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Author Topic: push/pull amp using IRF511  (Read 39310 times)
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ssbothwell KJ6RSG
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« on: October 16, 2011, 05:19:46 PM »

i found this amp design in "Experimental Methods in RF Design": http://i.imgur.com/vrwAY.png

K1 is 12 trifilar turns on a FT50-43
K2 is 12 bifilar turns on a stack of two FT37-50
K3 is a 2:3 transformer but the number of turns for each wire is unlisted

i have a couple questions about this amp:

1. each transformer poses a different challenge for simulation. how do i calculate the inductance values for a center tapped trifilar winding, how do i estimate the inductance caused by double stacking ferrite beads, and what values should i use in the 2:3 winding?

2. LTSpice does not have an IRF511 spice model. Furthermore IRF511 is difficult to find and unreasonably priced where i can find them. IRF510 is readily available and LTSpice includes an IRF510 spice model. would IRF510 be an acceptable substitute in this circuit?

3. what class is this amplifier and will it be able to amplify a low-level modulated AM signal without distortion?
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W4AMV
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« Reply #1 on: October 16, 2011, 06:00:03 PM »

Hi. The 510 is quite fine for CW. I suspect it will do well for AM when biased correctly. The amp pix I posted several weeks ago is a 510 and looking at an 830. They do 50-60 W at 1 W input, push-pull in the HF band.

On the xmfrs. You will need the AL factor from the data sheet. The AL is the inductance per unit turn. Assume the k factor is 0.9. If you stack them, you will get more L per unit turn. Without a measurement, for now assume the AL doubles (this really needs to be checked). Of course the dot convention and the start finish polarity needs to be observed, you have already learned that lesson!

Again on the 2:3 winding, implementing a 4:9 impedance transform, you are going to need the AL factor for that material mix... type 43. I believe all the AL factors are listed... check the AMIDON website (https://www.amidoncorp.com)  or FAIR RITE web site.

As a rule of thumb on these xmfrs, you would like the reactance of the xmfr to be 5x the impedance transformed. This requirement to minimize the insertion loss. So, if the drain push-pull impedance level were 25 ohms, then each xmfr drain side to CT should have a reactance of 125 ohms min. At the frequency of interest, calculate the required L.

73; GL with your new ticket!
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ssbothwell KJ6RSG
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« Reply #2 on: October 16, 2011, 06:27:29 PM »

oh good. i'm glad i can use IRF510.

here is the circuit with inductance values based on the info at toroids.info: http://i.imgur.com/YjGv6.png

in LTSpice, do i specify the reactance by setting the series resistance of the transformers? i assumed this to be the case and set rser to 125. is that not correct or is there something else wrong with my circuit? i am only getting 100mV Pk output from a 7mhz 7V Pk input.
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WA1GFZ
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« Reply #3 on: October 16, 2011, 08:59:31 PM »

Danger, this amp as drawn will blow up. The gates do not have a return path to ground so you will develope uneven voltages across the gates. Two options 1. put a little DC bias on C3 and C4 to run in AB mode. 2. tie R3 and R4 directly to ground for class C operation.
Download yourself a copy of Motorola App note AN749 on the design of broad band transformers. Check out CCI web site for a number of motorola app notes on broad band amplifiers.
A guy in EU  built a 1KW linear for 6 meters using 16 IRF511 a few years ago.
Good Luck Fun stuff
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« Reply #4 on: October 16, 2011, 09:33:47 PM »

The series resistance 125 Ohms is not the right way as it is like putting a resistor in series, but I don't know what the right way is for that.
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« Reply #5 on: October 16, 2011, 11:03:05 PM »

Quote
Danger, this amp as drawn will blow up. The gates do not have a return path to ground so you will develope uneven voltages across the gates. Two options 1. put a little DC bias on C3 and C4 to run in AB mode. 2. tie R3 and R4 directly to ground for class C operation.

You beat me to it.

In the simulation, for the bias to each gate, use a dc gen. to ground with the positive side of the gen to a >100 uH inductor and then to each gate. Try varyinging the DC gate biases to between 1.5 and 3.75 volts. I would also put a 10 ohm series resistor in the gate circuit just before the actual gate connection and after the bias node.

In real circuits, the reason for separate bias voltages is that each device will have different gains at different bias voltages. Series gate resistors reduce gate ringing.

