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Old 2nd January 2004, 08:25 AM   #11
sonnya is offline sonnya  Denmark
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Hi Andy.

A solution would be to change the voltage source from 5.6V down to 2.7V, Then add a 10R + 1uH in series with this "zenerdiode" like shown below. Now i only has a peaking of 3dB at 50MHz...

The phaseshift at 50MHz is only 24 degress. I think i could live with that....

---R10R----L1uH----Z2.7V---

Regards

Sonny
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Old 2nd January 2004, 10:57 AM   #12
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Quote:
Originally posted by andy_c
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.
Andy, this is to be expected as the super pair has a small amount
of + feedback to the emitter of OP BJT. I haven't yet played with
super pair (will soon) but I think a suitable way to tame it may
be as shown with damping at the problem spot.
You need both R's to be able to get suitable voltage across
driver BJT. I have omitted base stoppers for simplicity, they
may still be needed but I don't think so.

Cheers,

Terry

ps - hope image attachment works

[IMG]http://[/IMG]
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Old 2nd January 2004, 10:59 AM   #13
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[QUOTE]Originally posted by Terry Demol
[B]
ps - hope image attachment works

Apologies, will try and sort this out.

T
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Old 2nd January 2004, 11:12 AM   #14
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Quote:
Originally posted by Terry Demol


Andy, this is to be expected as the super pair has a small amount
of + feedback to the emitter of OP BJT. I haven't yet played with
super pair (will soon) but I think a suitable way to tame it may
be as shown with damping at the problem spot.
You need both R's to be able to get suitable voltage across
driver BJT. I have omitted base stoppers for simplicity, they
may still be needed but I don't think so.

Cheers,

Terry

Insert one R at collector of IP BJT and another at Emitter
of OP BJT. OP is connected to Emitter of OP BJT above R.
Damping R's are connected together and to CCS load at other
end. Clear as mud :-)

Cheers,

Terry
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Old 2nd January 2004, 05:12 PM   #15
jcx is offline jcx  United States
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this seem to work, i think it can be considered a RC "stopper" across the input Q2_cb; at high frequencies the RC shorts out the gain of Q2

this is equivalent to my first schematic with the loop minimized by going directly across Q2_cb instead of including V3 cascode voltage source which would introduce parasitics at 100 MHz in the real world

to regain more audio frequency performance i split the C and bootstrapped the midpoint to Q2_e, this gives the expected 40+ dB improvement of output Z over the simple cascode with the mje140 up to 40 KHz, and 20 dB better performance to beyond 1 MHz; the R3, C2,4 chain must have very low parasitics at 100 MHz, NPO surface mount would be best

(i also fixed the output current, its now 10 mA as i intended – it was only ~3mA in my earlier schem)
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Old 2nd January 2004, 09:47 PM   #16
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Quote:
Originally posted by jcx
this seem to work, i think it can be considered a RC "stopper" across the input Q2_cb; at high frequencies the RC shorts out the gain of Q2

this is equivalent to my first schematic with the loop minimized by going directly across Q2_cb instead of including V3 cascode voltage source which would introduce parasitics at 100 MHz in the real world

to regain more audio frequency performance i split the C and bootstrapped the midpoint to Q2_e, this gives the expected 40+ dB improvement of output Z over the simple cascode with the mje140 up to 40 KHz, and 20 dB better performance to beyond 1 MHz; the R3, C2,4 chain must have very low parasitics at 100 MHz, NPO surface mount would be best

(i also fixed the output current, its now 10 mA as i intended – it was only ~3mA in my earlier schem)
JCX,

Try these mods, Q3 should have a base stopper, insert R between Q2 collector and Q3 collector, insert R between Q1 emitter and Q3 collector. Remove the compensation network.

Cheers,

Terry
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Old 2nd January 2004, 10:41 PM   #17
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This is completely off topic but I must say the knowledge of electronics being thrown around here is numbing and free.

What a great forum.
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Old 3rd January 2004, 01:04 AM   #18
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Quote:
Originally posted by andy_c
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.
The Baxandall It does seem like a lot of trouble, but that's
because it has quite a bit of gain and inherently positive local
feedback. It's begging to be an oscillator.

As to the "Hawksford" cascode, it is inherently pretty stable,
however I would categorize it as an ordinary cascode since
the topology has been around for a very long time. I used to
buy monolithic JFET differential pairs that were internally
cascoded in a like manner.

Also beware of trusting your simulator too much.....
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Old 3rd January 2004, 03:18 AM   #19
jcx is offline jcx  United States
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Actually I find the cascode that returns the base current to the lower transistor to be nearly equivalent to the super-pair (I agree that “Hawksford” cascode is inappropriate as this version of the connection appears long before his paper but we do need a concise way to differentiate between the subtle flavors of cascodes and Hawksford does provide a service in pointing out the difference in performance)

Below I show a cascode with the base current returned to the lower Q emitter rings but isn’t outright unstable, however in the interest of a fair comparison to the compound super-pair cascode, I have shown it with a pure current source emitter load and it spontaneously oscillates as does the compound circuit

The super-pair may be seen to be topologically nearly equivalent to the cascode with base current compensation, just follow the pairings of emitter-base, collector-emitter between the 2 Qs of each circuit – you simply have to “twist” the input Q from NPN to PNP (and eliminate the Vref) to go from cascode to super-pair

(I don’t trust my simulator, I just don’t have to leave my armchair to go for a ride)

Terry: which drawing?
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Old 3rd January 2004, 06:04 AM   #20
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I like Terry's damper idea in the loop.
Don't forget the miller cap. You must have one of these in the final circuit so don't get too carried away until you've included this in your simulations.
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