JP3202100B2 - Variable gain amplifier - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a variable gain amplifier whose gain is controlled by a variable control voltage.

[0002]

2. Description of the Related Art FIG. 4 shows a conventional variable gain amplifier 1. In the figure, transistors Q11, Q13, Q14
, A positive voltage is applied in common to the collectors of the transistors Q12, and a positive voltage is applied to the collector of the transistor Q12 via a resistor _RL . The emitters of the transistors Q11 and Q12 are commonly grounded via a bias current source I _O , and an output voltage V _OUT is obtained between the collector of the transistor Q12 and the resistor _RL .

[0003] Each bases of the transistors Q13, Q14 is applied a voltage E ₁ common, with its emitter connected respectively to the bases of the transistors Q12, Q11, are connected to the collectors of the transistors Q15, Q16. The base of the transistor Q15 has the input voltage V _IN to the voltage E ₂ is applied, a voltage E ₂ is applied to the base of the transistor Q16. The emitters of the transistors Q15, Q16 are connected via respective resistors R _E of the same value to the control circuit 2 is controlled by a control current I _E in accordance with the variable control voltage V _CON, the input voltage V _IN is variable Gain is amplified.

In such a variable gain amplifier, the gain G _V [V / V] of the output voltage V _OUT is:

[0005]

G _V = R _L I _O / R _E I _E [V / V] (1)

The control circuit 2 includes a transistor Q17
Of the transistor Q18 is connected to the emitters of the transistors Q15 and Q16, and the control current _IE flows through the collector of the transistor Q18. Voltage E ₃ is applied to the base of transistor Q17.
And a variable control voltage V _CON is applied, and the transistor Q18
The voltage E ₃ are applied to the base. Transistors the emitters of Q17 and Q18, the resistor are connected to each other via a R _B, respectively constant current sources I _a
Grounded.

In this case, the control current _IE is

[0008]

[Number 2] is represented by _{I E = V CON / R B} , when it is substituted into equation (1),

[0009]

## EQU3 ## The relation is I _E ∝V _CON . Further, since the unit of the gain G _V shown in the equation (1) is represented by [V / V], when this is represented by the unit of [dB], the equation (5) is obtained.

[0010]

However, in the control circuit 2 in the conventional variable gain amplifier 1 described above, the control current _IE is proportional to the control voltage V _CON , and the gain G _{V as} shown in the equation (1). [V / V] is the control current I _E
, The gain G _V of the output voltage V _OUT with respect to the control voltage V _CON as shown in Equation (5) and FIG.
[DB] is proportional to the logarithm of the reciprocal of the control current _IE , and therefore, there is a problem that the gain G _V [dB] in the variable gain amplifier 1 becomes nonlinear.

The present invention has been made in view of the above-mentioned conventional problems, and has as its object to provide a variable gain amplifier capable of stably controlling the gain of an output voltage with respect to a control voltage to be linear.

[0012]

According to the present invention, there is provided a variable gain amplifier for amplifying a voltage of an input signal with a gain corresponding to a variable control voltage applied to a control input. The variable amplifier includes an amplification unit that amplifies the voltage of the input signal with a gain according to the control current, a control unit that controls a control current flowing through a control input of the amplification unit according to the variable control voltage, and an amplification unit. Output means for outputting a signal amplified by the control means, wherein the control means comprises a pair of first and second transistors having different junction areas between the emitter and the base, wherein the first and second transistors are variable. 1 that constitutes a differential circuit through which a control current according to a control voltage flows
A pair of first and second transistors, and a control circuit for applying a base voltage of a potential difference obtained by logarithmically converting a variable control voltage to the first and second transistors, wherein the first and second transistors are: The ratio of the junction area between the emitter and the base is selected so that the gain of the amplified voltage in the amplifying means is linear with respect to the variable control voltage.

