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Old 8th April 2004, 12:21 AM   #1
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Default oscillating...always oscillating

Alright, so I designed this simple symmetrical small signal amplifier and I can't for the life of me get it to stop oscillating. Image of circuit is attached; any ideas?
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Old 8th April 2004, 12:52 AM   #2
Magura is offline Magura  Denmark
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Sure, I got a great idea!!!

Attach the image


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Old 8th April 2004, 01:16 AM   #3
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My bad (obviously I'm new at this)
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Old 8th April 2004, 04:06 AM   #4
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okay, I guess I can't figure out how to upload an image, so here's a link to the circuitClick the image to open in full size.
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Old 8th April 2004, 07:45 AM   #5
sajti is offline sajti  Hungary
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This circuit has some problem. If You use this VAS stage, You can't put current mirror over the input stages. Replace them with simple resistors!
Is oscillation still exist, use compensation (c-b capacitors for Q10, and Q11), or use emitter degeneration resistors for the input stage, or resistors between the base of the outputs and the ground. The last two solution reduce the open loop gain.

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Old 8th April 2004, 08:01 AM   #6
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I agree. There is too much OL gain. The closed loop gain is 1x; it basically is a buffer. Do you want that? If you want gain, attenuate the feedback path, that will also decrease loop gain and increase stability.

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Old 8th April 2004, 08:07 AM   #7
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You need to add feedback compensation in one form or another. The bulletproof way is to put a 100pf or so cap from base to emitter of both VAS transistors. Another way of doing it is to use feedforward, a small 20pf or so cap from the VAS collector (either) to the negative feedback node, bases of both transistors.

In passing, you have way too much output bias. Only two diodes are needed to compensate for 2 Vbe drops of the output devices. And that's perhaps too high, depending on the actual part variations. You really need an adjustable Vbe multiplier to set bias. Most amps have them.
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Old 8th April 2004, 08:16 AM   #8
sajti is offline sajti  Hungary
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I think the biggest problem is the current mirrors. With current mirrors, there is nothing to set the bias for Q10, and Q11. Solve this problem first! You have too high bias currant for the input stages (I count it about 8mA!!!) Reduce it about 2mA, or lower. Set resistors instead of current mirrors, with about 1kohm. This will results 7mA for the Q10 and Q11.
Use 330-470ohm emitter degeneration resistors for the input stages. This will increase the input impedance, and lower the distortion in the first stage, and also reduce the OL gain.
I think that 30mA bias current is OK for the output stage. They works in class A in most of the application. If the oscillation still exist, use some 22-47pF between the c-b of Q10 and Q11.

I don't know if You interest but I think non feedback buffer would be better.

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Old 8th April 2004, 09:09 AM   #9
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Thumbs up Oscillating amp Design

Hi WoulYaShassu,
That's funny as you designed an amp just like the one I was going to try. I don't see a problem with the current mirrors. I would use a Darlington connected pair for the VAS transistors.
I agree with slowhands that you should add some compensation from the collector of the VAS transistors to the inverting input. Is this feedforward? I would could this feedback. Also a few 10's of pF over the feedback resistor may help.
It is absolutely important to decouple the VAS and inputstage from the outputstage by means of a small resistor of 10-30 Ohm and a decoupling cap or by a capmultiplier as in John Curls Parasound amplifiers.
Your circuit has a gain of one but could easily modified to a circuit with gain by adding a resistor Rg from the inverting input to ground. This will also improve the stability situation! Gain = 1+ R15/Rg.
I calculated some 6mA for the current sources for the inputstage which is a bit on the high side. Something like 2mA will do. See this thread for an idea making current sources with a LED and a transistor but it could also be done with FETs as Sonnya does.
Jfet/bipolar PCB and Schematic (finaly)
Two diodes will do for bias setting of the outputstage unless you are after class A. These diodes should be in close thermal contact with the output transistors or else you will get huge thermal runaway. Diode connected (base tied to the connector) transistors make better thermal contact. An amplified diode can be made adjustable but should be bypassed with a good cap.
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Old 8th April 2004, 09:54 AM   #10
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Hi WoulYaShassu,
I like the current mirrors also, Do try the phase compensation from one VAS collector to the bases of the feedback transistors. You will probably also need a global phase compensation capacitor across the global feedback resistor, as Elso mentioned. Its value may be about half VAS stage one is. They may be about 10pF and 4.7pF, respectively, give or take several pF. Also, place a 0.1uF capacitor across your bias diodes. If you are not using diode references for your constant current sources, you may as well just use a single resistor to each power rail. Also, don't forget to voltage divide the global feedback so that your circuit has gain. You can use a capacitor from the lower resistor to ground to preserve a unity DC gain.

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