JPH0983456A - Parasitic capacitance control circuit - Google Patents

Abstract

(57) [Summary] [Construction] A bias level for controlling the temperature dependence of the base-collector voltage of the transistor 17 is given to the signal for driving the current switch. [Effect] The temperature change of the parasitic capacitance between the base and collector of the transistor 17 can be controlled, and a good output waveform can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a parasitic capacitance control circuit.

[0002]

2. Description of the Related Art FIG. 2 shows a conventional example (Japanese Patent Laid-Open No. 2-215239). The pulse voltage signal input to the input terminals 19 and 20 and the anti-phase pulse voltage signal are input to the transistor 2
Waveform shaping is performed by a differential pair composed of 3, 24, a constant current source 25 and resistors 21, 22, and voltage level shifting is performed by an emitter follower composed of transistors 26, 27, diodes 28, 29 and resistors 30, 31. The current drive circuit 36 (current switch) of the stage is driven.

[0003]

In the conventional example, the potential between the diode 28 and the resistor 30 or the diode 29 and the resistor 3 is used.
The potential between 1 has a temperature characteristic of two diodes, and rises as the temperature rises and falls as the temperature falls. That is, the base potentials of the transistors 32 and 33 forming the current switch in the next stage have temperature characteristics. On the other hand, the collector potential of the transistor 33 is Vcc minus the forward voltage of the light emitting element diode 34. Normally, in order to keep the light output constant, the value of the current flowing through the light emitting element diode 34 is changed depending on the temperature. Therefore, when compared with the temperature dependence of the base-emitter voltage of the transistors 26 and 27 and the forward voltage of the diodes 28 and 29 in the emitter follower in which a substantially constant current is flowing, the temperature characteristic of the light emitting diode 34 is small. As a result, the base-collector voltage of the transistor 33 forming the current switch has temperature dependence. FIG. 3 shows the temperature characteristics of the base potential, the collector potential, and the base-collector voltage of the transistor 33.

A parasitic junction capacitance exists between the base and collector of the transistor, and the parasitic capacitance depends on the base-collector voltage as shown in FIG. Even in the conventional example, when the power supply voltage is sufficiently high and is 5 V or higher or the operating temperature range is narrow, the parasitic capacitance between the base and collector of the transistor forming the current switch is in the region where the voltage between the base and collector is large in FIG. The fluctuation amount of can be ignored. However, as a recent trend, lower power supply voltage and operation in a wide temperature range of optical communication devices are progressing. Therefore, if the circuit of the conventional example is used as it is, the base-collector voltage of the transistor constituting the current switch has temperature dependence, and the variation amount of the base-collector parasitic capacitance cannot be ignored. As a result, there is a temperature-dependent parasitic capacitance, and at high temperatures the base-collector voltage is small, so the parasitic capacitance is large, and at low temperatures, the base-collector voltage is large, so the parasitic capacitance is small. Become. This temperature dependence of the parasitic capacitance makes waveform shaping difficult.

[0005]

Means for Solving the Problems A bias having a temperature characteristic is applied to a signal for driving a current switch, and a base potential of a transistor forming the current switch is controlled to control temperature dependence of a base-collector voltage. To do.

[0006]

By using the above means, the temperature dependence of the base-collector parasitic capacitance of the transistor forming the current switch can be made smaller or constant than that of the conventional example, or the temperature characteristic opposite to that of the conventional example can be provided. Can be done. As a result, waveform shaping becomes easier and a good output waveform can be obtained.

[0007]

FIG. 1 shows an embodiment of the present invention. In this embodiment, terminals 1 and 2 for inputting a signal, a differential pair composed of resistors 5 and 6, transistors 7 and 8, and a constant current source 9 and a positive pair for biasing a signal passing through the differential pair. Current source 3 and resistor 4 having the temperature characteristic of 1, emitter follower composed of transistors 10 and 11 and constant current sources 12 and 13, and resistor 14
A current switch composed of the transistors 16 and 17 and the current source 18 that follows the temperature dependence of the current-light conversion efficiency of the laser diode 15, and the laser diode 15.

The base potentials of the transistors 16 and 17 change due to the temperature dependence of the base-emitter voltage of the transistors 10 and 11, and the collector potential of the transistor 17 is the laser diode 15 when driven with a constant light output.
Changes depending on the temperature dependence of the forward voltage of. By using the current source 3 and the resistor 4 to give a signal a bias level having two temperature dependences and a temperature dependence opposite thereto, the base-collector voltage of the transistor 17 is made constant regardless of the temperature. That is, the base-collector parasitic capacitance of the transistor 17 is kept constant regardless of the temperature. FIG. 5 shows the temperature dependence of the base potential and collector potential of the transistor 17, and the temperature characteristic of the bias level applied to the signal. FIG. 7 shows the output current waveform when the simulation is performed by the circuit (FIG. 6) using the equivalent circuit instead of the laser diode, and at the same time, the simulation result when the current source 3 is deleted from the circuit of FIG. 6 is also shown. At room temperature, no difference is found between the two simulation results, but at the circuit where the current source 3 is omitted, the base-collector voltage is the smallest at 85 ° C., a significant difference is seen.

Not only is the base-collector parasitic capacitance of the transistor constituting the current switch controlled to be constant irrespective of temperature, but the parasitic capacitance is controlled to have a negative temperature coefficient, and a small positive temperature coefficient is applied. It is also effective to control to have it.

[0010]

By using the present invention, it is possible to control the temperature change of the parasitic capacitance between the base and collector of the transistor forming the current switch, and to obtain a good output waveform.

[Brief description of drawings]

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

FIG. 2 is a circuit diagram showing an example of a conventional example.

FIG. 3 is a characteristic diagram showing temperature dependence of a base potential, a collector potential, and a base-collector potential in a current switch transistor of a conventional example.

FIG. 4 is a characteristic diagram showing the base-collector voltage dependence of the parasitic capacitance between the base and collector of a transistor.

FIG. 5 is a temperature characteristic diagram of temperature dependence of a base potential and a collector potential and a bias level given to a signal in a current switch transistor according to an embodiment of the present invention.

FIG. 6 is a circuit diagram in which a laser diode is replaced with an equivalent circuit for performing a simulation of an embodiment of the present invention.

7 is an output current waveform diagram when a simulation is performed with the circuit of FIG. 6 and when a simulation is performed with a circuit in which the current source 3 is removed from the circuit of FIG.

[Explanation of symbols]

 1, 2 ... Input terminal, 3 ... Current source, 4, 5, 6, 14 ... Resistor, 7, 8, 10, 11, 16, 17 ... Transistor, 9, 12, 13 ... Constant current source, 15 ... Laser diode , 18 ... Current source.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location H01L 27/04 21/822

Claims (4)

[Claims] 1. A parasitic capacitance control circuit for controlling a parasitic capacitance between a base and a collector of a transistor forming a current switch. 2. A temperature level dependence of a base-collector voltage is controlled by giving a bias level having a temperature characteristic to a signal for driving a current switch and controlling a base potential of a transistor forming the current switch, A parasitic capacitance control circuit for controlling temperature dependence of a parasitic capacitance between a base and a collector of the transistor. 3. A semiconductor laser driving circuit using the circuit according to claim 1. 4. An optical transmission module using the semiconductor laser drive circuit according to claim 3.