Nanosecond Avalanche Transistor Pulse Generator

Nov 02, 2012

The 2N2369a avalanche transistor circuit below can be used to generate sub-nanosecond electrical pulses for high-speed testing and other purposes:

Using an avalanche transistor to generate short electrical pulses has been discussed in detail by Jim Williams in the Linear Technology Corp. application note AN61 in the section Triggered 250 Picosecond Rise Time Pulse Generator. Subsequent articles by this author particularly AN79, AN94, AN98 and AN113, and EDN Diode-turn-on-time-induced failures in switching regulators provide circuits with greater control and information on sub-nanosecond avalanche pulse circuits. The intent here is to provide the simplest version of a free-running circuit without trigger or pulse amplitude control .

Output pulses of 10 -20 Vpeak with 600 ps tr/tf into 50Ω loads are achievable as shown below for a typical 2N2369a transistor with an avalanche breakdown voltage of 60V. The scope trace was taken using a Tektronix 400 MHz 2465B oscilloscope with 50Ω input termination. The output pulse height shown is ~ 10V but will depend somewhat on the exact breakdown voltage of the 2N2369a with a range of 60 to 85V typical, depending on the manufacturer and lot number. A suitable low current 100 VDC power supply can be assembled easily using a 115VAC/25VAC transformer or adapter and a voltage tripler circuit (see example circuit below):

An assembled circuit is shown below. Four 220Ω resistors are used in the emitter circuit to lower inductance and provide a source impedance of ~ 55Ω

The collector capacitor C1, which is typically 2 - 5 pF for subnanosecond pulse generation, charges through the collector resistor R5 until the transistor breaks down, discharging C1 through the transistor and paralleled emitter resistors thereby generating a short voltage pulse. The collector voltage trace below shows the charging and avalanche-breakdown and discharge cycle. A x10 attenuation probe was used and the voltage source was 100 VDC. The breakdown voltage for this device was 82V.

Simple Voltage-Tripler Power Supply

A simple fixed voltage tripler power supply with ~ 100VDC output suitable for driving the avalanche transistor pulse circuit above is shown below. A commercial floating 24VAC 60Hz (34Vpeak) adapter was used. A voltage doubler supply with the same input voltage would provide ~ 65VDC output. Depending on the 2n2369a, 65VDC may not be sufficient to trigger avalanche breakdown: