JPS63284913A - Variable gain amplifier - Google Patents

Abstract

PURPOSE:To prevent the DC potential of an output terminal from being changed even when a total gain is closed tight, by changing a ratio to mix the outputs of two amplifiers by a current source controllable electrically and the gain of a differential amplifier with a high gain. CONSTITUTION:A variable gain amplifier is constituted by providing a first differential amplifier consisting of transistors (Tr) 16 and 17 and resistors 13 and 14, and a second differential amplifier consisting of Trs 18 and 19. In such constitution, constant current sources 7 and 8 are controllable electrically and generate equal voltages for a control voltage. In such a case, it is assumed that the currents of the current sources 7 and 8 are I7 and I8, and the resistance values of the resistors 13-15 and 20-21 are R13-R15 and R20-R21. And when it is I7=I8 and I7=0 and I8=0, by setting the emitter currents of the Trs 16-19 at IE16-IE19 and the voltage of a power source 1 at V1 and the forward direction voltages of the bases and the emitters of the Trs 16 and 18 at VBE16 and VBE18, a result is shown in equation I if it is I7=I8not equal to 0. Therefore, equations II and III are satisfied, and it is possible to prevent the DC potential of the output terminals 22 and 23 from being changed by the current I7.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a transistor differential amplifier, and in particular to an AG
This invention relates to a variable gain amplifier that is used in C (automatic gain control) amplifiers and whose gain can be changed depending on current and voltage.

[Conventional technology]

Conventionally, as a circuit of this type, a configuration as shown in FIG. 1 is well known. A signal input from the signal source 102 is current-amplified by a differential amplifier composed of transistors 106 and 107 and a constant current source 103, and is then amplified by a differential amplifier composed of transistors 106 and 107 and a constant current source 103.
The signal is input from the emitter to a differential type common base amplifier configured with 109.

At this time, the voltage V of the base bias voltage sources 105 and 104
, and v18 are equal, the signal is split in half between transistors 108 and 109. By changing V2O3, the current flowing through the load resistor 110 is increased or decreased and the gain is changed. Note that 101 is a power source.

Fig. 4 is a schematic diagram showing the relationship between input amplitude and output when the gain of the conventional example is increased, and Fig. 5 is a schematic diagram showing the relationship between input amplitude and output when the gain of the conventional example is decreased. be.

[Problem that the invention seeks to solve]

In the circuit shown in FIG. 1 described above, as the gain decreases, the amplitude of the output signal that can be taken out across the resistor 110 as a load decreases, and since the operating points of transistors 106 and 107 are independent of V and o4, V1O4 is increased. The disadvantage is that even if the gain of the entire circuit is lowered, the maximum allowable input will not increase. Further, in the conventional circuit, not only the signal component but also the collector DC current of the transistor 108 changes depending on V2O3, so there is a drawback that not only the gain but also the DC potential of the output terminal 111 changes.

[Failure to solve the problem]

The variable gain amplifier of the present invention comprises a first
．． a second transistor, and the emitters of the first and second transistors each have the same value. Second
is connected to the first constant current source through a resistor, and the third. a fourth transistor, the base of the third transistor is connected to the base of the first transistor, and the base of the third transistor is connected to the base of the first transistor;
The collector of the transistor is connected to the collector of the first transistor, the base of the fourth transistor is connected to the base of the second transistor, and the collector of the fourth transistor is connected to the collector of the second transistor. connected to the first. A load or a load circuit for the purpose of taking out an output is provided to at least one of the collector of the third transistor and the second and fourth collectors, A connection point between the emitters of the fourth transistor and the first constant current source are connected via a second resistor, and the third. A second current source having a current equal to that of the first current source is connected to the connection point of the fourth emitter, and a second current source having a current equal to that of the first current source is connected to the connection point of the fourth emitter. It is characterized by having means for electrically controlling and changing the current value of the second current source.

〔Example〕

Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a circuit diagram of an embodiment of the present invention.

Transistor 24, power supply 1, constant current source 3, resistor 9.10
is the base DC bias circuit. transistor 16,
17, and resistors 13 and 14 constitute a first differential amplifier. The transistors 18 and 19 have their emitters directly connected to each other and have a higher gain than the first differential amplifier.
It forms a differential amplifier.

The constant current sources 7 and 8 are electrically controllable and both generate the same current with respect to the control voltage vc. The current value of current source 7°8 is I? , L. The resistance values of resistors 13, 14, 15゜20.21 are R13, RI41 RI51 R
4°, R2, I, = I, and I, = O, 1, = O
When , the emitter currents of transistors 16 to 19 are respectively I
g□6. Engineering 8□7゜II Engineering 8 + I 119, Engineering E16・R13=ΔVag+2Rts・Igxa
-=(1)I E16°R13) If ΔVBg is IE16@R13 2R15lB18 ・
-・・・(2) When Iy=Is#O, VI VBB18 Rls(2Igtg+Iy)=
Vt-■Bgts-R13Igxs...(3
) Vt is the t source 1o voltage, vBgts *VBB16 is the base-emitter forward voltage of the transistor 18.16.

is the Boltzmann constant, T is the absolute temperature, and q is the charge element. Here, by making the same approximation as in (2), VBIB 18
Medium VBE16 r Organize equation (3) and compare equations (4) and (5): Rls"I, R1s+Rts)
It can be seen that the term increases and decreases by the term, output terminal 22゜2
It turns out that the DC potential of No. 3 does not change due to engineering.

