JPH0230202B2 - RITOKUKAHENZOFUKUKAIRO - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a variable gain amplifier circuit, and particularly to an amplifier circuit suitable for externally controlling the gain.

As shown in Fig. 1, the preamplifier circuit of an optical receiver usually consists of an emitter-grounded circuit using a transistor _Q1 and a collector-grounded circuit using a transistor _Q2 , which are connected in series, and the output of this collector-grounded circuit is connected in series. A transimpedance amplifier is used in which feedback is fed back to the input side of the grounded emitter circuit (base of transistor _Q1 ) through a feedback circuit consisting of a feedback resistor Rf. Since this circuit has the function of converting the signal current of the light receiving element into a voltage, the ratio of the output voltage to the input voltage (transimpedance) becomes a parameter indicating the efficiency of the amplifier.

Furthermore, the transimpedance is the feedback resistance
Since it is almost equal to the resistance value of Rf, the feedback resistance Rf
By changing the resistance value, the transimpedance can be changed.

FIG. 2 is a circuit diagram showing an example of a circuit in which the feedback resistance value of FIG. 1 is changed equivalently. Second
In the figure, the collector and emitter of transistor _Q3 are connected to both ends of feedback resistor Rf.

Therefore, when the base potential of transistor Q ₃ rises, transistor Q ₃ enters the active region,
Since the impedance between the collector and emitter is reduced, the feedback resistance value is equivalently reduced, and the transimpedance amplifier is reduced, so the gain of the amplifier can be controlled by the base voltage of transistor _Q3 .

However, in the circuit shown in Figure 2, lowering the transimpedance of the amplifier increases the amount of feedback, so as the base voltage of transistor Q ₃ is increased from V ₁ to V ₂ to V ₃ , the 1 of 3 diagrams
~ As shown in 3, a peak occurs in the frequency characteristics,
In some cases, it may oscillate, making it impractical.

In addition, in FIG. 1 and FIG. 2, R ₁ and R ₂ are resistances.

The present invention has been made in view of the above, and an object of the present invention is to provide a variable gain amplifier circuit that can limit the gain and suppress changes in frequency characteristics.

A feature of the present invention is that an emitter-grounded circuit and a collector-grounded circuit are connected in series, and the output of the collector-grounded circuit is fed back to the input side of the emitter-grounded circuit through a feedback circuit, and the output is fed back to the input side of the emitter-grounded circuit through a feedback circuit. A first variable impedance element is connected, and a second variable impedance element is connected between the output of the collector grounding circuit and the ground, and the impedance of the two variable impedance elements increases and decreases in the same direction. The main point lies in the control circuit configuration.

The present invention will be described in detail using the embodiment shown in FIG.

FIG. 4 is a circuit diagram showing an embodiment of the variable gain amplifier circuit of the present invention. The same parts as in FIGS. 1 and 2 are designated by the same reference numerals, and the explanation thereof will be omitted here. In Fig. 4, in addition to a transistor Q ₃ whose collector and emitter are connected to both ends of a feedback resistor Rf, a level shift diode D ₁ and an emitter resistor R ₂ of a transistor Q ₂ , that is, whose collector is connected to the output of a common collector circuit. Transistor Q ₄ connected and emitter grounded is added, and transistor Q ₃ ,
These are connected as shown so that the impedance increases and decreases between each collector and emitter of _Q4 in the same direction.

According to FIG. 4, when the potential of the control voltage input terminal C is low, the transistors Q ₃ , Q ₄ and the diode D ₁ are cut off, and the transistors Q ₃ , Q ₄
The impedance between the collector and emitter becomes sufficiently large, and it can be considered as an open circuit. Output end B in this case
The output voltage at is the input current from input terminal A
It is approximately equal to the product of Iin and the resistance value of feedback resistor Rf.

Furthermore, when the voltage at terminal C rises and transistor Q ₃ becomes active, the impedance between the collector and emitter of transistor Q ₃ decreases, meaning that an impedance is inserted in parallel with feedback resistor Rf. , equivalently the feedback resistance value decreases, and therefore the output voltage decreases.

On the other hand, at this time diode _D1 and transistor
Since Q ₄ also becomes active, the impedance between the collector and emitter of transistor Q ₄ , which is connected in parallel with the emitter resistor R ₂ of transistor Q ₂ , decreases, thereby reducing the amount of feedback and narrowing the band.

Therefore, the functions of increasing the amount of feedback and expanding the band by the transistor _Q3 are canceled, and the output can be attenuated to the extent that the frequency characteristics remain flat.

As described above, according to the embodiment of the present invention, it is possible to compensate for the change in frequency characteristics caused by changing the feedback resistance value, and to change the gain while maintaining a flat frequency characteristic.

Furthermore, since the feedback resistance value can be changed by an external voltage, the band required for the signal can be set without changing the circuit. moreover,
Since the circuit can be configured with a relatively small number of elements, it is easy to form an integrated circuit.

As described above, according to the present invention, it is possible to limit the gain and suppress changes in frequency characteristics.

[Brief explanation of drawings]

Figure 1 is a circuit diagram showing the basic form of a transimpedance amplifier, Figure 2 is a circuit diagram for controlling the feedback resistor, Figure 3 is a diagram showing an example of the frequency characteristics of the circuit in Figure 2, and Figure 4 is a diagram showing an example of the frequency characteristics of the circuit in Figure 2. The figure is a circuit diagram showing an embodiment of the variable gain amplifier circuit of the present invention. Q ₁ ~ _{Q 4} : Transistor, Rf: Feedback resistor, D ₁ :
level shift diode.

Claims (1)

[Claims] 1. In an amplifier circuit in which an emitter grounded circuit and a collector grounded circuit are connected in cascade, and the output of the collector grounded circuit is fed back to the input side of the emitter grounded circuit through a feedback circuit, a first amplifier circuit is connected in parallel to the feedback circuit. A second variable impedance element is connected between the output of the collector grounding circuit and the ground, and the two variable impedance elements are controlled so that the impedance increases and decreases in the same direction. Features a variable gain amplifier circuit.