Modified Heising for Class C transistor final

[ka1tdq]

I just finished my 75 meter AM transmitter and I'm posting the modulation scheme for some final input before I test it on Tuesday.  The RF section was already tested and I'm getting a clean 15 watt sine wave on the output.  Someone advised that I bias the driver stage to prevent thermal runaway.  Thanks, done.

The audio modulation is modified heising and I've borrowed the idea from elsewhere on the web.  It just consists of an 8200uF electrolytic in series with an audio modulated NPN transistor.  The bias on the transistor is adjusted via a 10k pot. 

The modulated audio is run through a 25mH choke.

Jon
KA1TDQ

[AB2EZ]

Jon

1. 25mH corresponds to an impedance of around j8 ohms at 50Hz. Just to be sure, is the modulation resistance of the RF stage 8 ohms or less (to obtain a flat modulation characteristic down to 50 Hz)? For a 15W (at carrier) transistor transmitter... I would be surprised if the modulation resistance is 8 ohms or less.

2. Just to verify... is the 25mH inductor capable of handling the current that will flow through it without saturating?

3. Some people like to make the Heising capacitor as large as possible... but I believe (as also derived in the literature) that the flattest low frequency response (down to the point of rolloff) will result if the impedance of the Heising capacitor is equal to the impedance of the Heising inductor at the audio frequency, f,  at which the impedance of the Heising inductor is equal to the modulation resistance. I always pick the value of the Heising capacitor using that approach. For example, when I externally modulate my Ranger using a modified Heising configuration, I use the following values:

Ranger modulation resistance = 600V/0.120A = 5000 ohms
Heising choke: 20H => j5000 ohms at f= 40Hz
Heising capacitor = 0.8uF => -j5000 ohms at 40Hz

Looking forward to hearing more about your results.

Stu

[ka1tdq]

Oh, I see.

The choke is capable of 4 amps, so it can definitely handle the current.

I see by your numbers how to get the L and C values. My RF resistance is 50 ohms, so my low range roll-off is 318 hz for the inductor and 400 hz for the capacitor. It'll work, but the low range will suffer.

[WD5JKO]

Jon,

  Usually Heising has both a class A audio stage and the RF stage getting DC current through a common Heising choke. Adding the capacitor in series with the collector of the power transistor does not allow any DC operating point to be established.

  To run "Modified Heising" the modulator transistor needs to be able to provide power to the load through the Heising capacitor. Your modulator cannot do that unless you supply it power via another means. Perhaps an NPN-PNP complimentary output audio amplifier would do where the audio runs off the same 33v supply, and the audio output feeds the Heising capacitor.

Maybe something like this, but with a single supply:
http://diyaudioprojects.com/Solid/Jean-Hiraga-Class-A-Amplifier/

Jim
WD5JKO

[ka1tdq]

Yeah, my idea seemed too simple.

Another way would be to skip the on-board modulator and run an external PA amp. Just connect the positive speaker output lead to the electrolytic (-) lead and the speaker (-) lead to ground. That is also 8 ohms.

Also, rather than wasting my 6 volt regulator that's on-board, I can use it trick out the transmitter with bright blue LED's like the computer guys do with their home built computers.

[kb3ouk]

Actually, your idea is on the right track, but just needs a few slight changes. If the audio transistor can take the 33 volts of the power supply, then just remove the capacitor. If it can't, then keep the capacitor but add a dropping resistor from the power supply side of the choke to the audio transistor side of the capacitor, then add a second choke after the dropping resistor. The varying current flowing throught the choke developes an AC voltage across the choke, which feeds through the capacitor (basically the same concept as an impedance coupled audio output circuit).

[AB2EZ]

Jon

You need to verify that the RF stage that you plan to modulate can, in fact, be modulated by modulating the B+ applied to that stage. I.e. if you double the B+, then the current drawn by the RF stage will double, and the RF power output will be 4x bigger. You don't have to run the stage at 15W carrier.... but whatever the carrier output level is, the power should be 4x larger when you double the B+. Similarly, you need to see how low you can drive the B+ on downward portions of the modulation cycle before the RF stage enters cutoff. You can test all of this by just varying (by hand) the B+, and measuring the associated current and RF output power.

