Unusual amplifier oscillation

[Duo]

This has been the subject of extreme hair pulling for me for a while now and I've decided to give in and ask someone else about it.

An amplifier I've put together has the most interesting characteristic of oscillation that I've ever seen.

To start, it has a darlington output stage and operates in class B.

First off, with a turnoff resistor from base to base of the last main output transistors, the amplifier oscillates with a clean sine wave of about 200mV at 7MHz.

Second, with this resistor removed from the circuit, there is no oscillation during idle; it is when peaks of audio appear that the oscillation shows up. It appears on only the positive swings, and is proportional to power output. (This only occurs when loaded with a reactive load; if loaded resistively, the amplifier does not have this unusual 'peak' oscillation.)

It is more interesting because with the turnoff resistor in place, the 'peak' oscillation does not manifest itself, nor does the signal look different on the scope when the load is installed. It simply remains a good audio signal with 7MHz on it and doesn't change no matter what load I put on.

Does anyone possibly have any ideas at all???

Thanks

[Duo]

No ideas anyone?

[darkfenriz]

Originally posted by Duo
No ideas anyone?

Rather no ideas
I only suspect it is not related to the amplifier itself as 7MHz is pretty high. Not also output stage as it is (not triple) darlinton. Probably something with grounds and supplies (did you use 100nF for supplies?).

[anatech]

Hi Duo,
Are the supplies bypassed locally (near the outputs)? Is the bias circuit bypassed?

-Chris

[Duo]

Yes, everything is bypassed as far as I can tell.

I have done more testing and observed the problem in greater detail.

(The following testing was done with a turnoff resistance double what it was before. It seems that by doubling the resistance, the amplifier stopped oscillating when unloaded.)

I have determined that the oscillation occurs when any power is delivered to the load, at any frequency.

To do this, I connected a variable oscillator to the input and set the frequency to 2Hz. On a dual channel scope, I watched the loaded output and an unloaded output. The results were highly interesting.

The result was a line moving up and down at 2Hz (the loaded output) and a 7MHz oscillation on the other (only when the 2Hz signal peaked.) As the 2Hz sine passed zero, there was no oscillation.

If the load was removed, no matter what input, the amplifier would not oscillate at all.

Of conclusions I draw from this, an important one is the loading of the bias network for the output stage; it seems that when current flows in this network, the system begins to oscillate beautifully.

I'll enter another fact here: This amplifier, as I stated earlier, is biased to be class B. The same design, when operated in class AB or A, does not exhibit such oscillation at all; even when loaded heavily or with highly complex loads.

My future plan was to turn this into a class AB amplifier, but for testing purposes, I operated it in class B with diode bias.

So far, I believe something fishy is happening inside that bias network.

[anatech]

Hi Duo,
It could be that the GBP of the NPN's increases enough with current to become unstable. Try a little bias with no signal and no load. Increase the bias if it doesn't oscillate breifly to find out what approx current level the oscillations start.
-Chris

[Duo]

Well, this is a little confusing since the bias is class B. (No idle current flow.)

[megajocke]

Do you have a schematic? What output devices and drivers are you using?

The "dead-band" of class B will cause problems. Especially with reactive loads. Why do you want it to run in class B?

Some other things it could be:

• Fast output devices without base resistors
• Incorrect compensation

[anatech]

Hi Duo,
Exactly. with no bias the oscillation gets killed. When current flows the gain of the transistor increases, so the Gain Bandwidth Product goes up. That's why I suggested you allow for some bias current with no load and no input signal. You may just need a zobel network from the output to speaker common.

-Chris

[Duo]

Okay, well, first off, I have changed my point first stated about reactive loads.

I missed something earlier, but the amplifier does the same thing into reactive or resistive loads; therefore, it's not a reactive load causing the oscillation.

The amplifier uses four pairs of MJ15003/15004 outputs per channel, and each channel uses a single pair of MJE340/350 for drivers.