RF Feedback

[W1RKW]

got some limited success with negative feedback.  Got to jockey some values.  Still have a crappy looking output waveform but will mess with an attenuator for the Retro next.

Is it normal for bias to go to zero when RF is applied?  I don't remember it doing that and then again I can't remember if I ever watched the bias with RF applied.

[AB2EZ]

Bob

Great news!

Some types of DC meters will not read correctly in the presence of RF. The base bias voltage should not go to zero... but remember: once you draw average collector current, you will also draw average base current... and, depending upon R4... the base voltage will drop somewhat.

Did you try reversing to input transformer leads to see if there is positive feedback coming in via the primary of the transformer? [Reversing the leads will convert that feedback, if present, from positive to negative feedback].

Stu

[W1RKW]

Stu,
What's your take on the meter readings I was seeing when checking the transistors CE junctions?   See reply #66: http://amfone.net/Amforum/index.php?topic=24494.msg183410#msg183410

[W1RKW]

It seems the input of the amp is not 50ohms.  There is a mismatch with at least a 3:1 SWR.  A tuner does not improve the output signal and I'm beginning to wonder if the transistors are toast.

Tried a 4dB attenuator.  that didn't do anything except lower the output.  Waveform is still poor.

Still poking.

[AB2EZ]

Bob

With respect to your measurements on the transistors:

I think that you should assume, for now, based on the behavior you are observing (rather than the multimeter measurements) that one or both transistors are still working. If there was a collector-to-emitter short, there would be smoke rising due to the excessive collector-to-emitter current that would flow. If there was an emitter-to-base short, on either transitor... the bias voltage would be zero on both transistors. If one of the bases were open... you would see little or no effect from disconnecting the collector of that transistor (see below).

I suggest that you try the following:

Oscillation occurs because there is positive feedback somewhere. Adding a series RC between collector and base will introduce negative feedback (to cancel some or all of the positive feedback); and/or will introduce enough phase shift in the existing feedback to stop the oscillation. The value of C needs to be large enough to both: block the DC (which any capacitor will do), and to present an impedance that is less than the value of R. It is the R that provides the negative feedback. Since the RF output impedance of the amplfier is a few Ohms, and since the current gain is around 100: a relatively small value of R will be required... probably around a few hundred Ohms... maybe less. I.e. you want the fraction of the output current that flows into the feedback resistor (the rest flows into the RF load) to be comparable to the current gain of the transistor. However... before introducing negative feedback (which isn't in the existing design), it would be very helpful to know where the existing positive feedback is coming from.

a. Is the existing positive feedback coming from the output signal feeding back into the input transformer's input winding? To check, either reverse the input winding polarity, or disconnect the side of the input winding that is not grounded. [Shorting the input winding is inconclusive, because doing so also produces an RF short across the base of each transistor to ground]

If reversing the input winding (or disconnecting the side of the winding that is not grounded) has no significant effect on the oscillation... then that would indicate that the existing feedback, causing the oscillation, is not coming through the input winding of the input transformer.

b. You might want to see if the oscillation goes away (or changes significantly) when there is only one transistor being driven. Turning off one transistor might reduce the gain in the feedback path by 6dB... depending on what the feedback path is. This would also allow you to check whether one of the two transistors is "toast". You can turn off one transistor by removing the connection to it's collector.

Stu

[W1RKW]

Flipped winding of the primary on the input transformer.  Note: Secondary is not flippable.  Had a significant reduction in oscillation amplitude.   Reinstalled the RC networks for the Base Collector. (.01uf and 100ohm) and have a flat line again.  Also tried running with 0.0V bias by shunting the anode of the bias diode. Still can't seem to improve the output linearity.

Freq: 3880KHz
Upper waveform is output
Lower waveform is input

Scope settings:
x10 probes
Channel 1 (output) 5v/div
Channel 2 (input) 1v/div

The image doesn't reveal this but the input waveform isn't clean when the transmitter is keyed.

[AB2EZ]

Bob

Ok... making progress!

In the picture you posted, where is the output probe? (I.e. is it across the dummy load?)

Also... please refresh my memory: what is the turns ratio of the output transformer (total turns on output v. total turns on input).

I.e. if the peak-to-peak output voltage across the dummy load is 100 Volts... and the turns ratio is X...  then the peak-to-peak voltage from center tap to either collector is 100/2X = 50/X (assuming both transistors are working and operating as a matched, push-pull pair). But the peak voltage from center tap to either collector cannot exceed the supply voltage (~ 13 volts)... so the square wave may be a result of "running out of headroom"

If (taking into account the 10x probe) you have a signal that is a 50 volt (peak) square wave across 50 Ohms... then you are running the amp at 50 watts of r.f. output (continuous). This amplifier may not be able to sustain that level of continuous output power without overheating the output transistors.

