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

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Discussion Overview

The discussion centers around selecting suitable BJTs (Bipolar Junction Transistors) for a pulse input application operating at 17.5 MHz, with specific voltage levels and timing requirements. Participants explore criteria for BJT selection, including rise and fall times, load capacitance, and current handling capabilities.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant inquires about the criteria for selecting NPN and PNP transistors for a pulse input of +15V to -3V at 17.5 MHz, with a load capacitor of 150pF and rise/fall times of 10ns.
  • Another participant mentions using a 2N5770 NPN transistor for 1-2 ns rise time pulses but questions the feasibility of achieving the required rise/fall times with the specified load and voltage levels.
  • There is a suggestion to keep the transistor conducting but not in saturation for achieving faster pulses.
  • A participant asks for clarification on the application, speculating it could be related to an electro-optic modulator or polarization scrambler.
  • The application is later clarified as an image sensor driver.

Areas of Agreement / Disagreement

Participants have not reached a consensus on the best BJTs for the application, and multiple viewpoints regarding the feasibility of achieving the required specifications remain. The discussion includes both technical suggestions and questions about the application context.

Contextual Notes

There are uncertainties regarding the maximum collector current capabilities of RF transistors and whether they can handle the specified load while meeting the rise/fall time requirements. The impedance into which the pulse is applied has not been specified, which may affect the selection criteria.

Who May Find This Useful

Individuals working on high-frequency pulse applications, particularly in the context of image sensor drivers or similar electronic circuits, may find this discussion relevant.

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?
 
Image Sensor Driver
 

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