JPH0720969Y2 - Variable gain circuit of differential amplifier - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to a variable gain circuit for a differential amplifier, which is required to change the gain used in a vertical preamplifier of an oscilloscope.

(Prior Art) An example of a gain variable circuit which has been conventionally used is constructed as shown in FIG. That is, in the circuit shown in FIG. ₃ , the signals are amplified by the transistors Q ₂ and Q ₃ , but at that time, the transistors Q ₁ and Q ₄ are transmitted by the variable resistor VR _1.
Are voltage V ₁ applied to the base of the variable, the magnitude of the signal corresponding to the voltage V ₁ is applied to the opposite phase to the output signal of the Yotsute transistor Q _2, Q ₃ to the transistor Q _1, Q _4. As a result, the gain can be continuously changed by continuously changing the voltage V ₁ .

(Problems to be Solved by the Invention) In the conventional variable gain circuit described above, the gain can be continuously varied, but if the characteristics of the transistors Q _{1 to} Q ₄ are not completely matched, temperature drift or the like may occur. Therefore, there is a problem that the output becomes unbalanced.

Therefore, the configuration using the discrete circuit is not realistic, and the frequency characteristic is deteriorated due to the large number of components because four transistors are used.

It is an object of the present invention to provide a variable gain circuit which can be configured by a discrete circuit, has a small number of parts, and whose frequency characteristic does not change when the gain is changed.

(Means for Solving the Problem) A variable gain circuit for a differential amplifier according to the present invention has first and third cascode-connected transistors and second and fourth cascode-connected transistors. 1
A variable gain circuit for a differential amplifier configured to connect a transistor emitter and a second transistor emitter and to apply a predetermined DC voltage to a base of a third transistor and a base of a fourth transistor. A first resistor connected between the collector of the first transistor and the emitter of the third transistor, and having the same resistance value as the first resistor, and the collector of the second transistor and the emitter of the fourth transistor. The second connected between and
And a first and a second diode connected in anti-series between the connection point between the collector of the first transistor and the first resistance and the connection point between the collector of the second transistor and the second resistance. , A variable DC voltage source connected to a common connection point of the first and second diodes and applying a DC voltage to the common connection point.

(Operation) At the time of a small signal, the first and second diodes are equivalent resistors according to the forward voltage-current characteristics, and their resistance values are variable depending on the voltage applied to the first and second diodes. . Therefore, the first diode and the first resistor function as the variable attenuator, and the second diode and the second resistor function as the variable attenuator, and the collector of the first transistor and the first resistor are changed by changing the output voltage of the variable DC voltage source. The potential at the connection point between the second resistor and the collector of the second transistor and the second resistance changes, and the current flowing through the first and second diodes changes due to the change in the potential at the connection point. As a result, of the third and fourth emitter currents, the currents shunting the first and second diodes are controlled by the output voltage of the variable DC voltage source, and the gain of the differential amplifier as a whole is controlled.

In order to make the attenuation rates of both variable attenuators that are changed by the change of the output voltage of the variable DC voltage source the same, the characteristics of the first and second diodes must match and the frequency characteristics must be good. However, the former condition is satisfied by using a pair diode and the latter condition by using a diode having a good switching characteristic. This is easier than matching the characteristics of the four transistors as in the conventional example.

Further, the change of the attenuation rate of the variable attenuator described above with temperature is small. The reason for this will become clear from the following explanation.

(Example) Hereinafter, the present invention will be described with reference to an example.

FIG. 1 is a block diagram showing the construction of an embodiment of the present invention.

One embodiment of the present invention is a cascode-connected transistor.
Q ₁₁ and Q ₁₃ and cascode-connected transistor Q ₁₂
And Q ₁₄ , a constant current source CS connected to the commonly connected emitters of transistors Q ₁₁ and Q ₁₂ , and a transistor Q.
_A differential amplifier is configured with the voltage dividing resistors R ₃ and R ₄ that divide the power supply voltage V _S to generate the voltage applied to the bases of ₁₃ and Q ₁₄ .

Here, the transistor Q ₁₁ is the first transistor, the transistor Q ₁₂ is the second transistor, and the transistor Q ₁₃ is the third transistor.
To the transistor, the transistor Q ₁₄ corresponds to the fourth transistor.

The variable gain circuit consists of a resistor R ₁ connected between the emitter of transistor Q ₁₃ and the collector of transistor Q ₁₁ , a resistor R ₂ connected between the emitter of transistor Q ₁₄ and the collector of transistor Q ₁₂ , Variable DC voltage source VVS whose value is controlled, diode D ₁ connected between the collector of transistor Q ₁₁ and variable DC voltage source VVS, diode connected between the collector of transistor Q ₁₂ and variable DC voltage source VVS It is composed of D ₂ and a variable DC voltage source VVS
The gain of the differential amplifier can be continuously changed by changing the voltage of. This will be explained below.

As for the voltage-current characteristic of the diode, when the forward voltage applied to the diode is V and the forward current flowing in the diode when the voltage V is applied is I, Given in. I _o is the diode leakage current, k is the Boltzmann constant, q is the electron charge, and T is the absolute temperature.

When the formula (1) is differentiated by the voltage V and the reciprocal is taken, Is obtained. This becomes the operating resistance R _D of the diode when the signal is small.

If the diodes D ₁ and D ₂ and the variable DC voltage source VVS are taken out and shown, it is as shown in FIG. In FIG. 2 (a), a resistor R ₁ , a diode D ₁ and a variable DC voltage source VVS are shown. Here, the diode D ₁ can be equivalently replaced by the resistance R _D shown in the equation (2), and the variable DC voltage source VVS
Considering that the voltage can be changed, the resistance R _D becomes variable resistor VR ₂ equivalent, as shown in FIG. 2 (b), the resistance value of the variable resistor VR ₂ changes the output voltage of the variable DC voltage source It can be changed by doing. Therefore, the resistor R ₁ and the variable resistor VR ₂ constitute an attenuator. An attenuator is similarly constructed for the resistor R ₂ , the diode D _2, and the variable DC voltage source VVS.