Try reducing the input level (use an attenuator... e.g. 100 Ohms from the center pin of the Retro75 to ground and 50 Ohms in series between the center pin of the Retro 75 and the center pin of the amp ) to see the impact on the output of the amplifier (shape and size)

It looks to me like the amplifier is saturating because it is running out of headroom (the collector-to-base voltage is approaching zero).

Stu

[WA1GFZ]

Amp looks saturated due to overdrive

[W1RKW]

Stu,
Agree on input power being to high and causing saturation. Got a 4dB attenuator in place plus tried the 50 and 100 ohm in addition to the 4dB attenuator then took a different approach to reducing the input to the amp.  Instead of using the Retro75 as an RF source I used another source, an RF signal generator.  2.0Vp-p is just enough to key the COR.  The sad part is the output waveform looks exactly the same on all accounts as above just reduced in size.  I'm stumped.

Scope probe at output jack of amp.
Transformers are 16:1 according to the instructions which I'm assuming is impedance ratio rather than turns ratio.  There are actually 4 turns in each transformer assembly.

Any concern about the input impedance not being ideal?

[AB2EZ]

Bob

Assuming the output transformer has a ratio of total input turns to total output turns of 1:4; and if the peak-to-peak voltage across the dummy load at the output of the transformer is 100 volts...

Then the peak-to-peak voltage from collector to ground of either transistor should be 50 volts/4 = 12.5 volts. Therefore the peak voltage is 6.25 volts... and there should be no problem with saturation (assuming both transistors are working in a balanced way).

However... the picture you posted shows that there is a problem with saturation!

I am wondering if both transistors are, in fact, working.

Can you verify that both sides of the primary (the side facing the transistors) of the output transformer are properly connected to their respective collectors (i.e. the circuit board traces are ok, the solder joints are ok).

Likewise, can you verify that, in the absence of input rf, both transistors have approximately 0.6 volts between their base and their emitter (directly touching your probes to the transistor base and emitter).

Alternative: turn the power off, and check the continuity all the way from each transformer's center tap to the respective tabs on the transistors. Verify the each emitter has zero resistance to ground.

 Stu

[AB2EZ]

Bob

Also:

Turn the COR on... (i.e. don't use the input rf to do it)

Now put a signal into the amp that is smaller than 2V peak-to-peak (e.g. 1 volt peak to peak)... and see if the saturation is significantly improved

Stu

[W1RKW]

Stu,
I'm not convinced the transistors are bad, at least not yet. Afterall, the amp does amplify just not cleanly.

Continuity looks good on both xformers.  Looks good going through the xformers to each transistor leg.

0.650 on each BE junction.

12.5V on the collectors.

Also tried  operating the COR manually with much lower input signal. In fact dropped the input level to the point of drawing a 1/2 amp from the PS. No change in wave form.  Looks like I have a pretty powerful square wave generator.   

[AB2EZ]

Bob

When you dropped the input level.. how did that affect the output level (putting aside the shape of the output waveform)?

Stu

[W1RKW]

it reduced the amplitude as one would expect.  No change in shape of waveform.  I'm wondering about the oscilloscope.

[AB2EZ]

Bob

Swap the probes... so that the probe that is now attached to the input, and feeding scope channel 2... is connected to the output (and still feeding scope channel 2).

Separately: where are you connecting the input probe?

Stu

[W1RKW]

probes swapped. no change.  Input is at the input to the amp.  If the input probe is move to the Retro prior to the attenuator clean signal.  after the attenuator sinewave gets hacked up when amp is keyed.

[AB2EZ]

Bob

Okay... looking again at the waveforms you posted above... both the input and the output waveforms are mirror image symmetrical around V= 0 volts... which strongly suggests that the amplifier is balanced (no even harmonics)... and therefore both transistors are working in a balanced, push-pull configuration.

Could you put the output (channel 2) probe on the base of one of the transistors (set to DC coupled mode)... while leaving input probe (channel 1) where it is now... and post the waveforms. Please do this with around 1 volt peak on the input side of the input transformer(i.e. where channel 1 is connected).

Also, please let me know what the DC level is on channel 2 (the base of the transistor) when the rf input is 0 (shorted)

The above should be done with the COR manually activated, in all cases.

Stu

[AB2EZ]

Bob

P.S.

