[morzh]

Try any of these....use the one below 0.5W rating.

http://www.mouser.com/Semiconductors/Dis...bfZ1z0w7p6

This is one of them.

78-TZX12B


HOWEVER:

before you do this take a 100pF cap, put it in parallel to the transistor (C-E) and see if it degrades performance at high frequencies. You are about to get a scope so this would come handy.

There are low capacitance TVS type Zeners like NZL series but they only come below 6V clamp and you need 11-12V. Datasheets unfortunately do not list the capacitance for Zeners.

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Another way as Isaid,

take two 1N4148 or any small signal diodes

http://www.mouser.com/ProductDetail/Fair...bu6z6oTQTL

Put one from the Collector to +V (10V if it is convenient - it is better than 9V as the latter has resistor in series and a Zener) - put Plus to C and Minus to 10V.
Put another between C and GND - put minus to C and Plus to GND.

That is both diodes have to be reverse-biased.

What this achieve is it will clamp the voltage to GND/VCC should it exceed them by more than 0.7V. And because the capacitance of 4148 is 2 to 4pF you do ot have to worry much about degrading the performance.