifier with automatic gain control characteristics and a variable impedance circuit of the present invention, Fig. 2 is an input/output characteristic diagram of the amplifier with automatic gain control characteristics, and Fig. 3 is a block diagram of a conventional amplifier with automatic gain control characteristics;
FIG. 5 is a circuit diagram of the specific example of FIG. 3, and FIG. 6 is a diagram showing the Early effect of the transistor. DESCRIPTION OF SYMBOLS 1... Power supply, 2... Reference power supply, 3... AC signal source, 41, 42... Gain setting resistor, 5... Fixed gain amplifier, 6... Detection/rectification circuit, 7... Control circuit, 8... ...Resistor, 9,29...Capacitor, 1
0, 11, 12, 13, 14, 16, 17, 1
8, 19, 20, 21...transistor, 15,
22, 23, 24...diode, 25, 26...
...Emitter resistor, 27... Constant current source, 28... Variable current source, 30... Full feedback amplifier.

Claims (1)

[Claims for Utility Model Registration] A full feedback amplifier whose output impedance at the output terminal is controlled by a control output from a control circuit, comprising a first-stage differential amplifier and a second-stage differential amplifier, the first-stage differential amplifier a first transistor 21 whose base is connected to a reference voltage source and whose collector is connected to a common potential; a second transistor 20 whose base is connected to the output terminal and whose collector is connected to the common potential; a third transistor 19 connected to the emitter of the second transistor; a fourth transistor 18 having its base connected to the emitter of the second transistor; and resistors 25, 26 having one end connected to the emitters of the third transistor and the fourth transistor. a first current source 27 connected through the
a first diode 22 whose anode is connected to the collector of the third transistor; a second diode 23 whose anode is connected to the collector of the fourth transistor; and cathodes of the first diode and the second diode. a third diode 24 connected to an anode and a cathode connected to a common potential, the next stage differential amplifier includes a fifth transistor 16 whose base is connected to the collector of the fourth transistor; a sixth transistor 17 connected to the collectors of the three transistors and having its collector connected to the output end; a sixth transistor 17 having a current inflow end connected to the power supply and a current outflow end connected to the collector and output end of the fifth transistor; a first current mirror circuit 10, 11 serving as a load for the fifth transistor and the sixth transistor, and a second current having one end commonly connected to the emitters of the fifth transistor and the sixth transistor and the other end connected to a common potential. a variable impedance source 28, one of the first current source and the second current source is a variable current source whose current is controlled by the control output of the control circuit, and the other current source is a constant current source. In the circuit, second current mirror circuits 12 and 13 are inserted between the first current mirror circuit of the next stage differential amplifier and the collector and the output terminal of the fifth transistor, and an anode is connected to the output terminal. a fourth diode 15 whose cathode is connected to the collector of the sixth transistor of the next-stage differential amplifier, whose base is connected to the reference voltage power supply, whose collector is connected to the second current mirror circuit, and whose emitter is connected to the fifth transistor. The seventh connected to the collector of
A variable impedance circuit comprising a transistor 14.