GB2035742A - Driving circuit for non-linear threshold devices, more particularly power lasers - Google Patents

Abstract

A driving circuit for a non-linear threshold device L, for example a power semi-conductor laser comprises, a pair of identical transistors TR1, TR2 the bases of which are fed in common from the output of a pulse generator G, the emitters of which are connected in parallel to the threshold device L and the collectors of which are connected through respective identical RC networks to a common power supply Va, the transistors TR1, TR2 being of the type able to operate as high speed electronic switches when the operating point lies in a linear region of the collector characteristic but being biased into the region of the characteristic which corresponds to avalanche operation by the components of the driving circuit. <IMAGE>

Description

SPECIFICATION Driving circuit for non-linear threshold devices, more particularly power lasers The present invention relates to a driving circuit for non-linear threshold devices, for example semiconductor power lasers.

It is known that in driving a semiconductor power laser, e.g. for tests and measurements on optical fibres, the laser must receive pulses having very high peak current (e.g. in the range of 15-20 A), with good reproducibility and with very low switching times, that is with very steep edges. In addition, in order to limit the dissipated power, such pulses must have very short duration (e.g. up to a maximum of the order of 200 ns), with a duty cycle (i.e. ratio between the duration of a pulse and the repetition period of the pulses themselves) of the order of 1/1000 or even less.

Furthermore, the short duration of the pulses is a very important characteristic when measurements are effected, as it allows good time resolution.

These requirements are only in part met by known driving circuits that make use of SCR diodes (silicon controlled rectifiers), or medium power transistors or V-MOS devices.

In fact SCR diodes can supply high currents only at the cost of comparatively long switching times, whilst medium power transistors, which could operate at high speed, cannot supply the required peak currents.

Lastly, V-MOS devices present features that are a compromise between those of SCR diodes and the ones of medium power transistors, and as happens in every compromise solution, are satisfactory only in few cases; more particularly, V-MOS devices cannot achieve the switching speeds required by the tests.

To overcome these disadvantages, avalanche transistors have been used, which are known as components having a rather high switching speed.

Nevertheless the commercially available transistors which are expressly designed for use as avalanche transistors exhibit, when the avalanche break-down takes place, a rather high collector-to-emitter residual voltage; therefore the high current involved give rise to such an electrical power that it causes burning of the transistor itself.

We have unexpectedly discovered that some kinds of transistors which can be used as fast switches and are normally designed for use in the linear region of the collector characteristics (having collector-toemitter voltages not exceeding some thirty volts, and collector currents not higher than 300 mA) can be made to operate in the voltage region corresponding to avalanche breakdown and that they exhibit in this region better characteristics for a driving circuit than those of avalanche transistors.

More particularly, the collector-to-emitter residual voltage is considerably lower, and consequently the transistor can be utilized with very high currents, like those previously mentioned; without being burnt by the dissipated power.

According to the present invention there is provided a driving circuit for a non-linear threshold device comprising a pair of identical transistors each of the type able to operate as a high-speed electronic switch when the operating point lies in a linear region of the collector characteristic, each said transistor having its emitter connected to the device to be driven, its base connected, through a first RC network, to a pulse generator so that each transistor conducts every time a pulse is emitted from said generator, and its collector connected through a further RC network to a common voltage supply, the values of the supply voltage and of the resistance of each said further RC network being such as to move the operating points of said transistors out of said linear region of the collector characteristics andinto a region corresponding to the avalanche operation, and the said further RC networks being identical to one another.

An embodiment of the present invention will now be described by way of example with reference to the accompanying drawing, in which: Figure 1 is the circuit diagram of the embodiment; and Figure 2 shows the collector characteristics of a transistor which can be utilized in the circuit of Figure 1.

With reference to Figure 1, the driving circuit denoted by CP, is connected at its output to a semi-conductor laser L, schematically represented by the diode symbol, and has its control input connected to a pulse generator G.

Circuit CP basically consists of a pair of transistors, TR1,TR2, identical to each other, operating as high-speed electronic switches, which conduct when generator G emits a pulse, then connecting laser L to a common voltage supply Va.

