JPH0545088B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention relates to an improvement in a variable gain amplifier whose gain is controlled by an external signal.

2. Description of the Related Art Various circuit types are conventionally known as variable gain amplifiers whose gain is controlled by an external signal. For example, the bias currents of a high-gain differential amplifier and a low-gain differential amplifier that have a common load are switched complementary to each other to control the gain, or the bias current flowing through the feedback differential resistor of the emitter circuit of the differential amplifier is changed. For example, gain control is performed using

An example of the latter is a circuit as shown in Fig. 6 (1981 Telecommunications Society Optical and Radio Division National Conference, S1-12, Si Bipolar Analog IC Technology for Gb/S Optical Transmission, Kawarada et al.). In this diagram,
If transistors 81 to 84 are not provided and current source 7 is directly connected to resistors 5 and 6, transistors 1 and 2, resistors 3 to 6, and current source 7 constitute a normal differential amplifier. Therefore, if the control voltage applied to the base of transistor 84 is changed with respect to the reference voltage applied to the base of transistor 83, the current flowing through transistors 81 and 82 (operating as diodes) will change, and accordingly, transistors 1 and 82 will change. Since the feedback differential resistance of the emitter circuit 2 changes, the gain determined by the ratio with the load resistances 3 and 4 changes.

Problems to be Solved by the Invention However, in such a conventional variable gain amplifier, the bias current (supplied by the current source 7) for functioning as a differential amplifier is
The bias current for the AGC circuit must be taken into account when designing, which poses a problem in that the degree of freedom in design is restricted.

In this invention, the bias current of the differential amplifier is
The purpose of the present invention is to provide an improved variable gain amplifier that does not change due to AGC and does not impair design freedom.

Means for Solving the Problems A variable gain amplifier according to the present invention includes two transistors whose bases are input terminals, two load resistors connected to the collectors of the transistors, and the transistors. and a bias current source for differential amplification connected to the side opposite to the emitter connection side of each of the two resistors. It is characterized by comprising a diode connected in parallel to the resistor, and a variable current source for current injection and a variable current source for current output connected to both ends of the diode, respectively.

Operation The current flowing through the diode is caused by a variable current source for current inflow and a variable current source for current outflow connected to both ends of the diode, respectively. Adjusting these variable current sources to change the current flowing through the diode changes its resistance.

Then, the combined resistance value of the resistor and diode connected to the emitter of the transistor changes, which in turn means that the value of the feedback differential resistance forming the emitter circuit of the transistor changes. As a result, the gain of the differential amplifier, which is determined by the ratio between the load resistance value and this feedback differential resistance value, changes.

On the other hand, even if the current flowing through the diode is changed in this way, the current is provided by the variable current source for current inflow and the variable current source for current outflow, so it does not affect other currents. That is, the current flowing through the resistor connected to the emitter does not change. A current flows through the resistor connected to this emitter from a bias current source for differential amplification, and this current is independent of the current flowing through the diode.

As a result, the bias current of the differential amplifier is not affected by gain control, so it is possible to configure a variable gain amplifier without making any changes to the differential amplifier configuration, allowing for greater design freedom. The degree increases.

Embodiment In FIG. 1, except for diodes 8, 9 and current sources 10, 11, 12, transistors 1,
2. If we consider a configuration that includes only the resistors 3 to 6 and the current source 7, it has the configuration of a normal differential amplifier. In other words, the diodes 8 and 9 are connected in parallel with the resistors 5 and 6 to the emitters of the transistors 1 and 2 of a normal differential amplifier, and the diodes 8 and 9 are connected in parallel with the resistors 5 and 6. The variable gain amplifier shown in FIG. 1 is one in which a bias current is caused to flow through 9. That is, current source 1
A control current Ic is injected from 0 and 11, respectively, and a current 2Ic twice that amount is caused to flow out by a current source 12. Therefore, transistors 1 and 2
The bias currents Ic for AGC flowing into the emitters of , respectively, flow out to the current source 12 through the diodes 8 and 9, so the bias current as a differential amplifier, that is, the current Io flowing through the resistors 5 and 6, is the current Current determined by source 7
Io, and the bias current as this differential amplifier is
Io is not affected by changes in the AGC control current Ic.

Here, if the control current Ic is set to 0, it is the same as if there were no diodes 8, 9 and current sources 10 to 12, and at this time the gain becomes the minimum, and the minimum differential gain Amin is the same as that of a normal differential amplifier. Similarly, Amin≒Ra/Rb. However, Ra is the resistance value of each of the resistors 3 and 4, and Rb is the resistance value of each of the resistors 5 and 6.

Also, when the control current Io is maximized (Io=
Iomax) is the maximum gain Amax, and at this time, the differential resistance Rd of each of diodes 8 and 9 is Rd≒26/Iomax (mA), and the combined resistance Re on the emitter side of transistors 1 and 2 is Assuming that the emitter resistance of is re, Re=re+RdRb ≒re+Rd, so Amax≒Ra/Re ≒Ra/(re+Rd).

