Line driver circuit

[mark86]

Hi, apologies if this is a bit basic.. but I'm a bit new to electronics and have to design a circuit which can drive the following signal:

• up to +50v
• 25ns rise/fall time
• 8 ohm approx series resistance
• 33pF total capacitance

i dont really know where to start! would just two transistors do the job? I was worried about the speed and wasnt sure if a more advanced circuit would be required..

Any help or advice would be really appreciated!

Thanks!

Mark

[Hero999]

I disagree with you on this being basic, a 25ns rise time at 50V into an 8Ω 33pF load isn't going to be easy.

I would suggest MOSFETs.

What's it for?

[mark86]

Ok.. well i dont know if thats good or not! at least i dont feel stupid anymore.. if a little worried!

It's a CCD driver signal, the high voltage is required on one signal to multiply the signal before measuring.

So would you use the MOSFETs just as a pair? no special extra requirements?

Thanks!

Mark

[hotwaterwizard]

almost sounds like you could use an audio amplifier ic

[Hero999]

How many audio amplifiers are capable of giving a rise time of 25ns?

The circuit isn't too much of a problem, it's the PCB and the components that are the hard part.

[hotwaterwizard]

Ummmmm slew rate?

[audioguru]

The slew rate of 8V/us for the audio amplifier IC is much slower than 50V/25ns for the video amplifier circuit. It is 6250 times too slow.

[Hero999]

How many video amplifiers can switch voltage up to 50V into an 8Ω load?

[mark86]

Somebody suggested a BJT pair, with a diode to/from the rails to provide a bias at each collector. The a dc blocking capacitor, so that the edge on the driver signal would cause a spike that could turn on the transistors. I dont know if this could run fast enough.. and am not sure how to select the components for such a design.

I had not even thought of video amplifiers.. are they often used at this sort of voltage? Or fast enough?

Thanks for the help guys!

Mark

[Hero999]

Is there any DC in the signal?

I would suggest using MOSFETs for speed.

If the signal is AC then it does make things easier as a small pulse transformer could be used to drive the high side.

[mark86]

The driving signal would be from an FPGA, so between +5 and 0 v. Which is why I considered using a dc blocking capacitor to drive the circuit with edges. Pardon my ignorance, but what is the benefit of FETs over BJTs? Are they simply faster?

Thanks,

Mark

[Hero999]

Yes, MOSFETS are faster than BJTs.

[mark86]

Ok. Thanks for all your help. I will look into it!