3-Diode keep alive circuits for analog driven class E transmitters

[ka1tdq]

The class E transmitter I'm currently building uses 2 FETs in parallel and a drain voltage of 48 vdc. At 100 watts carrier, that should equal 2.32 amps drain current (assuming 90% efficiency).

Optimal load impedance for the FETs is .5 x (Edrain/Idrain). In my case it equals just over 10 ohms. The 50 ohm antenna and the loading cap match the output to that 10 ohm value.

My question is regarding the well known 3-diode keep alive schematic. I'm going to modulate this transmitter with a modified Heising circuit, and I want to use the keep alive circuit to prevent any negative peaks. On the AMWindow website, the schematic is applied to tube transmitters with the resistor's value in the circuit equal to plate load impedance. In a class E transmitter, would the value of that resistor be optimal drain impedance (10 ohms)?

In my 8-FET 75 meter rig, I think I used either 50 or 100 ohms in this same application. I have a modulation monitor and I've never gone over 100% negative, even with hitting over 125% positive.  

Jon

http://www.amwindow.org/tech/htm/3diodeka.htm

[steve_qix]

Your numbers  sound about right.  You need to use a BIG diode for the series diode (such as DSIE3006A or something like that).  The others should still be good for a few amperes.

If you hit 100% negative on your modulation monitor, you are overmodulating.  It will never go above 100% negative (because it can't - there's nothing less than 0 - the carrier is cut off).  I tend to try to never go over 95% to 97% negative.

[ka1tdq]

Hence my negative peak limiter on the big rig probably isn’t doing its job. I forget what I used as the series diode for it, but it was definitely strappingly big.

Maybe I should change the resistor value in it as well?

Jon

[ka1tdq]

Never mind, I thought about it. That resistor value isn’t as critical to the circuit as the series diode going from the keep-alive power source and the FETs. It’s just a current limiting part for the mod transformer.

As I remember, my measurements for the big rig used a 2.5vdc keep-alive power source. It was then that I was hitting 100% negative. I have since then increased that to a little over 5 volts. I did take another modulation monitor reading since then and it was at 100% positive and only 80% or so negative. But, that was also after my second heat sink assembly rebuild, and I’m nervous about hitting things hard again.

My 40 meter single FET rig uses the same circuit, but with values scaled down and with a 50 ohm (I think) resistor. I hit around 125% positive with it and see the negative limiting to around 90%.

I’ll use a 50 ohm resistor in my new rig and all should be well.

Jon

[steve_qix]

Never mind, I thought about it. That resistor value isn’t as critical to the circuit as the series diode going from the keep-alive power source and the FETs. It’s just a current limiting part for the mod transformer.

As I remember, my measurements for the big rig used a 2.5vdc keep-alive power source. It was then that I was hitting 100% negative. I have since then increased that to a little over 5 volts. I did take another modulation monitor reading since then and it was at 100% positive and only 80% or so negative. But, that was also after my second heat sink assembly rebuild, and I’m nervous about hitting things hard again.

My 40 meter single FET rig uses the same circuit, but with values scaled down and with a 50 ohm (I think) resistor. I hit around 125% positive with it and see the negative limiting to around 90%.

I’ll use a 50 ohm resistor in my new rig and all should be well.

Jon

I always forget to ask this: Do you have TransZorbs in the drain circuit (across the drains to ground) ?   If using analog drive, do you have TransZorbs across the gates?

[ka1tdq]

Drains yes, gates no. I’m using digital drive via a gate bus. Similar to the Partridge family bus, but not quite.

In the center of the bus sits a single IXDD614 which serves as the wave shaper from the DDS VFO. It also drives the rest of the bus (all other IXDD’s take their input from that first one’s output). It all seems to work. I generate RF.

Jon