If the small signal voltage gain of the second differential amplifier is Avz, then (
4) Find the gain AVI of the first differential amplifier 1 in the same manner as in equation 2.

Therefore, if each circuit constant is selected so that the overall gain AV of the circuit shown in FIG. 1 becomes Ay = AVI + AV2, the gain Av becomes a linear monotonically decreasing function of I.

Next, we will consider the maximum allowable input.

Equation (4) 10 equations (s) ” I 6 / 2 Therefore, load 2
0.21 of the direct current I1. It has nothing to do with Is.

Therefore, the maximum amplitude that can be extracted from both ends of the load 20.21 is I s x Rv (or 16XR1υ).

Here, assuming that A V = A vx + A V2 satisfies the condition of equation (6), the maximum allowable human power Vimax is R13 or the force, and K is the input signal or I? , can be regarded as a constant unrelated to IL, so K R1s > O or K. >0 From equation (7), it can be seen that Vimax increases as k increases.

That is, in the circuit shown in FIG. 1, the gain can be controlled by the terminals, the DC potential at the output terminal is constant regardless of the gain, and the lower the gain, the wider the dynamic range.

FIG. 2 is a schematic diagram of the voltage output and input voltage of the load when the gain is reduced.

FIG. 3 is a schematic diagram showing input and output when the gain is increased, similar to FIG. 2.

FIG. 6 is a circuit diagram of another embodiment of the present invention using a PNP transistor. Elements that are the same as in FIG. 1 are given the same numbers. Transistor 24.25 and resistor 13.14 constitute a first differential amplifier with low gain, and transistor 26.2
7 constitutes a second differential amplifier with high gain.

A constant current source composed of a transistor 34 and a resistor 35 is
A reference current for a pair of current mirrors is generated, and this current is
It is controlled by a variable voltage source 37. Transistor 29. A constant tg source consisting of a resistor 32 and a transistor 38. resistance 3
The constant current sources 37 and 9 have the same current value and the voltage source 37
The current value changes depending on the

〔Effect of the invention〕

As explained above, the present invention provides a differential amplifier that has a high gain and whose gain itself strongly depends on the emitter current, and a differential amplifier that has a resistor inserted in the emitter and has a low gain and whose gain is has a differential amplifier that is approximately determined by the value of and is almost independent of the emitter current, and changes the ratio at which the outputs of the two amplifiers are mixed by an electrically controllable current source and the gain of the high gain differential amplifier. This makes it possible to realize a high maximum allowable input amplitude even when the overall gain is reduced, and to realize a variable gain amplifier in which the DC potential at the output terminal does not change.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of an embodiment of the present invention, and FIG. 2 is a circuit diagram of an embodiment of the present invention. FIG. 3 is an explanatory diagram showing the operation of FIG. 1, FIG. 6 is a circuit diagram of another embodiment of the present invention, FIG. 1 is a circuit diagram of a conventional example, and FIGS. 4 and 5 are the diagrams shown in FIG. It is an explanatory diagram showing operation of. 1...Transistor base bias IZ6'
X, 2...Power supply, 3...Constant current source, 4
, 5... Signal sources whose phases are shifted by 180° from each other,
6... Constant current source forming a differential amplifier, 7, 8...
... Constant current source, 9.10 ... Resistor, 11,
12... Coupling capacitor, 13.14 Old...
Emitter resistance, 15...Resistance, 16.17)
Transistor, 18, 19...Transistor, 2
0.21 Old... Load resistance, 22.23... Output terminal, 24, 25... Transistor, 26.
27...Transistor, 28...Transistor, 29,30,33.38...Transistor, 31,32,36,39...Resistor, 3
4...Transistor, 35...Resistor,
37...Variable voltage source. Fig. 1 Fig. 2 Xi'an 3 Round ■; Feng χi Infiltration 憬翳 4 Zuting 5 Sendai 6 v

Claims (1)

[Claims] It has first and second transistors forming a differential amplifier, and a first constant current source is connected to the emitters of the first and second transistors through first and second resistors having the same value, respectively. and has third and fourth transistors whose emitters are connected to each other forming a second differential amplifier, the base of the third transistor is connected to the base of the first transistor, and the base of the third transistor is connected to the base of the first transistor. The collector of the third transistor is connected to the collector of the first transistor, and the collector of the fourth transistor is connected to the collector of the first transistor.
The base of the transistor is connected to the base of the second transistor, the collector of the fourth transistor is connected to the collector of the second transistor, and the collectors of the first and third transistors are connected to the base of the second transistor. A load or a load circuit for the purpose of taking out an output is provided on at least one of the collectors of the fourth transistor, and a connection point between the emitters of the third and fourth transistors and the first constant current source are connected to a second resistor. and a second current source having a current equal to that of the first current source is connected to the connection point of the third and fourth emitters.
A variable gain amplifier characterized in that a current source is connected to the variable gain amplifier, and the variable gain amplifier has means for electrically controlling and changing the current values of the first and second current sources.