By adjusting the control current Ic in this way, the gain can be changed from Amin to Amax.
The current Io flowing through the current source 7 is constant regardless of changes in the control current Ic. This means that the bias current of the differential amplifier can be adjusted independently of the AGC current Ic.
This means that Io can be determined by the current source 7, increasing the degree of freedom in design. As a result, it is also possible to configure a variable gain amplifier using the configuration of an existing differential amplifier as is.

Next, let's look at ICs commonly used as differential video amplifiers.
An example in which a variable gain amplifier is constructed using a circuit μA733 (manufactured by Fairchild) will be described. First, the equivalent circuit of this IC circuit 20 is as shown in FIG. That is, transistor 2
1 to 31 and resistors 32 to 47, but basically it is configured as a differential amplifier by transistors 21 and 22, and the resistor 32 is the resistor 3 in FIG. 1, and the resistor 33 is the first Resistor 4 in the figure, resistors 34, 35, resistor 5 in Figure 1, resistors 36, 37
correspond to the resistor 6 in FIG. 1, and the transistor 27 functions as the current source 7 in FIG. The IC circuit shown in FIG. 2 is provided with gain adjustment terminals G1 and G2 at the emitters of transistors 21 and 22, so that the IC circuit is constructed as shown in FIG. 3 using these terminals. That is, as shown in FIG. 3, the IC circuit 20 includes diodes 51 to 54 and transistors 55 to 54.
62, connect the resistors 63 to 71.

Here, two diodes 51 and 52 connected in series are used instead of diode 8 in FIG. are connected in parallel. This is to alleviate distortion caused by nonlinearity of the diode when the input signal becomes large. Transistor 55~
Reference numeral 58 forms a current mirror circuit, and transistor 59 and transistor 60 also form a current mirror circuit. These transistors 55-6
0 form current sources 10-12 of FIG.

When a control voltage Vc for controlling the gain is applied to the base of the transistor 62, a current Ic corresponding to this voltage Vc flows to the collector of the transistor 61. Since this current Ic flows to the collector of transistor 58, the same current also flows to the collectors of transistors 55, 56, and 57 due to the action of the current mirror circuit.
Ic flows respectively. In this way, the diode 51,
52, a current Ic is flowed into the diodes 53 and 54. On the other hand, the collector current Ic of the transistor 57
is the collector current of the transistor 60, but the resistor 69 connected to the emitter of the transistor 59
Since the resistance value of is 1/2 of the resistance value of the resistor 70 connected to the emitter of the transistor 60,
Due to the action of the current mirror circuit, twice as much current 2Ic flows through the emitter of the transistor 59.
In this way, the two current mirror circuits
The current sources 10-12 shown in the figure are formed. Therefore, by adjusting the control voltage Vc, the first
The gain can be controlled by the operation shown in the figure.

Note that the circuit of the variable gain amplifier shown in FIG.
It can be transformed as shown in the figure and Figure 5. That is, the fourth
In the figure, only one diode 8 is used, and the current source 1
0 has been changed, and the current source 12 has been removed. Fifth
In this figure, the connection directions of the diodes 8 and 9 of FIG. 1 are reversed, and the directions of the current sources 10 to 12 are also reversed accordingly. In the case of Figure 5, the gain is
Although it is exactly the same as the figure, the combined resistance Re on the emitter side of transistors 1 and 2 in Figure 4 is slightly different from that in Figure 1, and becomes Re = (Rb + Rb) //Rd, so A = 2 × (Ra The differential gain expressed as /Re) will also differ slightly.

Effects of the Invention According to the variable gain amplifier of the present invention, the bias state of the differential amplifier is not affected by gain control, so there is a large degree of freedom in design. In addition, since the gain is controlled by the emitter circuit of the differential amplifier,
The influence of stray capacitance and circuit capacitance on frequency characteristics can be reduced.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of one embodiment of the present invention, Fig. 2 is an equivalent circuit diagram of an IC circuit used in another embodiment, and Fig. 3 is a circuit diagram of another embodiment using the IC circuit of Fig. 2. 4 and 5 are circuit diagrams of modified examples, and FIG. 6 is a circuit diagram of a conventional example. 10-12...Current source, 20...IC circuit, G
1, G2...Gain adjustment terminal.

Claims (1)

[Claims] 1 Two transistors whose bases are input terminals, two load resistors each connected to the collector of the transistor, two resistors each connected to the emitter of the transistor, In a differential amplifier having a bias current source for differential amplification connected to the side opposite to the emitter connection side of each of the resistors, a diode connected in parallel to the emitter side resistor, and a diode connected to both ends of the diode. 1. A variable gain amplifier comprising: a variable current source for current injection and a variable current source for current output, each connected to a variable gain amplifier.