Phil - AC0OB
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ssbothwell KJ6RSG
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« Reply #6 on: October 17, 2011, 05:08:20 AM »

here is the circuit with the bias mods phil and wa1gfz suggested: http://i.imgur.com/ANSER.png

in the real circuit how do you guys suggest i produce the bias voltage? should i use a voltage divider, regulator ic (lm317?), or some other bias method?

is the voltage variable on spice signal sources Peak to Peak, RMS, or something else? i have it set to 7V but it looks like 3.4V Pk-Pk. also, is there a way to display RMS voltage in LTSpice?

i also had to add resistance to the 28V input to L5. without a resistor, ltspice was reporting a loop with L5 and L6. will the series resistance of the real life inductors negate the need to add an additional resistor to the circuit?
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« Reply #7 on: October 17, 2011, 10:39:41 AM »

Quote
in the real circuit how do you guys suggest i produce the bias voltage? should i use a voltage divider, regulator ic (lm317?), or some other bias method?


It depends on your PS system. For Class A and AB, I would recommend a +,- 5 volt biasing system using 78L05 and 79L05's with a 5k potentiometer such that the wiper of the pot goes to the RF choke inductor. A 0.1 cap is usually place from the wiper to ground for various reasons.

Phil - AC0OB
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WA1GFZ
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« Reply #8 on: October 17, 2011, 11:52:43 AM »

You don't need to go negative 0 to 5 volts is good. The gate threshold is around 3.5V positive. 5V on the high side of the pot is plenty. Also if you want to run in AB mode put 1 ohm in series whth each source to make the bias more stable over temperature. A resistive divider is fine as long as the PS voltage is stable.
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ssbothwell KJ6RSG
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« Reply #9 on: October 18, 2011, 12:12:39 AM »

i think i'll go the 78L05 route. can i put two pots in parallel off of one 78L05 or will one not produce enough current for both bias circuits?
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WA1GFZ
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« Reply #10 on: October 18, 2011, 08:29:05 AM »

Put a diode in series with 5 volt end each pot to protect the system if 1 FET fails drain to gate. This way you won't take out the regulator and other FET.
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ssbothwell KJ6RSG
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« Reply #11 on: October 18, 2011, 11:46:15 AM »

i'm attaching a schematic that includes all the suggested modifications so far.

with a 7V (Pk?) 7mhz input i get a 25W output. With a 3.5MHz imput i get a 64W output! is that going to be way too much power for the mosfets? will i burn them out?

my goal is a general purpose final amp stage for low level modulated AM signals on all the HF bands.

i want to try to put together a transmitter that uses a diode ring mixer to modulate the carrier, then a push/pull AB amp to get to ~1W and then this IRF510 push/pull amp as the final. will this work or should i be trying to do series modulation of the final amp stage?
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WA1GFZ
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« Reply #12 on: October 18, 2011, 12:24:08 PM »

The output power will be limited by the size of your heat sink and the current rating of the device. I would think 50 watts is a good power rating for a pair.
I run 14 IRF840s push pull parallel class e on 160 and can hit close to 2 KW PEP output flat out. It has been on the air since 1996.
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ssbothwell KJ6RSG
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« Reply #13 on: October 18, 2011, 12:57:38 PM »

woah! 14 IRF840s! thats crazy. do you have any photos of your rig?

i just ordered all the parts i needed for this amplifier. hopefully everything will arrive soon and i will be able to assemble it over the weekend.

the only power supply i have that can do 28V is a variable supply with a max current of 750mA. does that mean my max power output would be 21W?

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« Reply #14 on: October 18, 2011, 01:42:52 PM »

i want to try to put together a transmitter that uses a diode ring mixer to modulate the carrier, then a push/pull AB amp to get to ~1W and then this IRF510 push/pull amp as the final. will this work or should i be trying to do series modulation of the final amp stage?

Series mod on the final is much more efficient, and I'd expect to see more Pout.  Class C,D, etc. are MUCH more efficient.  BUT, I'm also enjoying watching this project take effect.  Smiley

BUT, low level mod has it's bennies, too.  And it's problems.  Don't be expecting 150 percent peaks from a low level stage...

--Shane
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« Reply #15 on: October 18, 2011, 02:35:20 PM »

the only power supply i have that can do 28V is a variable supply with a max current of 750mA. does that mean my max power output would be 21W?

Nope.  That would be your power input.

Output, figure half to three quarters, Class AB to C.