In this case, the control circuit includes third and fourth transistors for logarithmically converting the variable control voltage, and the third and fourth transistors respectively determine the potential difference between the converted control voltage by the first and fourth transistors. It may be applied to the base of each of the second transistors.

[0014] The amplifying means may be constituted by a differential amplifying circuit.

Further, the first transistor and the second transistor
It is preferable that the ratio of the junction area between the emitter and the base is set to 3 or 4.

[0016]

According to the present invention, the base voltage of the potential difference obtained by logarithmically converting the variable control voltage is applied to the pair of first and second transistors, and the pair of first and second transistors are arranged so that the gain in the amplifying means becomes linear. The ratio of the respective junction areas of the first and second transistors is selected. Thereby, the second
The gain in the amplifying means is controlled by the control current flowing through the transistor.

[0017]

Embodiments of the present invention will be described below with reference to the accompanying drawings. Referring to FIG. 2, there is shown one embodiment of a variable gain amplifier to which the present invention is applied. This variable gain amplifier 10 has a variable control voltage V _CON according to
This amplifier amplifies the input voltage V _IN corresponding to the input signal by variable gain. Note that the configuration may be the same as that of the variable gain amplifier 1 shown in FIG. 4 except for the control circuit 2 except for the control circuit 12 in the variable gain amplifier 10 shown in FIG. The control circuit 12 of the variable gain amplifier 10 will be described in detail. As shown in FIG. 1, the control circuit 12 includes transistors Q1 and Q2, and the emitter size (the number of emitters) of the transistors Q1 and Q2, that is, between the emitter and the base. The bonding area ratio is m: 1
(M does not have to be an integer). Each collector of the transistors Q1, Q3 and Q4 are positive voltage is applied, the collector of the transistor Q2, the transistor Q15 and Q16 via the resistor R _E shown in FIG. 2
Are connected to the respective emitters.

Returning to FIG. 1, transistors Q1 and Q
Each of the emitters 2 is commonly grounded via a bias current source I _EO . As will be described later, a control current _IE corresponding to the potential difference ΔV obtained by logarithmically converting the variable control voltage V _CON and the junction area ratio of the transistors Q1 and Q2 flows through the emitter of the transistor Q2. Therefore, the output voltage V _OUT is obtained between the collector of the transistor Q12 and the resistor _RL shown in FIG.

[0019] Each base of the transistors Q3 and Q4 are voltage E ₁ commonly applied, the emitters thereof is connected to the bases of transistors Q1 and Q2, respectively, are connected to the collectors of the transistors Q5 and Q6 ing. The base of the transistor Q5 and the control voltage V _CON of the voltage E ₂ and the variable is applied, the voltage E ₂ is applied to the base of the transistor Q6. The emitters of the transistors Q5 and Q6 are
They are connected to each other via a resistor _Rb , and are also grounded via _a constant current source Ia.

In the variable gain amplifier having such a configuration, the control voltage V _CON is determined by the transistor Q3 as shown in the equation (2).
And Q4 to logarithmically convert to a potential difference .DELTA.V at each base of transistors Q1 and Q2.

[0021]

Equation 4] _{ΔV = V T ln [{I} a + (V CON / R b)} / {I a - (V CON / R B)}] = V T ln {(1+ kV CON) / (1- kV _CON)} ... (2) _{where, V T = KT / q (} k is the Boltzmann constant, T is the temperature, q is one charge amount _{electronic) 1 / I a R b =} k (-1 / k <V _CON <1 / k) The control current _IE is obtained by applying the base voltage of this potential difference .DELTA.V to the bases of the transistors Q1 and Q2. In this case, the relationship between the potential difference .DELTA.V and the control current _IE is expressed by equation (3) ), And the control current _IE is the potential difference ΔV of the base e of the natural logarithm.
It is represented by the reciprocal of a value obtained by multiplying the power by the junction area ratio m between the emitter and the base of the transistors Q1 and Q2.

