Avalanche Mode Switching Circuits
Avalanche mode switching relies on avalanche multiplication of current flowing through the collector-base junction as a result of impact ionization of the atoms in the semiconductor crystal lattice. Avalanche breakdown in semiconductors has found application in switching circuits for two basic reasons
- it can provide very high switching speeds, since current builds-up in very small times, in the picosecond range, due to avalanche multiplication.
- It can provide very high output currents, since large currents can be controlled by very small ones, again due to avalanche multiplication.
The two circuits considered in this section are the simplest examples of avalanche transistor circuits for switching purposes: both the examples detailed are monostable multivibrators. There are several more complex circuits in the literature, for example in the books Roehr (1963) and Дьяконов (Dyakonov) (1973).
Most circuits employing an avalanche transistor are activated by the following two different kinds of input:
- Collector triggering input circuit: the input trigger signal is fed to the collector via a fast switching diode, possibly after being shaped by a pulse shaping network. This way of driving an avalanche transistor was extensively employed in first generation circuits since the collector node has a high impedance and also collector capacitance behaves quite linearly under large signal regime. As a consequence of this, the delay time from input to output is very small and approximately independent of the value of control voltage. However, this trigger circuit requires a diode capable of resist to high reverse voltages and switch very fast, characteristics that are very difficult to realize in the same diode, therefore it is rarely seen in modern avalanche transistor circuits.
- Base triggering input circuit: the input trigger signal is fed directly to the base via a fast switching diode, possibly after being shaped by a pulse shaping network. This way of driving an avalanche transistor was relatively less employed in first generation circuits because the base node has a relatively low impedance and an input capacitance which is highly nonlinear (as a matter of fact, it is exponential) under the large signal regime: this causes a fairly large, input voltage dependent, delay time, which was analyzed in detail in the paper Spirito (1974). However, the required inverse voltage for the feed diode is far lower respect diodes to be used in collectior trigger input circuits, and since ultra fast Schottky diodes are easily and cheaply found, this is the driver circuit employed in most modern avalanche transistor circuit. This is also the reason why the diode in the following applicative circuits is symbolized as a Schottky diode.
Avalanche transistor can also be triggered by lowering the emitter voltage, but this configuration is rarely seen in the literature and in practical circuits.: in reference Meiling & Stary (1968), paragraph 3.2.4 "Trigger circuits" one such configuration is described, where the avalanche transistor is used itself as a part of the trigger circuit of a complex pulser, while in reference Дьяконов (Dyakonov) (1973, pp. 185) a balanced level discriminator where a common bipolar junction transistor is emitter-coupled to an avalanche transistor is briefly described.
The two avalanche pulser described below are both base triggered and have two outputs. Since the device used is an NPN transistor, is a positive going output while is a negative going output: using a PNP transistor reverses the polarities of outputs. The description of their simplified versions, where resistor or is set to zero ohm (obviously not both) in order to have a single output, can be found in reference Millman & Taub (1965). Resistor recharges the capacitor or the transmission line (i.e. the energy storage components) after commutation. It has usually a high resistance to limit the static collector current, so the recharging process is slow. Sometimes this resistor is replaced by an electronic circuit which is capable of charging faster the energy storage components. However this kind of circuit usually is patented so they are rarely found in mainstream application circuits.
- Capacitor discharge avalanche pulser: a trigger signal applied to the base lead of the avalanche transistor cause the avalanche breakdown between the collector and emitter lead. The capacitor starts to be discharged by a current flowing through the resistors and : the voltages across those resistors are the output voltages. The current waveform is not a simple RC discharge current but has a complex behavior which depends on the avalanche mechanism: however it has a very fast rise time, of the order of fractions of a nanosecond. Peak current depends on the size of the capacitor : when its value is raised over a few hundred picofarads, transistor goes in to second breakdown avalanche mode, and peak currents reach values of several amperes.
- Transmission line avalanche pulser: a trigger signal applied to the base lead of the avalanche transistor cause the avalanche breakdown between the collector and emitter lead. The fast rise time of the collector current generates a current pulse of approximatively the same amplitude, which propagates along the transmission line. The pulse reaches the open circuited end of the line after the characteristic delay time of the line has elapsed, and then is reflected backward. If the characteristic impedance of the transmission line is equal to the resistances and, the backward reflected pulse reaches the beginning of the line and stops. As a consequence of this traveling wave behavior, the current flowing through the avalanche transistor has a rectangular shape of duration
In practical designs, an adjustable impedance like a two terminal Zobel network (or simply a trimmer capacitor) is placed from the collector of the avalanche transistor to ground, giving the tramission line pulser the ability to reduce ringing and other undesidered behavior on the output voltages.
It is possible to turn those circuits into astable multivibrators by removing their trigger input circuits and
- raising their power supply voltage until a relaxation oscillation begins, or
- connecting the base resistor to a positive base bias voltage and thus forcibly starting avalanche breakdown and associated relaxation oscillation.
A well-detailed example of the first procedure is described in reference Holme (2006). It is also possible to realize avalanche mode bistable multivibrators, but their use is not as common as other types described of multivibrators, one important reason being that they require two avalanche transistors, one working continuously in avalanche breakdown regime, and this can give serious problems from the point of wiev of power dissipation and device operating life.
A practical, easily realised, and inexpensive application is the generation of fast-rising pulses for checking equipment rise time.
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