You may need to add some type of negative peak limiting circuit to the modulated B+ to prevent the generation of distortion products when the B+ is driven down to near the cutoff level of the RF stage.

Assuming that modulating the B+ does, if fact, result in a proportional modulation of the average current drawn by the RF stage: the modulation resistance is not the same was what is (perhaps confusingly) referred to as the RF output impedance. What is often referred to as the RF output impedance is the optimum value of the RF load impedance at the fundamental frequency (with the antenna or a dummy load connected). The modulation resistance, R, is defined as: (1/R) = the change in average current drawn by the RF output stage / the change in B+ that produces that change in average current drawn by the RF output stage. It is approximately equal to the B+ at carrier / the average RF output stage current at carrier.

The modulation resistance is usually greater than what is referred to as the RF output impedance. In a typical class C RF amplifier, the modulation resistance is a little less than 2x the RF output impedance. In a class E amplifier, the modulation resistance is also a little less than 2x the RF output impedance. In a class B RF amplifier, the modulation resistance is around the pi/2 = 1.57  x  the RF output impedance

Stu

[ka1tdq]

Great explanation! Thanks, I didn't know lots of that.

I'm getting things set up tonight for a bench test tomorrow. Hopefully things go well. If they do, I will get some good measurements to incorporate a negative peak limiter to clamp the voltage before it gets too negative.

Jon

[steve_qix]

Since you're running such low power, you could get DC to (whatever your high end is) audio with near 0 distortion by using a class A series modulator.  That would certainly be the easiest and cleanest modulator you could use.

Place the modulator in series with the positive voltage going to the RF amplifier.  If your RF amplifier can handle, say 6x the power you're running at carrier, use a power supply that's 2.5 times higher than the carrier DC, and set the carrier DC appropriately.  This will give you up to 150% positive modulation.  A power supply of 3x the DC will allow up to 200% positive modulation.

Stu's comments about whether the RF amplifier can handle the power (and voltage) created by adding a modulator cannot be over stated.

[ka1tdq]

I haven't had a chance to test it yet, but probably will late this afternoon. My guess is that 33 volts will be too much carrier voltage considering the added modulation voltage. I can easily rearrange the power supply configurations for say 12 volts carrier and 33 volts modulation.

I may do 12 anyway to be safe. I tried testing 66 volts on the final during the RF testing and immediately popped the transistor.

When I make the negative peak limiter, what negative voltage am I limiting? I'm sure it's more than the 12 volt supply and I wouldn't simply just clip it at say .5 volts.

Also, the circuit would be between the output of the choke and ground... correct?

[ka1tdq]

Oh no... I just thought about it... its in parallel with the choke to limit negative excursions the other way.

[steve_qix]

Keep the supply at 33 volts (or whatever it is), and use a series MOSFET or transistor and do the modulation that way and set the carrier DC at 12V using the modulator. Heising is not a great modulation system - there are many limitations, and it is ultimately class A, so it is not more efficient than a class A series modulator, but has many more problems and limitations.

[kb3ouk]

Since he is planning on not running much in the way of carrier power, it might be better to run heising and modulate only up to 95% or so. Running 200% modulation on a 15 watt or so carrier is just asking for trouble. It might be ok when you have an abundant amount of carrier power, but when you're running low power like that, you're not gonna be be heard. The trick to getting 100% or more from heising modulation is to run more voltage on the modulator than on the RF amplifier. Another way of doing it would be to reduce the voltage down to say maybe 24 volts. Hook up the modulator and final like I said, cap between the final amp and the modulator, modulator fed voltage through a second choke. Butinstead putting the dropping resistor between the modulator and power supply, put it on the RF amplifier side, to reduce the RF amp voltage down to 12 volts or so. Reducing the voltage on the RF amp makes it easier for the modulator to go up to 100%, since there's a little less carrier power.

