Base Stoppers

[lumanauw]

I notice that base stoppers may help to stop oscilation.

What is the mechanism with this 10R-100R in series with base that makes it stops oscilation? What is wrong with transistor's plain base?

When searching here, some members explaining about "high impedance" meeting "low impedance" terms, but I don't get it. How to understand this "impedance" terms more easily?

Some designs "Replace" this base resistor with a B-E resistor about 470ohm (without series base resistors here).

Is this 470R B-E resistor stops oscilation by shooting at the same cause (that series base resistor targets)?

[lineup]

RC filter can be used to limit higher frequency.

MOSFETs have an input capacitance.
Gate resistor provides the R and so we have RC filter.
Gate resistor or Base resistor should be put as close as possible
to Gate or Base.

This way we can stop high freq oscillation, if we need.

The higher value of R, the lower this filters cut off freq will be.

[HarryDymond]

Hi everyone,

I have a similar question. I have constructed an amplifier (schematic here), and was surprised to discover that the output stage suffers from parasitic oscillation (surprised since it is an EF, and I didn't think they suffered from parasitic oscillation, and there are base-stopping resistors). The oscillation is at approximately 3.57 MHz with an amplitude of just over 5 Volts peak to peak.

Please note the following:

The power supply is ±45 V regulated (coming from a TTi CPX400A bench P.S.U.)

The output stage suffers from parasitic oscillation even with only one pair of output transistors (2SA1302/2SC3281) connected.

With no output transistors connected, no load connected (apart from oscilloscope) and the junction of R17 and R28 (i.e., the output junction) connected to the junction of Q14 emitter and R16 (i.e. feedback applied around input stage, VAS and drivers), the amplifier works as expected with no unwanted oscillations. This suggests a problem with the output stage

With all output transistors connected, but with the feedback point moved to the junction of D4 and D5 (i.e. running the amplifier with the output stage open-loop), the oscillation is still present.

MBR150 diodes are schottky types with a Vf of 0.3 V at the VAS operating current. In conjunction with the 1N4148 diodes in the bias chain, they result in a Vq across the output resistors of around 80 mV, resulting in an Iq of about 364 mA per output device. There is a 47 uF cap across the diodes which I forgot to put in the schematic.

The amp. is DC coupled as I'm not intending to use this amplifier for audio. Rather, it is destined to become part of my test equipment and needs to be able to output arbitrary waveforms that may or may not have DC components.

If any one has any comments on this, please let me know. I hope that any insights you have will also assist lumanauw.

[Upupa Epops]

Try to connect R15+C3 from middle between D4 and D5, not from output....

[lumanauw]

In one thread "Miller or not miller compensation", some member said an interesting thing.

At a certain frequency, the transistor base acts like a inductor, and this base stopper gives real impedance to the base.

What is the meaning of this? How to understand it easily?

The predriver is usually emitor follower (common collector). Also from the same thread, this predriver needs base stoppers because the VAS output (collector) is low impedance.

I cannot imagine what is low and high impedance. VAS collector gives low impedance? Is there an easy way to understand this?

[lumanauw]

Hi, HarryDymond,

I'm not sure, but with oscilation you must try and error

-put 1k in base of Q3
-change the double pole (c6,c7,r44) to ordinary single pole compensation. Search for value
-before Q24, you can try putting a buffer (emitor follower), because you use current mirror in differential
-try feedforward cap, ie : pf cap from VAS output to inverting input of differential
-put base stoppers infront of Q6-Q14, values up to 470R or put 470R between those B-E (This is what I asked )
-Split R16 to 2x100ohm, with junction connected to output, if you are biasing the output stage in classAB.

[mzzj]

Originally posted by lineup
RC filter can be used to limit higher frequency.

Ferrite beads are sometimes also used on gate for same purpose. Maybe more common in SMPS than amps tho. Choosing right one is more tricky and availlability is not nearly as good as resistors..

[AndrewT]

Hi,
what if the output stage is not causing the problem?

Could the regulated lab type PSU be the source of the oscillation?

[lineup]

You could try to put resistor in supply rail to VAS/input,
maybe 10-47 Ohm between R2 and collector of Q6.
And add a capacitor 100uF or so from the R2 side to ground.
Same for the negative rail.

In this way we protect input/VAS to be effected by supply variations
caused by the output stage.
Stop unwanted 'feedback' to reach input stage.

I dont know if this is a problem,
but you use very high value resistors in feedback, 91k / 10 k.
Same to set the input impedance. (= 100k )
This is not often seen.

[poobah]

Hi Harry,

Remove R25 & C3, this will run the amp open loop. Check to see if your output is stable. If not, then you are dealing with a parasitic. If it is stable, then you are dealing with a loop compensation problem... different beast.

You can then maesure the gain and phase response of the amp. Can you measure the phase response? Need a primer??? You will need a sine wave generator that will go to 10 mHz and a scope...