What Are the Best BJTs for a 17.5 MHz Pulse Input Application?

AI Thread Summary
The discussion focuses on selecting BJTs for a 17.5 MHz pulse input application with a voltage range of +15V to -3V and a load capacitor of 150pF, requiring rise and fall times of 10ns. Participants suggest considering both NPN and PNP transistors, noting that RF transistors may struggle with the required load and current specifications. A specific mention of the 2N5770 NPN transistor is made, which is suitable for faster rise times but operates at lower voltage levels. The importance of maintaining transistor conduction without saturation for optimal pulse performance is highlighted. Overall, the conversation emphasizes the need for careful selection of transistors to meet the stringent timing and voltage requirements.
VASANSNS
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Hello everybody,
I have an application which needs a pulse input ranging from +15v to -3v operating at 17.5 Mhz. The load capacitor is 150pF and the required rise and fall time of the pulse is 10nS. I am building a circuit with BJT's (both NPN and PNP) . I need some idea about the criteria for selection of BJT's available in the market.

Can somebody suggest some available NPN and PNP transistors which work in this range.
Also i tried searching for some RF transistors, but i was not sure if they are capable of handling such huge loads. The max collector current for RF transistors was around 50mA. I was not whether its possible to get such a strict rise/fall time with the given load. Can somebody comment on this calculations.

Also any other suggestion or reference links are also welcomed.

Regards
SNS
 
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I used to use a 2N5770 npn for 1-2 ns risetime pulses, but it was only about a 0.75 volt to 1.5 volt pulse into 50 ohms. Are you talking about a pulse that goes from 15 volts to -3 volts? Into what impedance? To get the fastest pulses, always keep the transisitor conducting, but never in saturation.
Bob S
 
What is your application? Electro-optic modulator? Polarisation scrambler maybe?
 
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