--Shane
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WA1GFZ
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« Reply #16 on: October 18, 2011, 02:47:24 PM »

check out QEX Nov. 2005 my 22 FET push pull parallel for 80m article
nicer looking. Same layout though.
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ssbothwell KJ6RSG
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« Reply #17 on: October 18, 2011, 07:55:59 PM »

the only power supply i have that can do 28V is a variable supply with a max current of 750mA. does that mean my max power output would be 21W?

Nope.  That would be your power input.

Output, figure half to three quarters, Class AB to C.


--Shane
KD6VXI


well in that case i would be better off with a 12V 3A power supply versus 28V 750mA?

i am going to give low level modulation a shot but i would like to also try series mod. would that look like this?: http://www.eie.polyu.edu.hk/~ensurya/lect_notes/commun_cir/Ch8/Fig8-19.jpg

i tried doing something like that but it doesnt seem to work (circuit image is attached to this post).


* seriesmod.jpg (187.14 KB, 1440x874 - viewed 1058 times.)
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WU2D
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CW is just a narrower version of AM


« Reply #18 on: October 18, 2011, 09:50:19 PM »

Frank that looks a lot like my IRF840 P-P modulator - same basic bias scheme. I get a lot of audio out of a pair.
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« Reply #19 on: October 18, 2011, 09:55:09 PM »

For the smulation, I would recommend you replace the 78L05 with a DC voltage generator and sweep the voltage from 1.5 to 4.5 volts to observe the effect of bias with output voltage, since I assume you have the 510 model imbedded within the simulation code. 

Phil - AC0OB

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WA1GFZ
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« Reply #20 on: October 18, 2011, 10:09:05 PM »

Then add 1 ohm source resistors and you will see how to tame the bias.
They also produce some negative feedback
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ssbothwell KJ6RSG
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« Reply #21 on: October 18, 2011, 10:27:13 PM »

WAIGFZ, the circuit i last posted has the 1ohm source resistors (R9/R10).

DMOD: it actually is using a dc voltage source. i couldn't find a 78L05 model so i just used a 5v dc source with a label placed over the symbol Grin adjusting the 5K resistors adjusts the bias.


the best documentation on high level modulation i could find is this: http://www.eie.polyu.edu.hk/~ensurya/lect_notes/commun_cir/Ch8/Fig8-19.jpg

the arrl handbook doesn't really get into it much.

My LTSPICE modulation circuit from the last image i posted where i have an audio input coupled to the DC power input via a transformer does not seem to be working. i have tried a variety of inductor ratios and audio power levels but no dice.

just for fun i tried using a signal generator as a power source for my LM3866 AB amp (from my other current thread). I set the signal generator in the audio range and cranked up the output voltage as high as it went. it modulated the amplifier decently.

that was a fun little experiment but i cant imagine it being a good idea to use an audio amp to drive a 50W final RF power amp.

in proper 'plate modulation' i am supposed to use the audio source to modulate the power supply right? what kind of transformer do i need and what sort of power levels do i need to run my audio source at?

i ripped a transformer out of a broken radio receiver and tried using it to do plate modulation. i hooked it up every way i could think of but had no success producing modulation. i'm attaching a photo of the transformer to this post.

[EDIT] reformatted text a little bit to clarify the post.


* 1018112205.jpg (324.17 KB, 1280x960 - viewed 782 times.)
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WA1GFZ
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« Reply #22 on: October 19, 2011, 08:55:39 AM »

Your circuit still has a problem. There needs to be a bleed resistor between each gate and ground. 10K will be fine. The gate is like a cap and will store a charge based on drive waveform. A bleed resistor will keep it at the right DC voltage.
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ssbothwell KJ6RSG
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« Reply #23 on: October 19, 2011, 02:24:26 PM »

Your circuit still has a problem. There needs to be a bleed resistor between each gate and ground. 10K will be fine. The gate is like a cap and will store a charge based on drive waveform. A bleed resistor will keep it at the right DC voltage.

do you mean like this?

also, can IRF510 really handle 50mA Pk gate current? that would destroy any other transistors i have experimented with.


* bleedresistrs_added.jpg (180.55 KB, 1440x874 - viewed 1869 times.)
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WA1GFZ
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« Reply #24 on: October 19, 2011, 07:16:39 PM »

No, Connect the resistors directly to ground, not in series with caps. DC current does not flow through a cap.

Gate power= CV^2F so the higher the frequency the higher the gate drive power.
The gate draws no DC current. The drive signal just has to charge and discharge the gate C to turn it on and off. At some point the gate can't handle being charged and discharged any faster so it over heats and fails
The gate power is due to ESL and ESR since there is some resistance and inductance in the gate structure.
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