Therefore, by changing the output voltage of the variable DC voltage source VVS, the voltage at the connection point between the resistor R ₁ and the anode of the diode D ₁ and the voltage at the connection point between the resistor R ₂ and the anode of the diode D ₂ are changed. However, by changing the voltage at the connection point,
The current flowing through the diode D ₁ (D ₂ ) changes, and the gain of the entire differential amplifier can be continuously controlled by changing the output voltage of the variable DC voltage source VVS.

Therefore, the gain of the differential amplifier shown in FIG. 1 can be continuously changed by changing the output voltage of the variable DC voltage source VVS.

The attenuation rate α of the attenuator shown in FIG. 2 (b) is Given in. Here, since I _o , q / k is constant by the diode D ₁ , an arbitrary attenuation rate can be obtained by the resistance R ₁ , the voltage V and the absolute temperature T. Therefore, the resistance value of the resistor R _{1 and} the resistance value of the resistor R ₂ are equal and set to an arbitrary value,
As described above, arbitrary attenuation can be continuously obtained by the output voltage V ₂ of the variable DC voltage source VVS.

In this case, the characteristics of the diodes D ₁ and D ₂ must match, which can be solved by using a pair of diodes. It is also necessary that the diodes D ₁ and D ₂ have good frequency characteristics, but if these diodes have good switching characteristics, the capacitance between the terminals is extremely small, so the current between the terminals of the diodes due to frequency changes The change is extremely small, and the use of a diode having a good switching characteristic does not cause a problem of having a good frequency characteristic. This is significantly easier than matching the characteristics of the _four transistors Q _{1 to} Q ₄ in the case of the conventional example.

Further, in the equation (3), the absolute temperature T exists as a parameter, which causes temperature dependence. However, since the currents flowing through the diodes D ₁ and D ₂ are almost constant unless the output voltage of the variable DC voltage source VVS is changed, as can be seen from the equation (1), the denominator of the equation (3) Is constant, and the effect of temperature is on the denominator There are only a few problems, and only the absolute temperature T at. Let's actually calculate. Now assuming that the attenuation rate is 0.4 times, from equation (3) And R ₁ = 100 [Ω], I _o = 10 × 10 ⁻⁹ [A], q = 1.60
× 10 ^-19 [C], k = 1.38 × 10 ^-23 [j ・ K ^-1 ], T = 297 °
[K] (20 ℃), V = 0.27V, Becomes

If the temperature rises to T = (298 °) [K] (21 ° C.) under this condition, the attenuation rate becomes α = 0.4008.

Therefore, if the ambient temperature changes by 1 ° C at an ambient temperature near 20 ° C, the attenuation factor will change (0.4008-0.4) /0.4×100% = 0.02%.

As described above, according to the present invention, the first resistor is provided between the collector of the first transistor and the emitter of the third transistor, and the collector of the second transistor and the fourth transistor are connected in cascode. A second resistor is connected between the emitter and the emitter, and is connected in anti-series between the connection point between the collector of the first transistor and the first resistor and the connection point between the collector of the second transistor and the second resistor. Since the connected first and second diodes are connected and the output voltage of the variable DC voltage source is applied to the common connection point of the first and second diodes, the output voltage of the variable DC voltage source is changed. The current flowing through the first and second diodes is controlled by, and the gain of the differential amplifier is changed.

Further, when the output voltage of the variable DC voltage source is changed, the attenuation rate of the variable attenuator including the first diode and the first resistance, and the second diode and the second resistance are included. In order to equalize the damping rate of the variable attenuator,
Even if the characteristics of the first diode and the second diode need to match and the frequency characteristics need to be good, it is easier than matching the characteristics of the four transistors as in the conventional example.

Further, the number of parts required is small, and the circuit configuration is simple.

Furthermore, unlike the conventional example, the signal lines do not cross each other, the circuit pattern can be easily designed, and the circuit pattern is simple, so that the frequency characteristics are not deteriorated by the circuit pattern.

Further, since the variable resistor for varying the gain is not inserted in the signal line, the change in the frequency characteristic that occurs when the slider position of the variable resistor is changed for varying the gain can be small.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing the configuration of an embodiment of the present invention. 2 (a) and 2 (b) are circuit diagrams for explaining the operation of one embodiment of the present invention. FIG. 3 is a circuit diagram of a conventional example. Q ₁₁ , Q ₁₂ , Q ₁₃ and Q ₁₄ …… Transistors, R ₁ and R ₂ …
… Resistors, VVS… Variable DC voltage sources, D ₁ and D ₂ … Diodes

Claims (1)

[Scope of utility model registration request] 1. A first and third cascode-connected transistor, and second and fourth cascode-connected transistors, wherein an emitter and a second of the first transistor are provided.
A variable gain circuit of a differential amplifier connected to the emitter of a transistor and configured to apply a predetermined DC voltage to the bases of the third transistor and the fourth transistor, the collector of the first transistor and the Three
A first resistor connected between the emitter of the transistor and a second resistor having the same resistance value as the first resistor and connected between the collector of the second transistor and the emitter of the fourth transistor. A resistor and first and second diodes connected in anti-series between a connection point between the collector of the first transistor and the first resistance and a connection point between the collector of the second transistor and the second resistance; A variable gain circuit for a differential amplifier, comprising a variable DC voltage source connected to a common connection point of first and second diodes and applying a DC voltage to the common connection point.