[0022]

(Equation 5) Therefore, the relationship between the control voltage V _CON and the control current _IE is expressed by Expression (4) when Expression (2) is substituted into Expression (3).

[0023]

I _E = I _EO / [1 + m {(1 + KV _CON ) / (1-KV _CON )}] (4) Here, the unit of the gain G _V shown in the equation (1) is [V / V ], So if this is expressed in units of [dB],

[0024]

Equation 7] becomes G (dB) = 20log (R L I O / R E I E) = 20log (R L / R E) + 20log (I O / I E) ... (5).

Therefore, the equation (4) is substituted into the second term of the equation (5), and the gain G (d
B) is plotted as shown in FIG. 3. As is apparent from FIG. 3, when m = 3 to 4, the second term of the equation (5) changes almost linearly with respect to the control voltage V _CON . ing. Since the first term of the equation (5) is constant irrespective of the control voltage V _CON , the gain G (dB) changes almost linearly with respect to the control voltage V _CON .

Therefore, the variable gain amplifier shown in FIG.
By configuring the ten control circuits 12 as shown in FIG. 1, it is possible to configure a variable gain amplifier in which the gain of the output voltage with respect to the control voltage V _CON changes linearly.

According to the present invention, since the equation (5) does not include parameters such as transistors and diodes that are greatly affected by the temperature T, the gain G (dB)
Has the effect of being stable with respect to the temperature T.

[0028]

As described above, according to the present invention, a base voltage of a potential difference obtained by logarithmically converting a variable control voltage is applied to a pair of first and second transistors. Is selected to a predetermined value, a control current is generated such that the gain in the amplifying means becomes linear. Therefore, the amplifying means can amplify the input signal with a gain according to the control current generated by the control means, and operate so that the gain of the output voltage becomes linear. Further, in this case, a variable gain amplifier that is stable against a change in temperature can be configured.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing one embodiment of a control circuit in a variable gain amplifier according to the present invention.

FIG. 2 is a circuit diagram showing one embodiment of a variable gain amplifier according to the present invention.

FIG. 3 is a graph showing a gain in a variable gain amplifier for each junction area ratio between an emitter and a base in the control circuit shown in FIG. 1;

FIG. 4 is a circuit diagram showing a conventional example of a variable gain amplifier.

FIG. 5 is a graph showing a gain in a conventional variable gain amplifier.

[Explanation of symbols]

Q1, Q2, Q3, Q4, Q5, Q6 Transistor V _CON control voltage _IE control current

Claims (4)

(57) [Claims] 1. A variable gain amplifier that amplifies a voltage of an input signal with a gain according to a variable control voltage applied to a control input, wherein the variable gain amplifier adjusts a voltage of the input signal according to a control current. Amplifying means for amplifying by a gain; control means for controlling a control current flowing through a control input of the amplifying means according to the variable control voltage; and output means for outputting a signal amplified by the amplifying means. Wherein the control means is a pair of first and second transistors having different emitter-base junction areas, and a control current according to the variable control voltage flows through the second transistor. A pair of first and second transistors forming a differential circuit; and a control circuit for applying a base voltage of a potential difference obtained by logarithmically converting the variable control voltage to the first and second transistors. The first and second transistors have a junction area ratio between the emitter and the base selected such that the gain of the amplified voltage in the amplifying means is linear with respect to the variable control voltage. A variable gain amplifier, characterized in that: 2. The variable gain amplifier according to claim 1, wherein the control circuit includes third and fourth transistors that logarithmically convert the variable control voltage, wherein the third and fourth transistors are: A variable gain amplifier, wherein a potential difference of the converted control voltage is applied to respective bases of the first and second transistors. 3. The variable gain amplifier according to claim 1, wherein said amplifying means is constituted by a differential amplifier circuit. 4. The variable gain amplifier according to claim 1, wherein said first transistor and said second transistor have an emitter-base junction area ratio of 3 or 4. Variable gain amplifier.