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Old 1st January 2004, 09:04 PM   #1
peufeu is offline peufeu  France
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Default Baxandall Super Pair

Hello,

I'm trying to use the Baxandall Super Pair, but everytime it will oscillate, in the simulator or on the bench.

As an emitter follower it's very stable (fixed collector voltage, CCS emitter load).

But if I try to use it with a varying collector voltage (typically in a VAS, low resistor emitter load, CCS collector load), I get oscillations.

Using the Super Pair as a Cascode also results in oscillations. Measured around 50-100 MHz on the scope.

Anyone has an idea on how to tame the beast ?

I've been playing with base stoppers to no avail... need help !

The attached circuit oscillates around 100 MHz.
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Old 1st January 2004, 10:24 PM   #2
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Resistance inserted in series with the Bases of the
devices is usually the first thing to try.
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Old 1st January 2004, 10:28 PM   #3
Pedja is offline Pedja  Serbia
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Hi Pierre,

Not really 100MHz per MicroCap and neglecting parasitic properties, but it seems like it doesn’t happen accidentally. Try the compensation capacitor (from the base of the first to the collector of the second transistor), say 68pF.

Pedja
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Old 1st January 2004, 10:48 PM   #4
jcx is offline jcx  United States
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at such a high frequency this is probably a very local phenomena, possibly similar to the oscillation of a capacitively loaded single transistor emitter follower where the capacitive load is reflected to the input as a negative resistance, resulting in oscillation for some source impedances - Nelson's suggestion can be appropriate to this type of oscillation

at some point an analysis using the full set of hybrid-pi transistor parameters is useful - the source and termination circuit impedances are important in determining the stable operation region

i have built the super pair a few times in current output circuits without obvious problems, but i haven't used it in a VAS circuit with high and complex base source impedance and possibly negative impedance active load - i believe you can find several simulations i have posted that suggest it can work as a VAS, but hardware results trump simulation every time
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Old 1st January 2004, 11:26 PM   #5
andy_c is offline andy_c  United States
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Okay, I played around with this a little more in simulation. I think my earlier emitter resistor mod was some kind of fluke. It doesn't seem to help this circuit. Using a circuit similar to yours in the simulator (shown below), I got 31 dB of peaking in the voltage gain at around 53 MHz. An oscillator for sure. Then I started putting various values of series resistance into the base of the PNP transistor. I was quite surprised at how large this resistor had to be. With a series base resistor of 1k, I still get peaking, about 5 dB at 34 MHz. This is probably enough to kill the oscillation. It make me nervous though. Even with values as large as 10k, I still get some peaking.
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Old 1st January 2004, 11:52 PM   #6
Pedja is offline Pedja  Serbia
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Quote:
Originally posted by jcx
at such a high frequency this is probably a very local phenomena, possibly similar to the oscillation of a capacitively loaded single transistor emitter follower where the capacitive load is reflected to the input as a negative resistance, resulting in oscillation for some source impedances - Nelson's suggestion can be appropriate to this type of oscillation
Quote:
Originally posted by andy_c
Then I started putting various values of series resistance into the base of the PNP transistor. I was quite surprised at how large this resistor had to be. With a series base resistor of 1k, I still get peaking, about 5 dB at 34 MHz. This is probably enough to kill the oscillation. It make me nervous though. Even with values as large as 10k, I still get some peaking.
JCX’s explanation seems right to me, and as it can be seen I showed already what is happening at the input. But, if the non-zero source impedance is the cause of the problem, is it really the added impedance here the way to go? I see that there is a critical range of the impedance, but as Andy pointed out, you should probably have very high base stopper value to tame the thing this way and thus the other things would come into the consideration. In the graph I posted above the source impedance is 1k and yes, the peak shown by the small signal analysis changes its amplitude and freq as the source impedance changes (graph below).

Btw, even if used like the emitter follower, “super pair” shows more problems with capacitive loads than any voltage follower without global (classic) f/b that I simulated.

Pedja
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Old 2nd January 2004, 12:37 AM   #7
andy_c is offline andy_c  United States
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I tried a couple more things. I set up a simulation similar to the circuit I showed in Post #5 above, except using the circuit peufeu codenamed "funky" in the "amplifier topology subjective effects" thread here http://www.diyaudio.com/forums/showt...698#post286698. The "funky" circuit also shows peaking in the frequency response in the 50-100 MHz range. Then I tried a different transistor for Q14 in the diagram of that earlier post, and varied its beta to vary the amount of compensating current fed back to the emitter of the main common base amp. With very large beta in the "sensing" transistor, the compensating current is almost zero and the peaking goes away. But when the transistors are nearly identical for beta, the peaking in the response again comes back.

So I thought, "what if the very act of feeding the base current back to the emitter is what's causing the instability?". This seems possible, since two circuits that use this technique (super pair and "funky") have the same problem. But there's another - the Hawksford cascode. I've shown the circuit below. How does it do?

This circuit has 34 dB of peaking at 57 MHz. The conclusion I'm reaching from all of this is quite unfortunate: Circuits which attempt to compensate the base current, such as the Hawksford cascode, the Baxandall super pair, and the "funky" circuit suck for high-frequency stability. Depressing.
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Old 2nd January 2004, 12:37 AM   #8
jcx is offline jcx  United States
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while i still would go for a thorough mathematical analysis before selling anything using this, i think i've found a correlation that makes sense - the input Q should be similar or slower speed than the output Q which explains my satisfaction with the circuit and the difficulties seen using it with higher V, slower devices in a power amp VAS

a possible approach below tames the worst case combination of andy's slow mje340 with the fast 2n3906 - i haven't looked at how much it compromizes performance

(i only kinda trust transient analysis in spice, .AC not at all, it can go wrong many ways)

(this is using trapazoidal in LT SwCadIII)

play with C2, R3 values; 500pF, 30 Ohms gives very good pulse
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Old 2nd January 2004, 01:10 AM   #9
andy_c is offline andy_c  United States
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As an added note here, I simulated the ft of the MJE340 at a bias current of 5 mA with VCE of 4 Volts - about the same conditions as what I used in the simulation above. ft comes out to 114 MHz. So while it's not a screamer by any means, it's not a dog either. Low-frequency AC beta is about 54 under these conditions. Unfortunately, the data sheet of these devices says so little it's hard to say if the models are realistic.
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Old 2nd January 2004, 06:23 AM   #10
PMA is offline PMA  Europe
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Default Re: Baxandall Super Pair

Quote:
Originally posted by peufeu
Hello,

I'm trying to use the Baxandall Super Pair, but everytime it will oscillate, in the simulator or on the bench.


I've been playing with base stoppers to no avail... need help !

The attached circuit oscillates around 100 MHz.
This kind of circuit needs some compensating capacitor. Try 470 pF between base and collector of the driver or output device .
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