The bases of TR1 and TR2 are connected in common to pulse generator G through a first RC network, having the task of adapting generator G to circuit CP and comprising capacitor C1 in series and resistance R1 to ground. The values of R1 and C1 depend on the kind of generator and components used in circuit CP.

The emitters of TR1, TR2 are connected in parallel to laser L, and the collectors are connected to supply Va through two further RC networks identical to each other, each comprising a resistance (R2 or respectively R3), in series with the collector, and a capacitor (C2 or respectively C3) to ground. The choice of the time constant of these RC networks determines the repetition period of the driving pulses.

To realise the operations of transistors TR1, TR2 some kinds of conventional transistors can be utilized which usually have linear collector characteristics till the collector-to-emitter voltages reach values of the order of some thirty volts and collectorto-emitter currents reach values of 300 mA, and are designed to be used as high-speed switches with the operating point in the linear region of the collector characteristic; as mentioned. We have discovered that, by increasing the collector-to-emitter voltage up to a value about twice the previously mentioned value or even higher, these transistors operate as avalanche transistors, with power dissipation much lower than that of the transistors specifically de signed to operate in the avalanche regions.Accordingly the power voltages, the values of resistances R2, R3, and the value of the internal resistance of laser L (when conducting) are such as to cause TR1, TR2 to operate just in said avalanche region.

By way of example, supposing that transistors TR1,TR2 are of the kind sold under the name BSX 30 by the firm SGS-ATES of Agrate Brianza (Italy), the collector characteristics of which are shown in Figure 2, the suitable values for the power supply Va, for R2, R3 and for C2, C3 are the following: R2,R3: notlowerthan 150 ohms, and preferably in the range from 180 ohms to 600 ohms; the lower limit depending on the condition that in absence of pulses from generator G (base current IB = 0 for transistors TR1,TR2) transistors TR1,TR2 be really cutoff; Va: from 60 V to about 80 V; C2, C3: The values of C2, C3 are determined on the basis of the pulse width, so as to obtain the required duty cycle; taking into account their intended use, with pulses of maximum duration of 200 ns, typical values can be from 100 pFto 10 nF.

The operation of the described device is the following: in absence of current from pulse generator G, transistors TR 1,TR2 are cutoff and capacitors C2, C3 become charged with the respective time constants R2 C2 and R3 C3, which are equal to each other. When a pulse is applied to the base of transistors TR1,TR2, making them conduct, a supply voltage is applied to the collector-to-emitter junc- tion, moving the operating point into the avalanche region, and capacitors C2, C3 discharge through TR1, TR2 Because transistor TR1, resistance R2 and capacitor C2 are exactly identical to TR2, TR3, C3, respectively the instants when the avalanche effect occurs in TR1 and TR2 coincide and therefore the currents outgoing from the two branches of the circuit are summed up thus ensuring the attainment of the current levels necessary to drive the laser.

Claims (5)

1. A driving circuit for a non-linear threshold device, comprising a pair of identical transistors each of the type able to operate as a high-speed electronic switch when the operating point lies in a linear region of the collector characteristic, each said transistor having its emitter connected to the device to be driven, its base connected, through a first RC network, to a pulse generator so that each transistor conducts every time a pulse is emitted from said generator, and its collector connected through a further RC network to a common voltage supply, the values of the supply voltage and of the resistance of each said further RC network being such as to move the operating points of said transistors out of said linear region of the collector characteristics and into a region corresponding to the avalanche operation, and the said further RC networks being identical to one another.

2. A driving circuit according to claim 1, wherein the supply voltage is in the range from 60 V to 80 V, the resistances of said further RC networks have values not lower than 150 ohms, and the capacitances of said further RC networks have values in the range 100 pFto 10 nF.

3. A driving circuit according to claim 2, wherein the resistances of said further RC networks have values in the range 180 to 600 ohms.

4. A driving circuit according to any preceding claim, wherein the device is a power semi-conductor laser.

5. A driving circuit as claimed in claim 1 and substantially as hereinbefore described with reference to the accompanying drawing.