It's time to revisit the large electrolytic capacitor across the bias supply. Please reinstall a 0.1uF or larger capacitor in parallel with it.... as you did last week.

Stu

[AB2EZ]

Bob

The reason I am concerned about the electrolytic capacitor is that the amplifier may be doing the following:

For some reason, there is no low impedance path from the center tap of the input transformer to ground (for RF).

Thus, the RF waveform being applied to the input winding of the input transformer cannot push current into one base unless it draws an equal and opposite current from the other base.


The current that can be drawn out of the base of either transistor is limited to the bias current.

Thus, as the input RF signal tries to push current into the top transistor's base, that current gets clipped at a level equal to the bias current in the bottom transistor.... and vice-versa.

This would result in the clipping (squaring) behavior you are seeing at the output of the amplifier.

In addition to adding the capacitor in parallel with the large electrolytic capacitor... make sure that the path from the center tap of the input transformer to the top of the electrolytic capacitor has a low DC resistance.  

Stu

[Gito]

Hi

In my opinion ,the square wave output ,it's not a problem ,maybe it must be like that,when I look at the schematic diagram it's a class D amplifier/broadband Amplifier.
Even if it's square ,it represent the amplitude of the carrier.
When we modulate ,the carrier amplitude goes up and down,now if we look at the square wave output,does the amplitude of  it follow as the input sine wave  up and downs,
like when You get a lower drive than You get a lower output,drive with a higher drive You got a higher output ,with the same square wave.

Remember,a square wave output is the most efficient  transmitter design,
It's the amplitude of it ,that's important does it follow the amplitude of the input amplitude after modulation as a whole.

In my opinion ,It's not "important" what the Carrier wave looks like,But the amplitude of the input carrier and the output carrier,does it follow liniearlly,when modulated.
Of course I may be wrong.

Gito.n

[W1RKW]

Hi Stu,
I never removed that capacitor.  It's still in place but it's not a 0.1uf but rather a 0.01uf.  You think I should up it by 10x?

I was looking at the app notes on CCI website.  They show this amp running an AM sig at about 50watts out with a pretty nice envelope with 1.5watts in.  What I also noticed in their app notes was the PCB layout.  The layout is much different than the current layout. I wonder if this has something to do with the waveform I'm seeing and even though there is a negative feedback network between the collector and base that this thing is still breaking out into oscillation during certain parts of the waveform cycle.  Maybe we haven't hit the sweet spot.  But then again maybe Gito has a point too.  Don't know.  Never worked on a broad band amp before.

I'm stumped.  This thing certainly looks like its running in saturation.  

[AB2EZ]

Bob

You need an RF path to ground on the input transformer center tap whose impedance is 1 ohm of less at 3.885 MHz (corresponding the the r.f. input impedance from each transistor's base to ground)

.01uF at 3.885 MHz is 4 ohms.

Try 0.1 uF

Stu

P.S. You're getting there.

[WA1GFZ]

you might try a 1uf ceramic at the input ct. seems like the gain is very high.

[AB2EZ]

Frank

With all due respect... I don't think its a gain problem. I think its a base current starvation problem.

One transistor's r.f. base current is being pulled from the other transistor (instead of from ground, via the center tap on the secondary of the input transformer) ... but that can only continue up to the level of the base bias. Beyond that, the transistor that is now starved for base current can only pull what it needs from the 10 ohm resistor from base-to-ground (10x the impedance of the base itself). This causes two effects.

1. The base current ... and thus the collector current abruptly levels off

2. The sudden change causes a glitch that stimulates oscillatory behavior

The 270uF electrolytic is, for some reason, not acting like a low enough impedance to ground. A 0.1uF is probably sufficient... but a 1.0 uF would be even better (if it behaves like 1uF at RF frequencies...i.e. 0.04 ohms to ground or so at 3.885 MHz)

Stu

[Gito]

Hi

Speaking of the bias suplly ,it used a MJE 243 conectet  as a replacement of 1N4497 diode ,and used as a "zener" diode to give a fix  + bias suplly ,to Make the Transmitter a class B or AB2 system.
It must have a stif bias supply.
So i'm curios  of the droping resistor is it R1 with a 100kohm value,if it so with a little current flowing in the base of the transmitter,the bias wiil go to Zero (droping resistor X current).

Maybe the right value is 100 ohm ,so there's about 100 ma flowing in the circuit of the bias supply that make it a Stif supply.

If i'm not mistaken 1 N4997 diode has a 50 watt power rating ,so it can disipated that current.

 Is the value of R1 and R4  wrong (interchanged)meaning the value of R1 is 33 ohm and R4 100 kohm

Gito.n