[ka1tdq]

I like the idea of a series mosfet for the frequency response. I plan on using a linear anyway. I'm on the road right now (parked) so any good links to a good mosfet audio circuit? I already have the preamp driver stage, so I just need the biasing setup.

[kb3ouk]

http://files.myopera.com/Frister/projects/modulator.jpg

Here's something to start with. I think this circuit was for an IRF510, but could probably be adapted for some more power if you need it.

[ka1tdq]

Thanks! I'm going to build it. I will keep the big choke and capacitor on the transmitter to help bring dow the DC closer to 30 volts and aid in regulation.

So I'm guessing that a negative peak limiter would go in parallel with the output NPN and the 10uH choke. After getting a good carrier level then build the limiter to suit.

[steve_qix]

What components do you have lying around the shack?  I could design a circuit around these, depending on what you have.  I need to know about MOSFETs, transistors, op-amps, etc.

[ka1tdq]

I think I have a standard 741 opamp, some 2N3866s, a high voltage N channel mosfet, a 250k variable resistor, 5 volt regulator, and a few 7984 tubes.

[AB2EZ]

Jon

Can you post the schematic of the Class C transistor RF stage that you will be modulating?

Stu

[ka1tdq]

This is fairly accurate.  The output network is for 50 ohms.

[steve_qix]

Hmmm... The schematic is a bit hard to read.  Looking at the transistor number, the NTE1127 is an IC.  The NTE112 is a diode.  I'm unable to discern whether the last character in the part number is an F or a 7 (looks like a 7), but either way I couldn't figure it out 

[ka1tdq]

Yeah, I made a mistake on that.  The final is an NTE291.  I've also attached a photo of my semiconductor stash.  It includes:

(5) TC4420 mosfet drivers
(2) 7805 voltage regulators
(1) 2N3907 power transistor (vintage 1960's I think)
(1) NTE4011B quad NAND gates (I think)
(1) SA602 for your oscillator/multiplexer needs
(2) good ole 2N3866's
(2) NTE291's
(1) NTE969
(1) NTE51
(5) IRF740's (insert "Halellujia" song here)
(1) 16006 audio NPN transistor I was originally using as a mic preamp
(2) SN74AC74DR dual positive edge triggered flip-flops
(1) LT6231CS8 dua opamp
(2) FST3253MX multiplexer/demultiplexer TTL

Also, there is more voltage available from the transmitter (around 66 vdc). So I can still maintain around a 10 watt carrier with a class A modulator. 

[steve_qix]

Ok, try this one.  It uses the MOSFETs you have.  Some of the values are very non-critical. If there is a part you have, and it's close but not the exact value, ask about it and chances are it will work.

[steve_qix]

The 50VDC power supply input value may be a bit too high for your application.  In ideal class C, the RF peaks will get up to about twice the applied DC, but you do want a safety factor and of course if the tuning is wrong or if the load disappears or changes, things happen!

The transistors in the input stages (2n3904) can be substituted with similar devices - 2n2222, etc.   The B (beta) is the important parameter.  You may have to adjust R19 if the transistors are different.  You want about 5 volts at the collector of Q5.

Adjust the carrier for about .3 times the applied DC.   This will give you good headroom for positive peaks.  Keep in mind, with this design, the output will not make the "rail" (full power supply voltage) due to the gate threshold value of the series modulator MOSFET.  It will be about 4 or 5 volts lower than the DC voltage.  So, for the values shown, you can expect about 45VDC max out of the modulator under the highest positive peak conditions.

[ka1tdq]

Thanks a bunch Steve! I feel honored... I have a personalized and customized schematic. 

I will heat sink the fet with no problem.  I have a metal chassis mount already with fins (electrically isolated) with a cooling fan as well.  I like the fact that the negative peak limiter is included too.

I'm going to build it as-is rather than modifying anything.  It'll be maybe a $10 trip to Fry's to get the little pieces. 

I've got lots going on right now until the weekend, but I will put it together and let you know how it turns out.

Thanks again!

Jon