# Best Circuit for Pulse Amplifier?

#### SiegeX1

Joined Mar 10, 2009
25
I have an application where I need to take two 0 - 3.3V pulses from an FPGA and combine them to produce a Vout which can swing from -15v to +15v.

Here are some requirements and specs

• The max switching speed is 1Mhz @ 25% duty cycle
• The 3.3v pulses from the FPGA must go into an optocoupler to isolate the sensitive FPGA from the higher voltages
• The load to this circuit will be a 75 Ohm cable that is transformer coupled
• I will have a total of 4 of these 2-input channels
I've included some images below to best explain what I'm trying to do. I'm wondering what type of circuit is recommended?

I should note that we had a few microchip TC4432 MOSFET drivers lying around and I used a pair of those, one for each input to directly drive each side of the transformer and it seemed to work. However, I'm fairly sure MOSFET drivers are not intended to source/sink a constant amount of current (~250mA in my case) but rather have short peak currents to charge up the gate of a FET; am I correct?

The other alternative I can think of is a full-bridge motor driver but I'm not real sure how to set that up.

Thanks!

Two examples of what the input -> output relationship looks like

What goes in the black box?

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#### ifixit

Joined Nov 20, 2008
652
Hi SiegeX1,

Questions:
2. Is this going to be a data line using AMI encoding?
3. How long is the coax line?
4. The transformer offers isolation, so why is the opto-coupler needed?
5. You could use a class B push-pull driver if the transformer primary was center-tapped. Can you get a center-tapped transformer?
15V into 75Ω at 25% duty cycle is 15/75 = 0.2A * 15 = 3Watts * .25 = 0.75 Watts. 10% of this would be dissipated in the TC 3144 (75mW). The TC3144 has a θja of 155 °C/W for the SOIC package, therefore the junction temperature would rise to 155 * .075 = 11.6 °C above ambient with the worst case data pattern of all ones. No problem there.

A continuous maximum current spec is missing since it is unimportant for most MOSFET drive applications. You require a 200mA drive that lasts for 250nS. The spec only mentions a 100mA test condition for the output level test, and a 3A pulse into a capacitive load for 25nS. Therefore you can't be sure that 0.2 Amp won't cause a failure later in life. My guess is that it would not be a problem for a hobby project.

The TC3144 output Z is 7 to 10Ω which is a significant part of the line Z. Depending on your requirements, this may, or may not be important.

Regards, Ifixit

#### SiegeX1

Joined Mar 10, 2009
25
Hi SiegeX1,

Questions:
This will be a digital protocol simulator to help in verification/validation testing. I wrote the FPGA code and now I need to condition the signals to match that of the various encoding schemes we use
2. Is this going to be a data line using AMI encoding?
I was not familiar with the 'AMI' term but after looking at wikipedia, yes that is what we use although we call it 'Bi-polar'. We will also do 'Uni-polar' encoding which is what my top input/output example shows. The bottom example is the AMI or bi-polar encoding as you mentioned.
3. How long is the coax line?
I believe it would be no longer then ~10ft or so
4. The transformer offers isolation, so why is the opto-coupler needed?

Regards,
Ifixit

#### SiegeX1

Joined Mar 10, 2009
25
Hi again! Sorry for the late reply, I was on travel for the last few days.

What is the worst case pulse level required? 15V into 75ohm? If so, you only need to select a small MOSFET with a 1A max Id spec. Its gate charge requirements will be easier to handle.
Yes, I was able to get a hard copy of the requirments and at ~1Mhz the requirement is to have a pulse amplitude between 9V and 15V across a load that has 75ohm in parallel with a 5mH inductor (see attachment)

In regards to the transformer, I'm having an awful hard time trying to wrap my head around what I really need in order to get a +/-15V pulse on the output using a +15V source. I started a separate thread regarding my troubles getting my head around modeling it. Would appreciate it if you could take a look at that as well.

Also, thanks for clearing up the gate charge vs gate capacitance. I found a really nice white paper by microchip that went into quite a bit of detail and now I feel I have a pretty strong grasp of what's going on.

P.S. Even though you nearly broke even, you still came home a winner because those new video slots with all the bonus levels have quite a bit of entertainment value

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